linaro-dev June 2011

linaro-dev@lists.linaro.org

149 participants
196 discussions

by Zygmunt Krynicki

Hello everyone. Since a few people started asking me questions about lava dashboard, reports and other things I though I we could benefit from sharing this knowledge. I created user forum for the dashboard at: http://fracture.suxx.pl/projects/lava-dashboard/boards Please go there and post questions you may have. You will need an account which you can quickly create here: http://fracture.suxx.pl/account/register I hope that will make the communication smoother. Thanks ZK

15 years, 1 month

Bug escalation procedure

by Fathi Boudra

Hi, This is an update to the bug management process. The wiki page is: https://wiki.linaro.org/Cycles/BugManagement Cheers, Fathi -- Bug escalation procedure ------------------------ It's important that bugs which affect the LEB's and other images are caught early and fixed as soon as possible. To help the Linaro Release Team in identifying and tracking the release blocker bugs, any individual could propose a bug as a blocker and follow the bug escalation procedure. Prerequisite ------------ * All bugs in a 'New' state should be sufficiently triaged to determine their importance. * All bugs in a 'High' or 'Critical' state should be highlighted to the Linaro Release Team: Please, add it to the wiki page 'Bugs' section for the next release meeting. The release meeting calendar can be found on the wiki. Meetings are held on Thursday. If possible, attend the meeting. Procedure --------- 1. Subscribe Linaro Release Team ('linaro-release') to the proposed bug. 2. Linaro Release Team evaluates the proposed bug. 2a. Linaro Release Team accepts the bug as a release blocker: * notify by a comment on the bug. * set the milestone to the next release. * assign to the appropriate team. 2b. Linaro Release Team rejects the bug as a release blocker: * unsubscribe Linaro Release Team from the poposed bug.

15 years, 1 month

[PATCH v4 00/12] mmc: use nonblock mmc requests to minimize latency

by Per Forlin

How significant is the cache maintenance over head? It depends, the eMMC are much faster now compared to a few years ago and cache maintenance cost more due to multiple cache levels and speculative cache pre-fetch. In relation the cost for handling the caches have increased and is now a bottle neck dealing with fast eMMC together with DMA. The intention for introducing none blocking mmc requests is to minimize the time between a mmc request ends and another mmc request starts. In the current implementation the MMC controller is idle when dma_map_sg and dma_unmap_sg is processing. Introducing none blocking mmc request makes it possible to prepare the caches for next job parallel with an active mmc request. This is done by making the issue_rw_rq() none blocking. The increase in throughput is proportional to the time it takes to prepare (major part of preparations is dma_map_sg and dma_unmap_sg) a request and how fast the memory is. The faster the MMC/SD is the more significant the prepare request time becomes. Measurements on U5500 and Panda on eMMC and SD shows significant performance gain for large reads when running DMA mode. In the PIO case the performance is unchanged. There are two optional hooks pre_req() and post_req() that the host driver may implement in order to move work to before and after the actual mmc_request function is called. In the DMA case pre_req() may do dma_map_sg() and prepare the dma descriptor and post_req runs the dma_unmap_sg. Details on measurements from IOZone and mmc_test: https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req Changes since v3: * Based on 2.6.39-rc7 * Add error check for testlist in mmc_test.c * Resolve in mmc-queue-thread that caused the mmc-thread to miss a wakeup. * Move parallel request handling to core.c. This simplifies the interface from 4 public functions to 1. This also gives access for SDIO to use the same functionallity, even though the function is not tuned for the SDIO execution flow yet. Per Forlin (12): mmc: add none blocking mmc request function omap_hsmmc: use original sg_len for dma_unmap_sg omap_hsmmc: add support for pre_req and post_req mmci: implement pre_req() and post_req() mmc: mmc_test: add debugfs file to list all tests mmc: mmc_test: add test for none blocking transfers mmc: add member in mmc queue struct to hold request data mmc: add a block request prepare function mmc: move error code in mmc_block_issue_rw_rq to a separate function. mmc: add a second mmc queue request member mmc: test: add random fault injection in core.c mmc: add handling for two parallel block requests in issue_rw_rq drivers/mmc/card/block.c | 452 +++++++++++++++++++++++++---------------- drivers/mmc/card/mmc_test.c | 361 ++++++++++++++++++++++++++++++++- drivers/mmc/card/queue.c | 184 +++++++++++------ drivers/mmc/card/queue.h | 32 +++- drivers/mmc/core/core.c | 165 ++++++++++++++- drivers/mmc/core/debugfs.c | 5 + drivers/mmc/host/mmci.c | 146 ++++++++++++-- drivers/mmc/host/mmci.h | 8 + drivers/mmc/host/omap_hsmmc.c | 90 ++++++++- include/linux/mmc/core.h | 6 +- include/linux/mmc/host.h | 19 ++ lib/Kconfig.debug | 11 + 12 files changed, 1187 insertions(+), 292 deletions(-) -- 1.7.4.1

