David Gibson
19 years ago
2 changed files with 640 additions and 0 deletions
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|
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@STRING{pub-IEEE = "IEEE Computer Society"} |
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@STRING{pub-IEEE:adr = "345 E. 47th St, New York, NY 10017, USA"} |
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|
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@BOOK{IEEE1275, |
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key = "IEEE1275", |
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title = "{IEEE} {S}tandard for {B}oot ({I}nitialization {C}onfiguration) {F}irmware: {C}ore {R}equirements and {P}ractices", |
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publisher = pub-IEEE, |
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address = pub-IEEE:adr, |
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series = "IEEE Std 1275-1994", |
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year = 1994, |
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} |
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|
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@BOOK{IEEE1275-pci, |
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key = "IEEE1275-pci", |
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title = "{PCI} {B}us {B}inding to: {IEEE} {S}td 1275-1994 {S}tandard for {B}oot ({I}nitialization {C}onfiguration) {F}irmware", |
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publisher = pub-IEEE, |
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address = pub-IEEE:adr, |
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note = "Revision 2.1", |
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year = 1998, |
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} |
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|
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@MISC{noof1, |
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author = "Benjamin Herrenschmidt", |
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title = "Booting the {L}inux/ppc kernel without {O}pen {F}irmware", |
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month = may, |
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year = 2005, |
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note = "v0.1, \url{http://ozlabs.org/pipermail/linuxppc64-dev/2005-May/004073.html}", |
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} |
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|
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@MISC{noof5, |
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author = "Benjamin Herrenschmidt", |
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title = "Booting the {L}inux/ppc kernel without {O}pen {F}irmware", |
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month = nov, |
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year = 2005, |
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note = "v0.5, \url{http://ozlabs.org/pipermail/linuxppc64-dev/2005-December/006994.html}", |
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} |
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|
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@MISC{dtcgit, |
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author = "David Gibson et al.", |
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title = "\dtc{}", |
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howpublished = "git tree", |
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note = "\url{http://ozlabs.org/~dgibson/dtc/dtc.git}", |
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} |
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\documentclass[a4paper,twocolumn]{article} |
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|
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\usepackage{abstract} |
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\usepackage{xspace} |
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\usepackage{amssymb} |
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\usepackage{latexsym} |
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\usepackage{tabularx} |
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\usepackage[T1]{fontenc} |
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\usepackage{calc} |
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\usepackage{listings} |
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\usepackage{color} |
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\usepackage{url} |
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|
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\title{Device trees everywhere} |
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\author{David Gibson \texttt{<{dwg}{@}{au1.ibm.com}>}\\ |
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Benjamin Herrenschmidt \texttt{<{benh}{@}{kernel.crashing.org}>}\\ |
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\emph{OzLabs, IBM Linux Technology Center}} |
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|
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\newcommand{\R}{\textsuperscript{\textregistered}\xspace} |
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\newcommand{\tm}{\textsuperscript{\texttrademark}\xspace} |
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\newcommand{\tge}{$\geqslant$} |
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%\newcommand{\ditto}{\textquotedbl\xspace} |
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|
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\newcommand{\fixme}[1]{$\bigstar$\emph{\textbf{\large #1}}$\bigstar$\xspace} |
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|
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\newcommand{\ppc}{\mbox{PowerPC}\xspace} |
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\newcommand{\of}{Open Firmware\xspace} |
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\newcommand{\benh}{Ben Herrenschmidt\xspace} |
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\newcommand{\kexec}{\texttt{kexec()}\xspace} |
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\newcommand{\dtbeginnode}{\texttt{OF\_DT\_BEGIN\_NODE\xspace}} |
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\newcommand{\dtendnode}{\texttt{OF\_DT\_END\_NODE\xspace}} |
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\newcommand{\dtprop}{\texttt{OF\_DT\_PROP\xspace}} |
