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diff --git a/Documentation/process/maintainer-soc.rst b/Documentation/process/maintainer-soc.rst new file mode 100644 index 0000000000..12637530d6 --- /dev/null +++ b/Documentation/process/maintainer-soc.rst @@ -0,0 +1,177 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +SoC Subsystem +============= + +Overview +-------- + +The SoC subsystem is a place of aggregation for SoC-specific code. +The main components of the subsystem are: + +* devicetrees for 32- & 64-bit ARM and RISC-V +* 32-bit ARM board files (arch/arm/mach*) +* 32- & 64-bit ARM defconfigs +* SoC-specific drivers across architectures, in particular for 32- & 64-bit + ARM, RISC-V and Loongarch + +These "SoC-specific drivers" do not include clock, GPIO etc drivers that have +other top-level maintainers. The drivers/soc/ directory is generally meant +for kernel-internal drivers that are used by other drivers to provide SoC- +specific functionality like identifying an SoC revision or interfacing with +power domains. + +The SoC subsystem also serves as an intermediate location for changes to +drivers/bus, drivers/firmware, drivers/reset and drivers/memory. The addition +of new platforms, or the removal of existing ones, often go through the SoC +tree as a dedicated branch covering multiple subsystems. + +The main SoC tree is housed on git.kernel.org: + https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git/ + +Clearly this is quite a wide range of topics, which no one person, or even +small group of people are capable of maintaining. Instead, the SoC subsystem +is comprised of many submaintainers, each taking care of individual platforms +and driver subdirectories. +In this regard, "platform" usually refers to a series of SoCs from a given +vendor, for example, Nvidia's series of Tegra SoCs. Many submaintainers operate +on a vendor level, responsible for multiple product lines. For several reasons, +including acquisitions/different business units in a company, things vary +significantly here. The various submaintainers are documented in the +MAINTAINERS file. + +Most of these submaintainers have their own trees where they stage patches, +sending pull requests to the main SoC tree. These trees are usually, but not +always, listed in MAINTAINERS. The main SoC maintainers can be reached via the +alias soc@kernel.org if there is no platform-specific maintainer, or if they +are unresponsive. + +What the SoC tree is not, however, is a location for architecture-specific code +changes. Each architecture has its own maintainers that are responsible for +architectural details, CPU errata and the like. + +Information for (new) Submaintainers +------------------------------------ + +As new platforms spring up, they often bring with them new submaintainers, +many of whom work for the silicon vendor, and may not be familiar with the +process. + +Devicetree ABI Stability +~~~~~~~~~~~~~~~~~~~~~~~~ + +Perhaps one of the most important things to highlight is that dt-bindings +document the ABI between the devicetree and the kernel. +Please read Documentation/devicetree/bindings/ABI.rst. + +If changes are being made to a devicetree that are incompatible with old +kernels, the devicetree patch should not be applied until the driver is, or an +appropriate time later. Most importantly, any incompatible changes should be +clearly pointed out in the patch description and pull request, along with the +expected impact on existing users, such as bootloaders or other operating +systems. + +Driver Branch Dependencies +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +A common problem is synchronizing changes between device drivers and devicetree +files. Even if a change is compatible in both directions, this may require +coordinating how the changes get merged through different maintainer trees. + +Usually the branch that includes a driver change will also include the +corresponding change to the devicetree binding description, to ensure they are +in fact compatible. This means that the devicetree branch can end up causing +warnings in the "make dtbs_check" step. If a devicetree change depends on +missing additions to a header file in include/dt-bindings/, it will fail the +"make dtbs" step and not get merged. + +There are multiple ways to deal with this: + +* Avoid defining custom macros in include/dt-bindings/ for hardware constants + that can be derived from a datasheet -- binding macros in header files should + only be used as a last resort if there is no natural way to define a binding + +* Use literal values in the devicetree file in place of macros even when a + header is required, and change them to the named representation in a + following release + +* Defer the devicetree changes to a release after the binding and driver have + already been merged + +* Change the bindings in a shared immutable branch that is used as the base for + both the driver change and the devicetree changes + +* Add duplicate defines in the devicetree file guarded by an #ifndef section, + removing them in a later release + +Devicetree Naming Convention +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The general naming scheme for devicetree files is as follows. The aspects of a +platform that are set at the SoC level, like CPU cores, are contained in a file +named $soc.dtsi, for example, jh7100.dtsi. Integration details, that will vary +from board to board, are described in $soc-$board.dts. An example of this is +jh7100-beaglev-starlight.dts. Often many boards are variations on a theme, and +frequently there are intermediate files, such as jh7100-common.dtsi, which sit +between the $soc.dtsi and $soc-$board.dts files, containing the descriptions of +common hardware. + +Some platforms also have System on Modules, containing an SoC, which are then +integrated into several different boards. For these platforms, $soc-$som.dtsi +and $soc-$som-$board.dts are typical. + +Directories are usually named after the vendor of the SoC at the time of its +inclusion, leading to some historical directory names in the tree. + +Validating Devicetree Files +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +``make dtbs_check`` can be used to validate that devicetree files are compliant +with the dt-bindings that describe the ABI. Please read the section +"Running checks" of Documentation/devicetree/bindings/writing-schema.rst for +more information on the validation of devicetrees. + +For new platforms, or additions to existing ones, ``make dtbs_check`` should not +add any new warnings. For RISC-V and Samsung SoC, ``make dtbs_check W=1`` is +required to not add any new warnings. +If in any doubt about a devicetree change, reach out to the devicetree +maintainers. + +Branches and Pull Requests +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Just as the main SoC tree has several branches, it is expected that +submaintainers will do the same. Driver, defconfig and devicetree changes should +all be split into separate branches and appear in separate pull requests to the +SoC maintainers. Each branch should be usable by itself and avoid +regressions that originate from dependencies on other branches. + +Small sets of patches can also be sent as separate emails to soc@kernel.org, +grouped into the same categories. + +If changes do not fit into the normal patterns, there can be additional +top-level branches, e.g. for a treewide rework, or the addition of new SoC +platforms including dts files and drivers. + +Branches with a lot of changes can benefit from getting split up into separate +topics branches, even if they end up getting merged into the same branch of the +SoC tree. An example here would be one branch for devicetree warning fixes, one +for a rework and one for newly added boards. + +Another common way to split up changes is to send an early pull request with the +majority of the changes at some point between rc1 and rc4, following up with one +or more smaller pull requests towards the end of the cycle that can add late +changes or address problems identified while testing the first set. + +While there is no cut-off time for late pull requests, it helps to only send +small branches as time gets closer to the merge window. + +Pull requests for bugfixes for the current release can be sent at any time, but +again having multiple smaller branches is better than trying to combine too many +patches into one pull request. + +The subject line of a pull request should begin with "[GIT PULL]" and made using +a signed tag, rather than a branch. This tag should contain a short description +summarising the changes in the pull request. For more detail on sending pull +requests, please see Documentation/maintainer/pull-requests.rst. |