15 years, 1 month

Generating ancillary sections with gas

by Dave Martin

Hi Michael, This thread about how to generate ancillary sections using gas has resurfaced again. Do you know who might be available from the toolchain group to take a look at this? It appears that this issue can best be solved by a change to gas (or, possibly, to gcc). Cheers ---Dave References: Dynamic patching in discarded sections <http://thread.gmane.org/gmane.linux.kernel/1152142> Generating ancilliary sections with gas <http://thread.gmane.org/gmane.linux.linaro.toolchain/686>

15 years, 1 month

Draft daily (hmm) cloud builds of Android toolchain from bzr

by Paul Sokolovsky

Hello, Alexander, I've set up builds of Android toolchain from bzr: https://android-build.linaro.org/builds/~pfalcon/linaro-toolchain-4.5-bzr/ https://android-build.linaro.org/builds/~pfalcon/linaro-toolchain-4.6-bzr/ Jim, I had to revamp bzr build in linaro-build.sh: http://git.linaro.org/gitweb?p=people/pfalcon/android/toolchain/build.git;a… Note that it now expects bzr branch with explicit version, e.g. lp:gcc-linaro/4.5 or lp:gcc-linaro/4.6 . Well, now the issue: the builds above are not quite ready to be daily. That's because bzr checkout is slow, and is also quite big. Normal bzr clone takes 1.1Gb and cloud build takes 1hr instead of 30min with a tarball. I tried to play with bzr checkout --lightweight, but ctrl+c'ed it when it showed that it dealt with 1.3Gb of stuff (surprisingly, after break, I saw almost empty local empty structure, so I wonder if that wasn't d/l counter, but even then it's awfully slow). So, where do we go from here? Unless there's tip export feature directly out of remote repo for bzr (bzr export seem to requite local working copy), we'd need to play mirror, this time bzr, again %). -- Best Regards, Paul

15 years, 1 month

[NOTES] Dev Platforms Status Meeting Notes and Actions 6/16

by Tom Gall

Enclosed you'll find a link to the agenda, notes and actions from the Linaro Developer Platforms Weekly Status meeting held on June 16th in #linaro-meeting on irc.freenode.net at 14:00 UTC. https://wiki.linaro.org/Platform/DevPlatform/Meetings/2011-06-16 New and Carried Over Actions: * rsalveti to create a RT so wookey can have access to people.linaro and git.linaro * aviksil to sync with npitre about http://lists.infradead.org/pipermail/linux-arm-kernel/2011-March/045283.html * rsalveti to discuss with gwg to see if they should host the smartt wiki and such * rename the team from foundations to devplatform and work with james_w to fix status.linaro.org once the team gets renamed * suihkulokki to see if he can merge his kernel wiki into the linaro HowTo section * rsalveti to propose a call for smartt to identify next steps * rsalveti to make sure all bps from the dev platform team are showing at status.linaro.org Regards, Tom Developer Platforms Team "We want great men who, when fortune frowns will not be discouraged." - Colonel Henry Knox w) tom.gall att linaro.org w) tom_gall att vnet.ibm.com h) tom_gall att mac.com IRC) Dr_Who, tgall_foo

