On 16/05/2023 10:57, Conor Dooley wrote: > On Tue, May 16, 2023 at 10:31:19AM +0200, Krzysztof Kozlowski wrote: >> On 15/05/2023 21:20, Conor Dooley wrote: >>> From: Conor Dooley <conor.dooley@xxxxxxxxxxxxx> >>> >>> Arnd suggested that adding maintainer handbook for the SoC "subsystem" >>> would be helpful in trying to bring on board maintainers for the various >>> new platforms cropping up in RISC-V land. >>> >>> Add a document briefly describing the role of the SoC subsystem and some >>> basic advice for (new) platform maintainers. >>> >>> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx> >>> Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx> > >>> +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. Once dt-bindings have >>> +been merged (and appear in a release of the kernel) they are set in stone, and >>> +any changes made must be compatible with existing devicetrees. This means that, >>> +when changing properties, a "new" kernel must still be able to handle an old >>> +devicetree. For many systems the devicetree is provided by firmware, and >>> +upgrading to a newer kernel cannot cause regressions. Ideally, the inverse is >>> +also true, and a new devicetree will also be compatible with an old kernel, >>> +although this is often not possible. >> >> I would prefer to skip it and instead: enhance >> Documentation/devicetree/bindings/ABI.rst and then reference it here. > > Sure. > >>> +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 file 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 >> >> I actually prefer such solution: >> >> - Duplicate defines in the devicetree file hidden by #ifndef section >> and remove them later in a following release >> >> We can keep both, but mine above leads to cleaner changes in DTS file. > > I think all of the options involved are either a bit ugly, or a bit of a > pain in the hole. > >>> + - 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 >> >> The policy told to me some time ago was that no merges from driver >> branch or tree are allowed towards DTS branch, even if they come only >> with binding header change. There are exceptions for this, e.g. [1], but >> that would mean we need to express here rules for cross-tree merges. > > I've got away with having an immutable branch for dt-binding headers! Of course, all is in an immutable branch, but in which tree? I talk about a case when driver tree, e.g. different clock maintainer, takes the binding. > That said, Arnd did actually have a look at this (and suggested some > changes) before I sent it & did not cry fowl about this section. IIRC, > this is actually his wording, not mine. Best regards, Krzysztof
