[Mesa-dev] [RFC] Concrete proposal to split classic

[Rob Clark]

On Mon, Mar 22, 2021 at 3:15 PM Dylan Baker <dylan at pnwbakers.com> wrote:
>
> Hi list,
>
> We've talked about it a number of times, but I think it's time time to
> discuss splitting the classic drivers off of the main development branch
> again, although this time I have a concrete plan for how this would
> work.
>
> First, why? Basically, all of the classic drivers are in maintanence
> mode (even i965). Second, many of them rely on code that no one works
> on, and very few people still understand. There is no CI for most of
> them, and the Intel CI is not integrated with gitlab, so it's easy to
> unintentionally break them, and this breakage usually isn't noticed
> until just before or just after a release. 21.0 was held up (in small
> part, also me just getting behind) because of such breakages.
>
> I konw there is some interest in getting i915g in good enough shape that
> it could replace i915c, at least for the common case. I also am aware
> that Dave, Ilia, and Eric (with some pointers from Ken) have been
> working on a gallium driver to replace i965. Neither of those things are
> ready yet, but I've taken them into account.
>
> Here's the plan:
>
> 1) 21.1 release happens
> 2) we remove classic from master
> 3) 21.1 reaches EOL because of 21.2
> 4) we fork the 21.1 branch into a "classic-lts"¹ branch
> 5) we disable all vulkan and gallium drivers in said branch, at least at
>    the Meson level

I'm +1 for the -lts branch.. the layering between mesa "classic" and
gallium is already starting to get poked thru in the name of
performance, and we've already discovered cases of classic drivers
being broken for multiple months with no one noticing.  I think a
slower moving -lts branch is the best approach to keeping things
working for folks with older hw.

But possibly there is some value in not completely disabling gallium
completely in the -lts branch.  We do have some older gallium drivers
which do not have CI coverage and I think are not used frequently by
developers who are tracking the latest main/master branch.  I'm not
suggesting that we remove them from the main (non-lts) branch but it
might be useful to be able to recommend users of those drivers stick
with the -lts version for better stability?

BR,
-R

> 6) We change the name and precidence of the glvnd loader file
> 7) apply any build fixups (turn of intel generators for versions >= 7.5,
>    for example
> 8) maintain that branch with build and critical bug fixes only
>
> This gives ditros and end users two options.
> 1) then can build *only* the legacy branch in the a normal Mesa provides
>    libGL interfaces fashion
> 2) They can use glvnd and install current mesa and the legacy branch in
>    parallel
>
> Because of glvnd, we can control which driver will get loaded first, and
> thus if we decide i915g or the i965 replacement is ready and turn it on
> by default it will be loaded by default. An end user who doesn't like
> this can add a new glvnd loader file that makes the classic drivers
> higher precident and continue to use them.
>
> Why fork from 21.1 instead of master?
>
> First, it allows us to delete classic immediately, which will allow
> refactoring to happen earlier in the cycle, and for any fallout to be
> caught and hopefully fixed before the release. Second, it means that
> when a user is switched from 21.1 to the new classic-lts branch, there
> will be no regressions, and no one has to spend time figuring out what
> broke and fixing the lts branch.
>
> When you say "build and critical bug fixes", what do you mean?
>
> I mean update Meson if we rely on something that in the future is
> deprecated and removed, and would prevent building the branch or an
> relying on some compiler behavior that changes, gaping exploitable
> security holes, that kind of thing.
>
> footnotes
> ¹Or whatever color you like your bikeshed_______________________________________________
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