15 years, 1 month

Power Management WG Weekly Status report 2011-06-17

by Mounir Bsaibes

== Meetings minutes == https://wiki.linaro.org/WorkingGroups/PowerManagement/Meetings/2011-06-15 == Weekly Status Report == https://wiki.linaro.org/WorkingGroups/PoweManagement/Status/2011-06-16<https://wiki.linaro.org/WorkingGroups/PowerManagement/Status/2011-06-16> == Summary == * finalize on cpu topology patch * preparing thermal framework for Arm on 2.6.39 * powedebug new and improved various areas of the code Regards, Mounir

15 years, 1 month

Kernel WG Weekly Status report 2011-06-17

by Mounir Bsaibes

== Meetings minutes == https://wiki.linaro.org/WorkingGroups/Kernel/Meetings/2011-06-13 == Weekly Status Report == https://wiki.linaro.org/WorkingGroups/Kernel/Status/2011-06-16 == Summary == * Linaro kernel will be freezed one week before the monthly release. * linaro-android tree is now available: http://git.linaro.org/gitweb?p=people/jstultz/android.git;a=shortlog;h=refs… * Was able to build 99% of randconfig on all our member's current platforms * Completed the power measurement with and without memory regions patches Regards, Mounir

15 years, 1 month

Linaro 11.05 weekly testing 2011-06-16

by Fathi Boudra

Hi, This is a message sent out once per week to call on our community to encourage all Linux/ARM enthusiasts to try out the latest preview images of our Linaro Evaluation Builds (LEBs) for Android Handset and Ubuntu Desktop/Netbook. These images showcase the current state of development on our LEBs images on their way to the next release: 11.06. * Android: http://snapshots.linaro.org/offspring/11.05-daily/linaro-android/leb-panda/… * Ubuntu: http://snapshots.linaro.org/offspring/11.05-daily/linaro-ubuntu-desktop/201… http://snapshots.linaro.org/offspring/11.05-daily/linaro-hwpacks/panda-x11-… On top of the official Linaro Evaluation Builds, the Platform Team hosts a set of functional images and board support packages that are typically released by Linaro engineers for developers that have different images requirements. These images are not officially supported release artifacts and are provided on a best-effort basis in the hope that they can be useful for the Linux/ARM enthusiasts community. The download locations for these so called "Developers and Community Builds" can be found below. Developer Images based on Android with vanilla AOSP experience using linux-linaro mainline kernel as well as linaro toolchain: * Android Mainline for BeagleBoardr-xM: http://snapshots.linaro.org/offspring/11.05-daily/linaro-android/beaglexm/2… * Android Mainline for PandaBoard: http://snapshots.linaro.org/offspring/11.05-daily/linaro-android/panda/2011… Developer Images based on Ubuntu: Note: Ubuntu images are distributed in two parts: 1. a SoC independent userspace rootfs (the image) 2. a Soc specific BSP (the hwpack) * Nano: http://snapshots.linaro.org/offspring/11.05-daily/linaro-nano/20110616/0/im… * ALIP: http://snapshots.linaro.org/offspring/11.05-daily/linaro-alip/20110616/0/im… * Developer: http://snapshots.linaro.org/offspring/11.05-daily/linaro-developer/20110616… Not officially supported community board support packages are also provided for either boards that don't meet our Linaro Evaluation Builds criterias. The idea is that you can combine them freely (e.g. combine an imx51 hwpack with Nano or Developer); though in practice UI images might not work. A list of all hardware packs hosted by Linaro Platform can be found below: http://snapshots.linaro.org/offspring/11.05-daily/linaro-hwpacks/ Make your way to the installation instructions: https://wiki.linaro.org/Platform/Android/ImageInstallation https://wiki.linaro.org/Platform/DevPlatform/Ubuntu/ImageInstallation For an explanation on how to test and submit your results to the QA tracker at: http://qatracker.linaro.org For an explanation of how to use the qatracker please see: https://wiki.linaro.org/QA/QATracker Cheers, Fathi -- Linaro Release Manager | Platform Project Manager