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\newcommand{\dtend}{\texttt{OF\_DT\_END\xspace}} |
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\newcommand{\dtc}{\texttt{dtc}\xspace} |
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\newcommand{\phandle}{\texttt{linux,phandle}\xspace} |
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\begin{document} |
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|
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\maketitle |
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|
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\begin{abstract} |
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We present a method for booting a \ppc{}\R Linux\R kernel on an |
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embedded machine. To do this, we supply the kernel with a compact |
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flattened-tree representation of the system's hardware based on the |
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device tree supplied by Open Firmware on IBM\R servers and Apple\R |
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Power Macintosh\R machines. |
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|
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The ``blob'' representing the device tree can be created using \dtc |
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--- the Device Tree Compiler --- that turns a simple text |
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representation of the tree into the compact representation used by |
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the kernel. The compiler can produce either a binary ``blob'' or an |
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assembler file ready to be built into a firmware or bootwrapper |
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image. |
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|
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This flattened-tree approach is now the only supported method of |
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booting a \texttt{ppc64} kernel without Open Firmware, and we plan |
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to make it the only supported method for all \texttt{powerpc} |
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kernels in the future. |
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\end{abstract} |
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|
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\section{Introduction} |
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|
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\subsection{OF and the device tree} |
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|
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Historically, ``everyday'' \ppc machines have booted with the help of |
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\of (OF), a firmware environment defined by IEEE1275 \cite{IEEE1275}. |
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Among other boot-time services, OF maintains a device tree that |
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describes all of the system's hardware devices and how they're |
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connected. During boot, before taking control of memory management, |
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the Linux kernel uses OF calls to scan the device tree and transfer it |
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to an internal representation that is used at run time to look up |
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various device information. |
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|
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The device tree consists of nodes representing devices or |
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buses\footnote{Well, mostly. There are a few special exceptions.}. |
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Each node contains \emph{properties}, name--value pairs that give |
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information about the device. The values are arbitrary byte strings, |
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and for some properties, they contain tables or other structured |
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information. |
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|
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\subsection{The bad old days} |
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|
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Embedded systems, by contrast, usually have a minimal firmware that |
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might supply a few vital system parameters (size of RAM and the like), |
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but nothing as detailed or complete as the OF device tree. This has |
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meant that the various 32-bit \ppc embedded ports have required a |
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variety of hacks spread across the kernel to deal with the lack of |
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device tree. These vary from specialised boot wrappers to parse |
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parameters (which are at least reasonably localised) to |
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CONFIG-dependent hacks in drivers to override normal probe logic with |
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hardcoded addresses for a particular board. As well as being ugly of |
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itself, such CONFIG-dependent hacks make it hard to build a single |
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kernel image that supports multiple embedded machines. |
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|
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Until relatively recently, the only 64-bit \ppc machines without OF |
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were legacy (pre-POWER5\R) iSeries\R machines. iSeries machines often |
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only have virtual IO devices, which makes it quite simple to work |
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around the lack of a device tree. Even so, the lack means the iSeries |
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boot sequence must be quite different from the pSeries or Macintosh, |
