Linux SOF drivers

Background

The Linux development is split by subsystem. All SOF contributions are merged through the sound/system (maintained by Takashi Iwai) and the sound/soc subsystem (maintained by Mark Brown).

All SOF patches merged by the two maintainers will be used for linux-next (as a first pass of integration to detect conflicts with other subsystems or compilation issues) and eventually merged by Linus Torvalds in the mainline.

Instructions for SOF developers

ABI changes

One fundamental and non-negotiable premise of Linux kernel development is “we don’t break the userspace.” More specifically, users may update their kernels at any time, while keeping the SOF firmware binary and topology files stored in the root filesystem unchanged. The expectation is that the SOF Linux driver does not generate any errors and that audio functionality remains unchanged.

Conversely, when a capability is introduced in a new firmware release, the expectation is that the kernel shall be updated as well. In other words, a new firmware does not need to include any backwards-compatibility code to interface with an older kernel.

When the ABI changes, the developer or maintainer shall tag it in GitHub, and the ABI level change will be recorded in the official ABI change tracker:

https://github.com/orgs/thesofproject/projects/2

When the ABI is not backwards-compatible, the Pull Requests on the kernel side shall include code that deals with older firmware and topology files.

Development branch

All SOF development takes place on the topic/sof-dev branch in the SOF tree:

git@github.com:thesofproject/linux.git

Developers are required to submit Pull Requests (PRs) against the topic/sof-dev branch. The Continuous Integration (CI) will run a set of static analysis, builds, and on-device testing.

Two approvers are required for each PR. SOF admins may in some exceptions use their privileges to merge PRs, such as to restore functionality and broken builds.

When a PR is submitted by an SOF admin, another admin must approves that PR. The PRs are integrated into the SOF tree using the ‘rebase-and-merge’ method which keeps the integrated patches in a linear order.

Rebasing tree

In addition to the topic/sof-dev branch, the SOF project maintains a parallel topic/sof-dev-rebase branch. This branch is not intended for development, but to make upstream contributions easier to manage.

Upstream merges

During Linux development, patches to the ALSA/ASoC cores, dependencies such as audio codecs, or bug fixes may be contributed by the community. SOF Linux maintainers will, on a regular basis (typically weekly), merge all upstream contributions into the SOF tree.

Development flow

The SOF Linux maintainers are:

Intel

Pierre Bossart

@plbossart

Intel

Ranjani Sridharan

@ranj063

Intel

Kai Vehmanen

@kv2019i

NXP

Daniel Baluta

@dbaluta

The SOF maintainers apply the following process

1. Mirror all SOF patches to topic/sof-dev-rebase

This mirroring consists in doing a set of “git cherry-pick” operations from topic/sof-dev to topic/sof-dev-rebase. Once all development patches are applied, SOF maintainers will add the relevant Signed-off-by and Reviewed-by ‘tags.’

In specific cases, incremental patches will be squashed to simplify upstream reviews, commit messages will be made clearer, and the order of patches will be changed, but in all cases the intent is that both topic/sof-dev and topic/sof-dev-rebase provide the same code (as seen with git diff or diff -r).

2. Upstream merge/rebase

When the two branches are integrated, the SOF maintainer will create an upstream baseline. This baseline is then merged locally on top of topic/sof-dev, then pushed as a dedicated PR and run through the CI tests. The merge may in some cases create conflicts that have to be resolved locally by the maintainer. Once the PR is deemed suitable for integration, the maintainer will use a ‘Commit merge’ operation (in contrast to the ‘rebase-and-merge’ used for development).

In parallel, the topic/sof-dev-rebase branch is rebased on top of the same baseline, and again compared to the topic/sof-dev branch. After the two separate operations of merge and rebase on the two branches, these two branches should again be identical. The net effect of the rebase is that all patches already integrated by ALSA/ASoC maintainers ‘disappear.’ In other words, the topic/sof-dev-rebase branch helps visualize all patches not currently merged upstream.

3. Upstream contributions

The SOF maintainer will generate patch sets and send them with a cover to the alsa-devel mailing list, with the maintainers in Cc:. In most cases the patches are approved without issues, but the ALSA/ASoC maintainers or members of the community may provide feedback and request some changes. In those cases, the changes are applied on topic/sof-dev, then mirrored and squashed on topc/sof-dev-rebase, and submitted again. Under no circumstances should the SOF maintainer handle changes to the topic/sof-dev-rebase directly.

4. Exceptions

In very specific cases, such as for HDMI-related patches, it might be easier for an SOF developer to submit the patches directly to alsa-devel. By default, though, the process should be that all patches are first submitted to the SOF GitHub, CI-tested. Only when maintainers provide a written agreement should developers submit SOF-related patches directly to the alsa-devel mailing list.

To avoid disrupting the development and rewriting its history, all upstream patches are integrated using the “Merge commit” option.

Development summary

     +----reject-----------+                      +--------merge----------------+
     |                     |                      |                             |
     v                     |                      v                             |
+----+------+        +-----+-------+       +------+--------+           +--------+----------+
| developer +------->+ SOF reviews +--ok-->+ topic/sof-dev |         +-+ upstream baseline |
| PR        |        | CI tests    |       |               |         | |                   |
+-----------+        +-----+-------+       +------+--------+         | +---------+---------+
                           |                      |                  |           ^
                           |                               +--rebase-+           |
                           |                      |        |             ALSA maintainers ok
                           |                      |        v                     |
                           |           +----------v--------+--+         +--------+----------+
                           |           | topic/sof-dev-rebase +-email-->+    alsa-devel     |
                           |           |                      |         |    mailing list   |
                           |           +----------------------+         +--------+----------+
                           |                                                     ^
                           |                                                     |
                           |                                                     |
                           +-----------------direct path (exceptions)------------+