15 years, 1 month

[RFC] (early draft) dt: Linux dt usage model documentation

by Grant Likely

Signed-off-by: Grant Likely <grant.likely(a)secretlab.ca> --- Hey all, This is an early draft of the usage model document for the device tree, but I wanted to get it out there for feedback, and so that some of the Linaro engineers could get started on migrating board ports. g. Documentation/devicetree/usage-model | 403 ++++++++++++++++++++++++++++++++++ 1 files changed, 403 insertions(+), 0 deletions(-) create mode 100644 Documentation/devicetree/usage-model diff --git a/Documentation/devicetree/usage-model b/Documentation/devicetree/usage-model new file mode 100644 index 0000000..4203119 --- /dev/null +++ b/Documentation/devicetree/usage-model @@ -0,0 +1,403 @@ +Linux and the Device Tree +The Linux usage model for device tree data + +Author: Grant Likely <grant.likely(a)secretlab.ca> + +This article describes how Linux uses the device tree. An overview of +the device tree data format can be found at the <a +href="http://devicetree.org/Device_Tree_Usage">Device Tree Usage</a> +page on <a href="http://devicetree.org">devicetree.org</a>. + + + All the cool architectures are using device tree. I want to + use device tree too! + +The "Open Firmware Device Tree", or simply Device Tree (DT), is a data +structure and language for describing hardware. More specifically, it +is an description of hardware that is readable by an operating system +so that the operating system doesn't need to hard code details of the +machine. + +Structurally, the DT is a tree, or acyclic graph with named nodes, and +nodes may have an arbitrary number of named properties encapsulating +arbitrary data. A mechanism also exists to create arbitrary +links from one node to another outside of the natural tree structure.. + +Conceptually, a common set of usage conventions, called 'bindings', +is defined for how data should appear in the tree to describe typical +hardware characteristics including data busses, interrupt lines, gpio +connections, and peripheral devices. + +As much as possible, hardware is described using existing bindings to +maximize use of existing support code, but since property and node +names are simply text strings, it is easy to extend existing bindings +or create new ones by defining new nodes and properties. + +<h2>History</h2> +The DT was originally created by Open Firmware as part of the +communication method for passing data from Open Firmware to a client +program (like to an operating system). An operating system used the +Device Tree to discover the topology of the hardware at runtime, and +thereby support a majority of available hardware without hard coded +information (assuming drivers were available for all devices). + +Since Open Firmware is commonly used on PowerPC and SPARC platforms, +the Linux support for those architectures has for a long time used the +Device Tree. + +In 2005, when PowerPC Linux began a major cleanup and to merge 32 bit +and 64 support, the decision was made to require DT support on all +powerpc platforms, regardless of whether or not they used Open +Firmware. To do this, a DT representation called the Flattened Device +Tree (FDT) was created which could be passed to the kernel as a binary +blob without requiring a real Open Firmware implementation. U-Boot, +kexec, and other bootloaders were modified to support both passing a +Device Tree Binary (dtb) and to modify a dtb at boot time. + +Some time later, FDT infrastructure was generalized to be usable by +all architectures. At the time of this writing, 5 mainlined +architectures (arm, mips, powerpc, sparc, and x86) and 1 out of +mainline architecture (nios) have some level of DT support. + +<h2>Data Model</h2> +If you haven't already read the +href="http://devicetree.org/Device_Tree_Usage">Device Tree Usage</a> +page, then go read it now. It's okay, I'll wait.... + +<h3>High Level View</h3> +The most important thing to understand is that the DT is simply a data +structure that describes the hardware. There is nothing magical about +it, and it doesn't magically make all hardware configuration problems +go away. What it does do is provide a language for decoupling the +hardware configuration from the board and device driver support in the +Linux kernel (or any other operating system for that matter). Using +it allows board and device support to become data driven; to make +setup decisions based on data passed into the kernel instead of on +per-machine hard coded selections. + +Ideally, data driven platform setup should result in less code +duplication and make it easier to support a wide range of hardware +with a single kernel image. + +Linux uses DT data for three major purposes: +1) platform identification, +2) runtime configuration, and +3) device population. + +<h4>Platform Identification</h4> +First and foremost, the kernel will use data in the DT to identify the +specific