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which is not ideal. |
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|
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The device tree also presents a problem for implementing \kexec. When |
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the kernel boots, it takes over full control of the system from OF, |
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even re-using OF's memory. So, when \kexec comes to boot another |
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kernel, OF is no longer around for the second kernel to query. |
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|
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\section{The Flattened Tree} |
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|
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In May 2005 \benh implemented a new approach to handling the device |
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tree that addresses all these problems. When booting on OF systems, |
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the first thing the kernel runs is a small piece of code in |
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\texttt{prom\_init.c}, which executes in the context of OF. This code |
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walks the device tree using OF calls, and transcribes it into a |
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compact, flattened format. The resulting device tree ``blob'' is then |
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passed to the kernel proper, which eventually unflattens the tree into |
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its runtime form. This blob is the only data communicated between the |
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\texttt{prom\_init.c} bootstrap and the rest of the kernel. |
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|
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When OF isn't available, either because the machine doesn't have it at |
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all or because \kexec has been used, the kernel instead starts |
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directly from the entry point taking a flattened device tree. The |
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device tree blob must be passed in from outside, rather than generated |
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by part of the kernel from OF. For \kexec, the userland |
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\texttt{kexec} tools build the blob from the runtime device tree |
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before invoking the new kernel. For embedded systems the blob can |
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come either from the embedded bootloader, or from a specialised |
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version of the \texttt{zImage} wrapper for the system in question. |
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|
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\subsection{Properties of the flattened tree} |
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|
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The flattened tree format should be easy to handle, both for the |
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kernel that parses it and the bootloader that generates it. In |
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particular, the following properties are desirable: |
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|
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\begin{itemize} |
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\item \emph{relocatable}: the bootloader or kernel should be able to |
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move the blob around as a whole, without needing to parse or adjust |
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its internals. In practice that means we must not use pointers |
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within the blob. |
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\item \emph{insert and delete}: sometimes the bootloader might want to |
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make tweaks to the flattened tree, such as deleting or inserting a |
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node (or whole subtree). It should be possible to do this without |
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having to effectively regenerate the whole flattened tree. In |
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practice this means limiting the use of internal offsets in the blob |
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that need recalculation if a section is inserted or removed with |
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\texttt{memmove()}. |
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\item \emph{compact}: embedded systems are frequently short of |
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resources, particularly RAM and flash memory space. Thus, the tree |
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representation should be kept as small as conveniently possible. |
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\end{itemize} |
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|
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\subsection{Format of the device tree blob} |
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\label{sec:format} |
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|
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\begin{figure}[htb!] |
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\centering |
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\footnotesize |
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\begin{tabular}{r|c|l} |
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\multicolumn{1}{r}{\textbf{Offset}}& \multicolumn{1}{c}{\textbf{Contents}} \\\cline{2-2} |
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\texttt{0x00} & \texttt{0xd00dfeed} & magic number \\\cline{2-2} |
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\texttt{0x04} & \emph{totalsize} \\\cline{2-2} |
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\texttt{0x08} & \emph{off\_struct} & \\\cline{2-2} |
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\texttt{0x0C} & \emph{off\_strs} & \\\cline{2-2} |
