2. SoC Subsystem

2.1. 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.

2.2. 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.

2.2.1. 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 Devicetree (DT) ABI.

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.

2.2.2. 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

2.2.3. 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.

2.2.4. 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 Writing Devicetree Bindings in json-schema 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.

2.2.5. 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 Creating Pull Requests.