machine. In a perfect world, the specific platform shouldn't +matter to the kernel because all platform details would be described +perfectly by the device tree in a consistent and reliable manner. +Hardware is not perfect though, and so the kernel must identify the +machine during early boot so that it has the opportunity to run +machine specific fixups. + +In the majority of cases, the machine identity is irrelevant, and the +kernel will instead select setup code based on the machines core +cpu or SoC. On ARM for example, setup_arch() in +arch/arm/kernel/setup.c will call setup_machine_fdt() in +arch/arm/kernel/devicetree.c which searches through the machine_desc +table and selects the machine_desc which best matches the device tree +data. It determines the best match by looking at the 'compatible' +property in the root device tree node, and comparing it with the +dt_compat list in struct machine_desc. + +The 'compatible' property contains a sorted list of strings starting +with the exact name of the machine, followed by an optional list of +boards it is compatible with sorted from most compatible to list. For +example, the root compatible properties for the TI BeagleBoard and its +successor, the BeagleBoard xM board might look like: + + compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3"; + compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3"; + +Where "ti,omap3-beagleboard-xm" specifies the exact model, it also +claims that it compatible with the OMAP 3450 SoC, and the omap3 family +of SoCs in general. You'll notice that the list is sorted from most +specific (exact board) to least specific (SoC family). + +Astute readers might point out that the Beagle xM could also claim +compatibility with the original Beagle board. However, one should be +cautioned about doing so at the board level since there is typically a +high level of change from one board to another, even within the same +product line, that it is hard to nail down exactly is meant when one +board claims to be compatible with another. For the top level, it is +better to err on the side of caution and not claim one board is +compatible with another. The notable exception would be when one +board is a carrier for another, such as a cpu module attached to a +carrier board. + +One more note on compatible values. Any string used in a compatible +property must be documented as to what it indicates. Add +documentation for compatible strings in Documentation/devicetree/bindings. + +Again on ARM, the for each machine_desc, the kernel looks to see if +any of the dt_compat list entries appear in the compatible property. +If one does, then that machine_desc is a candidate for driving the +machine. After searching the entire table of machine_descs, +setup_machine_fdt() returns the 'most compatible' machine_desc based +on which entry in the compatible property each machine_desc matches +against. If no matching machine_desc is found, then it returns NULL. + +The reasoning behind this scheme is the observation that in the majority +of cases, a single machine_desc can support a large number of boards +if that all use the same SoC, or same family of SoCs. However, +invariably there will be some exceptions where a specific board will +require special setup code that is not useful in the generic case. +Special cases could be handled by explicitly checking for the +troublesome board(s) in generic setup code, but doing so very quickly +becomes ugly and/or unmaintainable if it is more than just a couple of +cases. + +Instead, the compatible list allows a generic machine_desc to provide +support for a wide common set of boards by specifying "less +compatible" value in the dt_compat list. In the example above, +generic board support can claim compatibility with "ti,omap3" or +"ti,omap3450". If a bug was discovered on the original beagleboard +that required special workaround code during early boot, then a new +machine_desc could be added which implements the workarounds and only +matches on "ti,beagleboard". + +PowerPC uses a slightly different scheme where it calls the .probe() +hook from each machine_desc, and the first one returning TRUE is used. +However, this approach does not take into account the priority of the +compatible list, and probably should be avoided for new architecture +support. + +<h4>Runtime configuration</h4> +In most cases, a DT will be the sole method of communicating data from +firmware to the kernel, so also gets used to pass in runtime and +configuration data like the kernel parameters string and the location +of an initrd image. + +Most of this data is contained in the /chosen node, and when booting +Linux it will look something like this: + + chosen { + bootargs = "console=ttyS0,115200 loglevel=8"; + initrd-start = <0xc8000000>; + initrd-end = <0xc8200000>; + }; + +The bootargs property contains the kernel arguments, and the initrd-* +properties define the address