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\texttt{0x10} & \emph{off\_rsvmap} & \\\cline{2-2} |
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\texttt{0x14} & \emph{version} \\\cline{2-2} |
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\texttt{0x18} & \emph{last\_comp\_ver} & \\\cline{2-2} |
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\texttt{0x1C} & \emph{boot\_cpu\_id} & \tge v2 only\\\cline{2-2} |
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\texttt{0x20} & \emph{size\_strs} & \tge v3 only\\\cline{2-2} |
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\multicolumn{1}{r}{\vdots} & \multicolumn{1}{c}{\vdots} & \\\cline{2-2} |
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\emph{off\_rsvmap} & \emph{address0} & memory reserve \\ |
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+ \texttt{0x04} & ...& table \\\cline{2-2} |
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+ \texttt{0x08} & \emph{len0} & \\ |
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+ \texttt{0x0C} & ...& \\\cline{2-2} |
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\vdots & \multicolumn{1}{c|}{\vdots} & \\\cline{2-2} |
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& \texttt{0x00000000}- & end marker\\ |
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& \texttt{00000000} & \\\cline{2-2} |
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& \texttt{0x00000000}- & \\ |
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& \texttt{00000000} & \\\cline{2-2} |
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\multicolumn{1}{r}{\vdots} & \multicolumn{1}{c}{\vdots} & \\\cline{2-2} |
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\emph{off\_strs} & \texttt{'n' 'a' 'm' 'e'} & strings block \\ |
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+ \texttt{0x04} & \texttt{~0~ 'm' 'o' 'd'} & \\ |
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+ \texttt{0x08} & \texttt{'e' 'l' ~0~ \makebox[\widthof{~~~}]{\textrm{...}}} & \\ |
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\vdots & \multicolumn{1}{c|}{\vdots} & \\\cline{2-2} |
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\multicolumn{1}{r}{+ \emph{size\_strs}} \\ |
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\multicolumn{1}{r}{\vdots} & \multicolumn{1}{c}{\vdots} & \\\cline{2-2} |
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\emph{off\_struct} & \dtbeginnode & structure block \\\cline{2-2} |
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+ \texttt{0x04} & \texttt{'/' ~0~ ~0~ ~0~} & root node\\\cline{2-2} |
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+ \texttt{0x08} & \dtprop & \\\cline{2-2} |
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+ \texttt{0x0C} & \texttt{0x00000005} & ``\texttt{model}''\\\cline{2-2} |
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+ \texttt{0x10} & \texttt{0x00000008} & \\\cline{2-2} |
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+ \texttt{0x14} & \texttt{'M' 'y' 'B' 'o'} & \\ |
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+ \texttt{0x18} & \texttt{'a' 'r' 'd' ~0~} & \\\cline{2-2} |
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\vdots & \multicolumn{1}{c|}{\vdots} & \\\cline{2-2} |
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& \texttt{\dtendnode} \\\cline{2-2} |
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& \texttt{\dtend} \\\cline{2-2} |
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\multicolumn{1}{r}{\vdots} & \multicolumn{1}{c}{\vdots} & \\\cline{2-2} |
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\multicolumn{1}{r}{\emph{totalsize}} \\ |
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\end{tabular} |
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\caption{Device tree blob layout} |
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\label{fig:blob-layout} |
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\end{figure} |
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|
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The format for the blob we devised, was first described on the |
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\texttt{linuxppc64-dev} mailing list in \cite{noof1}. The format has |
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since evolved through various revisions, and the current version is |
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included as part of the \dtc (see \S\ref{sec:dtc}) git tree, |
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\cite{dtcgit}. |
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|
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Figure \ref{fig:blob-layout} shows the layout of the blob of data |
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containing the device tree. It has three sections of variable size: |
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the \emph{memory reserve table}, the \emph{structure block} and the |
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\emph{strings block}. A small header gives the blob's size and |
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version and the locations of the three sections, plus a handful of |
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vital parameters used during early boot. |
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|
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The memory reserve map section gives a list of regions of memory that |
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the kernel must not use\footnote{Usually such ranges contain some data |
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structure initialised by the firmware that must be preserved by the |
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kernel.}. The list is represented as a simple array of (address, |
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size) pairs of 64 bit values, terminated by a zero size entry. The |
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strings block is similarly simple, consisting of a number of |
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null-terminated strings appended together, which are referenced from |
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the structure block as described below. |
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|
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The structure block contains the device tree proper. Each node is |
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introduced with a 32-bit \dtbeginnode tag, followed by the node's name |