and size of an initrd blob. The +chosen node may also optionally contain an arbitrary number of +additional properties for platform specific configuration data. + +During early boot, the architecture setup code calls of_scan_flat_dt() +several times with different helper callbacks to parse device tree +data before paging is setup. The of_scan_flat_dt() code scans through +the device tree and uses the helpers to extract information required +during early boot. Typically the early_init_dt_scan_chosen() helper +is used to parse the chosen node including kernel parameters, +early_init_dt_scan_root() to initialize the DT address space model, +and early_init_dt_scan_memory() to determine the size and +location of usable RAM. + +On ARM, the function setup_machine_fdt() is responsible for early +scanning of the device tree after selecting the correct machine_desc +that supports the board. + +<h4>Device population</h4> +After the board has been identified, and after the early configuration data +has been parsed, then kernel initialization can proceed in the normal +way. At some point in this process, unflatten_device_tree() is called +to convert the data into a more efficient runtime representation. +This is also when machine specific setup hooks will get called, like +the machine_desc .init_early(), .init_irq() and .init_machine() hooks +on ARM. The remainder of this section uses examples from the ARM +implementation, but all architectures will do pretty much the same +thing when using a DT. + +As can be guessed by the names, .init_early() is used for any machine +specific setup that needs to be executed early in the boot process, +and .init_irq() is used to set up interrupt handling. Using a DT +doesn't materially change the behaviour of either of these functions. +If a DT is provided, then both .init_early() and .init_irq() are able +to call any of the DT query functions (of_* in include/linux/of*.h) to +get additional data about the platform. + +The most interesting hook in the DT context is .init_machine() which +is primarily responsible for populating the Linux device model with +data about the platform. Historically this has been implemented on +embedded platforms by defining a set of static clock structures, +platform_devices, and other data in the board support .c file, and +registering it en-masse in .init_machine(). When DT is used, then +instead of hard coding static devices for each platform, the list of +devices can be obtained by parsing the DT, and allocating device +structures dynamically. + +The simplest case is when .init_machine() is only responsible for +registering a block of platform_devices. Platform devices are concept +used by Linux for memory or io mapped devices which cannot be detected +by hardware, and for 'composite' or 'virtual' devices (more on those +later). While there is no 'platform device' terminology for the DT, +platform devices roughly correspond to device nodes at the root of the +tree and children of simple memory mapped bus nodes. + +About now is a good time to lay out an example. Here is part of the +device tree for the NVIDIA Tegra board. + +/{ + compatible = "nvidia,harmony", "nvidia,tegra250"; + #address-cells = <1>; + #size-cells = <1>; + interrupt-parent = <&intc>; + + chosen { }; + aliases { }; + + memory { + device_type = "memory"; + reg = <0x00000000 0x40000000>; + }; + + soc { + compatible = "nvidia,tegra250-soc", "simple-bus"; + #address-cells = <1>; + #size-cells = <1>; + ranges; + + intc: interrupt-controller@50041000 { + compatible = "nvidia,tegra250-gic"; + interrupt-controller; + #interrupt-cells = <1>; + reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >; + }; + + serial@70006300 { + compatible = "nvidia,tegra250-uart"; + reg = <0x70006300 0x100>; + interrupts = <122>; + }; + + i2s-1: i2s@70002800 { + compatible = "nvidia,tegra250-i2s"; + reg = <0x70002800 0x100>; + interrupts = <77>; + codec = <&wm8903>; + }; + + i2c@7000c000 { + compatible = "nvidia,tegra250-i2c"; + #address-cells = <1>; + #size-cells = <1>; + reg = <0x7000c000 0x100>; + interrupts = <70>; + + wm8903: codec@1a { + compatible = "wlf,wm8903"; + reg = <0x1a>; + interrupts = <347>; + }; + }; + }; + + sound { + compatible = "nvidia,harmony-sound"; + i2s-controller = <&i2s-1>; + i2s-codec = <&wm8903>; + }; +}; + +At .machine_init() time, Tegra board support code will need to look at +this DT and decide which nodes to create platform_devices for. +However, looking at the tree, it is not immediately obvious what kind +of device each node represents, or even if a node represents a device +at all. The /chosen, /aliases, and /memory nodes are informational +nodes that don't describe devices (although arguably memory could be +considered a device). The children of the /soc node are memory mapped +devices, but the codec@1a is an i2c device, and the sound node +represents not a device, but rather how other devices are connected +together to create the audio subsystem. I know what each device is +because I'm familiar with the board design, but how does the kernel +know what to do with each node? + +The trick is that the kernel starts at the root of the tree and looks +for nodes that have a 'compatible' property. First, it is generally +assumed that any node with a 'compatible' property represents a device +of some kind, and second, it can be assumed that any node at the root +of the tree is either directly attached to the processor bus, or is a +miscellaneous system device that cannot be described any other way. +For each of these nodes, Linux allocates and registers a +platform_device, which in turn may get bound to a platform_driver. + +Why is using a platform_device for these nodes a safe assumption? +Well, for the way that Linux models devices, just about all bus_types +assume that its devices are children of a bus controller. For +example, each i2c_client is a child of an i2c_master. Each spi_device +is a child of an spi bus. Similarly for USB, PCI, MDIO, etc. The +same hierarchy is also found in the DT, where i2c device nodes only +ever appear as children of an i2c bus node. Ditto for spi, mdio, usb, +etc. The only devices which do not require a specific type of parent +device are platform_devices (and amba_devices, but more on that +later), which will happily live at the base of the Linux /sys/devices +tree. Therefore, if a DT node is at the root of the tree, then it +really probably is best registered as a platform_device. + +Linux board support code calls of_platform_populate(NULL, NULL, NULL) +to kick of discovery of devices at the root of the tree. The +parameters are all NULL because when starting from the root of the +tree, there is no need to provide a starting node (the first NULL), a +parent struct device (the last NULL), and we're not using a match +table (yet). For a board that only needs to register devices, +.init_machine() can be completely empty except for the +of_platform_populate() call. + +In the Tegra example, this accounts for the /soc and /sound nodes, but +what about the children of the soc node? Shouldn't they be registered +as platform devices too? For Linux DT support, the generic behaviour +is for child devices to be registered by the parent's device driver at +driver .probe() time. So, an i2c bus device driver will register a +i2c_client for each child node, an spi bus driver will register +it's spi_device children, and similarly for other bus_types. +According to that model, a driver could be written that binds to the +soc node and simply registers platform_devices for each of it's +children. The board support code would allocate and register an soc +device, an soc device driver would bind to the soc device, and +register platform_devices for /soc/interrupt-controller, /soc/serial, +/soc/i2s, and /soc/i2c in it's .probe() hook. Easy, right? Although +it is a lot of mucking about for just registering platform devices. + +It turns out that registering children of certain platform_devices as +more platform_devices is a common pattern, and the device tree support +code reflects that. The second argument to of_platform_populate() is +an of_device_id table, and any node that matches an entry in that +table will also get it's child nodes registered. In the tegra case, +the code can look something like this: + +static struct of_device_id harmony_bus_ids[] __initdata = { + { .compatible = "simple-bus", }, + {} +}; + +static void __init harmony_init_machine(void) +{ + /* ... */ + of_platform_populate(NULL, harmony_bus_ids, NULL); +} + +"simple-bus" is defined in the ePAPR 1.0 specification as a property +meaning a simple memory mapped bus, so the of_platform_populate() code +could be written to just assume simple-bus compatible nodes will +always be traversed. However, we pass it in as an argument so that +board support code can always override the default behaviour. + +<h2>Appendix A: AMBA devices</h2> + +ARM Primecells are a certain kind of device attached to the ARM AMBA +bus which include some support for hardware detection and power +management. In Linux, struct amba_device and the amba_bus_type is +used to represent Primecell devices. However, the fiddly bit is that +not all devices on an AMBA bus are Primecells, and for Linux it is +typical for both amba_device and platform_device instances to be +siblings of the same bus segment. + +When using the DT, this creates problems for of_platform_populate() +because it must decide whether to register each node as either a +platform_device or an amba_device. This unfortunately complicates the +device creation model a little bit, but the solution turns out not to +be too invasive. If a node is compatible with "arm,amba-primecell", then +of_platform_populate() will register it as an amba_device instead of a +platform_device.

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linaro-dev June 2011