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as a null-terminated string, padded to a 32-bit boundary. Then |
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follows all of the properties of the node, each introduced with a |
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\dtprop tag, then all of the node's subnodes, each introduced with |
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their own \dtbeginnode tag. The node ends with an \dtendnode tag, and |
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after the \dtendnode for the root node is an \dtend tag, indicating |
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the end of the whole tree\footnote{This is redundant, but included for |
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ease of parsing.}. The structure block starts with the \dtbeginnode |
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introducing the description of the root node (named \texttt{/}). |
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|
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Each property, after the \dtprop, has a 32-bit value giving an offset |
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from the beginning of the strings block at which the property name is |
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stored. Because it's common for many nodes to have properties with |
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the same name, this approach can substantially reduce the total size |
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of the blob. The name offset is followed by the length of the |
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property value (as a 32-bit value) and then the data itself padded to |
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a 32-bit boundary. |
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|
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\subsection{Contents of the tree} |
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\label{sec:treecontents} |
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|
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Having seen how to represent the device tree structure as a flattened |
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blob, what actually goes into the tree? The short answer is ``the |
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same as an OF tree''. On OF systems, the flattened tree is |
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transcribed directly from the OF device tree, so for simplicity we |
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also use OF conventions for the tree on other systems. |
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|
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In many cases a flat tree can be simpler than a typical OF provided |
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device tree. The flattened tree need only provide those nodes and |
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properties that the kernel actually requires; the flattened tree |
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generally need not include devices that the kernel can probe itself. |
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For example, an OF device tree would normally include nodes for each |
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PCI device on the system. A flattened tree need only include nodes |
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for the PCI host bridges; the kernel will scan the buses thus |
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described to find the subsidiary devices. The device tree can include |
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nodes for devices where the kernel needs extra information, though: |
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for example, for ISA devices on a subsidiary PCI/ISA bridge, or for |
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devices with unusual interrupt routing. |
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|
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Where they exist, we follow the IEEE1275 bindings that specify how to |
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describe various buses in the device tree (for example, |
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\cite{IEEE1275-pci} describe how to represent PCI devices). The |
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standard has not been updated for a long time, however, and lacks |
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bindings for many modern buses and devices. In particular, embedded |
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specific devices such as the various System-on-Chip buses are not |
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covered. We intend to create new bindings for such buses, in keeping |
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with the general conventions of IEEE1275 (a simple such binding for a |
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System-on-Chip bus was included in \cite{noof5} a revision of |
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\cite{noof1}). |
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|
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One complication arises for representing ``phandles'' in the flattened |
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tree. In OF, each node in the tree has an associated phandle, a |
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32-bit integer that uniquely identifies the node\footnote{In practice |
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usually implemented as a pointer or offset within OF memory.}. This |
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handle is used by the various OF calls to query and traverse the tree. |
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Sometimes phandles are also used within the tree to refer to other |
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nodes in the tree. For example, devices that produce interrupts |
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generally have an \texttt{interrupt-parent} property giving the |
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phandle of the interrupt controller that handles interrupts from this |
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device. Parsing these and other interrupt related properties allows |
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the kernel to build a complete representation of the system's |
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interrupt tree, which can be quite different from the tree of bus |
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connections. |
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|
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In the flattened tree, a node's phandle is represented by a special |
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\phandle property. When the kernel generates a flattened tree from |
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OF, it adds a \phandle property to each node, containing the phandle |
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retrieved from OF. When the tree is generated without OF, however, |
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only nodes that are actually referred to by phandle need to have this |
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property. |
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|
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Another complication arises because nodes in an OF tree have two |
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names. First they have the ``unit name'', which is how the node is |
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referred to in an OF path. The unit name generally consists of a |
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device type followed by an \texttt{@} followed by a \emph{unit |
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address}. For example \texttt{/memory@0} is the full path of a memory |
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node at address 0, \texttt{/ht@0,f2000000/pci@1} is the path of a PCI |
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bus node, which is under a HyperTransport\tm bus node. The form of |
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the unit address is bus dependent, but is generally derived from the |
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node's \texttt{reg} property. In addition, nodes have a property, |
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\texttt{name}, whose value is usually equal to the first path of the |
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unit name. For example, the nodes in the previous example would have |
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\texttt{name} properties equal to \texttt{memory} and \texttt{pci}, |
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respectively. To save space in the blob, the current version of the |
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flattened tree format only requires the unit names to be present. |
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When the kernel unflattens the tree, it automatically generates a |
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\texttt{name} property from the node's path name. |
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|
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\section{The Device Tree Compiler} |
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\label{sec:dtc} |
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|
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\begin{figure}[htb!] |
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\centering |
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\begin{lstlisting}[frame=single,basicstyle=\footnotesize\ttfamily, |
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tabsize=3,numbers=left,xleftmargin=2em] |
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/memreserve/ 0x20000000-0x21FFFFFF; |
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|
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/ { |
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model = "MyBoard"; |
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compatible = "MyBoardFamily"; |
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#address-cells = <2>; |
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#size-cells = <2>; |
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|
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cpus { |
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#address-cells = <1>; |
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#size-cells = <0>; |
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PowerPC,970@0 { |
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device_type = "cpu"; |
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reg = <0>; |
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clock-frequency = <5f5e1000>; |
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timebase-frequency = <1FCA055>; |
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linux,boot-cpu; |
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i-cache-size = <10000>; |
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d-cache-size = <8000>; |
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}; |
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}; |
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|
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memory@0 { |
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device_type = "memory"; |
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memreg: reg = <00000000 00000000 |
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00000000 20000000>; |
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}; |
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|
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mpic@0x3fffdd08400 { |
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/* Interrupt controller */ |
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/* ... */ |
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}; |
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|
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pci@40000000000000 { |
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/* PCI host bridge */ |
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/* ... */ |
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}; |
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|
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chosen { |
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bootargs = "root=/dev/sda2"; |
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linux,platform = <00000600>; |
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interrupt-controller = |
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< &/mpic@0x3fffdd08400 >; |
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}; |
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}; |
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\end{lstlisting} |
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\caption{Example \dtc source} |
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\label{fig:dts} |
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\end{figure} |
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|
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As we've seen, the flattened device tree format provides a convenient |
||||
way of communicating device tree information to the kernel. It's |
||||
simple for the kernel to parse, and simple for bootloaders to |
||||
manipulate. On OF systems, it's easy to generate the flattened tree |
||||
by walking the OF maintained tree. However, for embedded systems, the |
||||
flattened tree must be generated from scratch. |
||||
|
||||
Embedded bootloaders are generally built for a particular board. So, |
||||
it's usually possible to build the device tree blob at compile time |
||||
and include it in the bootloader image. For minor revisions of the |
||||
board, the bootloader can contain code to make the necessary tweaks to |
||||
the tree before passing it to the booted kernel. |
||||
|
||||
The device trees for embedded boards are usually quite simple, and |
||||
it's possible to hand construct the necessary blob by hand, but doing |
||||
so is tedious. The ``device tree compiler'', \dtc{}\footnote{\dtc can |
||||
be obtained from \cite{dtcgit}.}, is designed to make creating device |
||||
tree blobs easier by converting a text representation of the tree |
||||
into the necessary blob. |
||||
|
||||
\subsection{Input and output formats} |
||||
|
||||
As well as the normal mode of compiling a device tree blob from text |
||||
source, \dtc can convert a device tree between a number of |
||||
representations. It can take its input in one of three different |
||||
formats: |
||||
\begin{itemize} |
||||
\item source, the normal case. The device tree is described in a text |
||||
form, described in \S\ref{sec:dts}. |
||||
\item blob (\texttt{dtb}), the flattened tree format described in |
||||
\S\ref{sec:format}. This mode is useful for checking a pre-existing |
||||
device tree blob. |
||||
\item filesystem (\texttt{fs}), input is a directory tree in the |
||||
layout of \texttt{/proc/device-tree} (roughly, a directory for each |
||||
node in the device tree, a file for each property). This is useful |
||||
for building a blob for the device tree in use by the currently |
||||
running kernel. |
||||
\end{itemize} |
||||
|
||||
In addition, \dtc can output the tree in one of three different |
||||
formats: |
||||
\begin{itemize} |
||||
\item blob (\texttt{dtb}), as in \S\ref{sec:format}. The most |
||||
straightforward use of \dtc is to compile from ``source'' to |
||||
``blob'' format. |
||||
\item source (\texttt{dts}), as in \S\ref{sec:dts}. If used with blob |
||||
input, this allows \dtc to act as a ``decompiler''. |
||||
\item assembler source (\texttt{asm}). \dtc can produce an assembler |
||||
file, which will assemble into a \texttt{.o} file containing the |
||||
device tree blob, with symbols giving the beginning of the blob and |
||||
its various subsections. This can then be linked directly into a |
||||
bootloader or firmware image. |
||||
\end{itemize} |
||||
|
||||
For maximum applicability, \dtc can both read and write any of the |
||||
existing revisions of the blob format. When reading, \dtc takes the |
||||
version from the blob header, and when writing it takes a command line |
||||
option specifying the desired version. It automatically makes any |
||||
necessary adjustments to the tree that are necessary for the specified |
||||
version. For example, formats before 0x10 require each node to have |
||||
an explicit \texttt{name} property. When \dtc creates such a blob, it |
||||
will automatically generate \texttt{name} properties from the unit |
||||
names. |
||||
|
||||
\subsection{Source format} |
||||
\label{sec:dts} |
||||
|
||||
The ``source'' format for \dtc is a text description of the device |
||||
tree in a vaguely C-like form. Figure \ref{fig:dts} shows an |
||||
example. The file starts with \texttt{/memreserve/} directives, which |
||||
gives address ranges to add to the output blob's memory reserve table, |
||||
then the device tree proper is described. |
||||
|
||||
Nodes of the tree are introduced with the node name, followed by a |
||||
\texttt{\{} ... \texttt{\};} block containing the node's properties |
||||
and subnodes. Properties are given as just {\emph{name} \texttt{=} |
||||
\emph{value}\texttt{;}}. The property values can be given in any |
||||
of three forms: |
||||
\begin{itemize} |
||||
\item \emph{string} (for example, \texttt{"MyBoard"}). The property |
||||
value is the given string, including terminating NULL. C-style |
||||
escapes (\verb+\t+, \verb+\n+, \verb+\0+ and so forth) are allowed. |
||||
\item \emph{cells} (for example, \texttt{<0 8000 f0000000>}). The |
||||
property value is made up of a list of 32-bit ``cells'', each given |
||||
as a hex value. |
||||
\item \emph{bytestring} (for example, \texttt{[1234abcdef]}). The |
||||
property value is given as a hex bytestring. |
||||
\end{itemize} |
||||
|
||||
Cell properties can also contain \emph{references}. Instead of a hex |
||||
number, the source can give an ampersand (\texttt{\&}) followed by the |
||||
full path to some node in the tree. For example, in Figure |
||||
\ref{fig:dts}, the \texttt{/chosen} node has an |
||||
\texttt{interrupt-controller} property referring to the interrupt |
||||
controller described by the node \texttt{/mpic@0x3fffdd08400}. In the |
||||
output tree, the value of the referenced node's phandle is included in |
||||
the property. If that node doesn't have an explicit phandle property, |
||||
\dtc will automatically create a unique phandle for it. This approach |
||||
makes it easy to create interrupt trees without having to explicitly |
||||
assign and remember phandles for the various interrupt controller |
||||
nodes. |
||||
|
||||
The \dtc source can also include ``labels'', which are placed on a |
||||
particular node or property. For example, Figure \ref{fig:dts} has a |
||||
label ``\texttt{memreg}'' on the \texttt{reg} property of the node |
||||
\texttt{/memory@0}. When using assembler output, corresponding labels |
||||
in the output are generated, which will assemble into symbols |
||||
addressing the part of the blob with the node or property in question. |
||||
This is useful for the common case where an embedded board has an |
||||
essentially fixed device tree with a few variable properties, such as |
||||
the size of memory. The bootloader for such a board can have a device |
||||
tree linked in, including a symbol referring to the right place in the |
||||
blob to update the parameter with the correct value determined at |
||||
runtime. |
||||
|
||||
\subsection{Tree checking} |
||||
|
||||
Between reading in the device tree and writing it out in the new |
||||
format, \dtc performs a number of checks on the tree: |
||||
\begin{itemize} |
||||
\item \emph{syntactic structure}: \dtc checks that node and property |
||||
names contain only allowed characters and meet length restrictions. |
||||
It checks that a node does not have multiple properties or subnodes |
||||
with the same name. |
||||
\item \emph{semantic structure}: In some cases, \dtc checks that |
||||
properties whose contents are defined by convention have appropriate |
||||
values. For example, it checks that \texttt{reg} properties have a |
||||
length that makes sense given the address forms specified by the |
||||
\texttt{\#address-cells} and \texttt{\#size-cells} properties. It |
||||
checks that properties such as \texttt{interrupt-parent} contain a |
||||
valid phandle. |
||||
\item \emph{Linux requirements}: \dtc checks that the device tree |
||||
contains those nodes and properties that are required by the Linux |
||||
kernel to boot correctly. |
||||
\end{itemize} |
||||
|
||||
These checks are useful to catch simple problems with the device tree, |
||||
rather than having to debug the results on an embedded kernel. With |
||||
the blob input mode, it can also be used for diagnosing problems with |
||||
an existing blob. |
||||
|
||||
\section{Future Work} |
||||
|
||||
\subsection{Board ports} |
||||
|
||||
The flattened device tree has always been the only supported way to |
||||
boot a \texttt{ppc64} kernel on an embedded system. With the merge of |
||||
\texttt{ppc32} and \texttt{ppc64} code it has also become the only |
||||
supported way to boot any merged \texttt{powerpc} kernel, 32-bit or |
||||
64-bit. In fact, the old \texttt{ppc} architecture exists mainly just |
||||
to support the old ppc32 embedded ports that have not been migrated |
||||
to the flattened device tree approach. We plan to remove the |
||||
\texttt{ppc} architecture eventually, which will mean porting all the |
||||
various embedded boards to use the flattened device tree. |
||||
|
||||
\subsection{\dtc features} |
||||
|
||||
While it is already quite usable, there are a number of extra features |
||||
that \dtc could include to make creating device trees more convenient: |
||||
\begin{itemize} |
||||
\item \emph{better tree checking}: Although \dtc already performs a |
||||
number of checks on the device tree, they are rather haphazard. In |
||||
many cases \dtc will give up after detecting a minor error early and |
||||
won't pick up more interesting errors later on. There is a |
||||
\texttt{-f} parameter that forces \dtc to generate an output tree |
||||
even if there are errors. At present, this needs to be used more |
||||
often than one might hope, because \dtc is bad at deciding which |
||||
errors should really be fatal, and which rate mere warnings. |
||||
\item \emph{binary include}: Occasionally, it is useful for the device |
||||
tree to incorporate as a property a block of binary data for some |
||||
board-specific purpose. For example, many of Apple's device trees |
||||
incorporate bytecode drivers for certain platform devices. \dtc's |
||||
source format ought to allow this by letting a property's value be |
||||
read directly from a binary file. |
||||
\item \emph{macros}: it might be useful for \dtc to implement some |
||||
sort of macros so that a tree containing a number of similar devices |
||||
(for example, multiple identical ethernet controllers or PCI buses) |
||||
can be written more quickly. At present, this can be accomplished |
||||
in part by running the source file through CPP before compiling with |
||||
\dtc. It's not clear whether ``native'' support for macros would be |
||||
more useful. |
||||
\end{itemize} |
||||
|
||||
\bibliographystyle{amsplain} |
||||
\bibliography{dtc-paper} |
||||
|
||||
\section*{About the authors} |
||||
|
||||
David Gibson has been a member of the IBM Linux Technology Center, |
||||
working from Canberra, Australia, since 2001. Recently he has worked |
||||
on Linux hugepage support and performance counter support for ppc64, |
||||
as well as the device tree compiler. In the past, he has worked on |
||||
bringup for various ppc and ppc64 embedded systems, the orinoco |
||||
wireless driver, ramfs, and a userspace checkpointing system |
||||
(\texttt{esky}). |
||||
|
||||
Benjamin Herrenschmidt was a MacOS developer for about 10 years, but |
||||
ultimately saw the light and installed Linux on his Apple PowerPC |
||||
machine. After writing a bootloader, BootX, for it in 1998, he |
||||
started contributing to the PowerPC Linux port in various areas, |
||||
mostly around the support for Apple machines. He became official |
||||
PowerMac maintainer in 2001. In 2003, he joined the IBM Linux |
||||
Technology Center in Canberra, Australia, where he ported the 64 bit |
||||
PowerPC kernel to Apple G5 machines and the Maple embedded board, |
||||
among others things. He's a member of the ppc64 development ``team'' |
||||
and one of his current goals is to make the integration of embedded |
||||
platforms smoother and more maintainable than in the 32-bit PowerPC |
||||
kernel. |
||||
|
||||
\section*{Legal Statement} |
||||
|
||||
This work represents the view of the author and does not necessarily |
||||
represent the view of IBM. |
||||
|
||||
IBM, \ppc, \ppc Architecture, POWER5, pSeries and iSeries are |
||||
trademarks or registered trademarks of International Business Machines |
||||
Corporation in the United States and/or other countries. |
||||
|
||||
Apple and Power Macintosh are a registered trademarks of Apple |
||||
Computer Inc. in the United States, other countries, or both. |
||||
|
||||
Linux is a registered trademark of Linus Torvalds. |
||||
|
||||
Other company, product, and service names may be trademarks or service |
||||
marks of others. |
||||
|
||||
\end{document} |
||||
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Reference in new issue