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

[Jason Ekstrand]

I'm bringing this up via e-mail so it gets a wider audience. Given how will 
crocus is working at this point, is like to propose we hold off for about 
three more releases before we drop classic. This next release, 21.2, we'll 
have crocus as an option with i965 as the default. There will also be a 
-Dprefer-crocus meson options so distros or individuals can attempt to flip 
it on. The release after that, 21.3, we'll keep i965 in the tree but have 
crocus be the default (assuming things are going well.) Some time in 2022, 
probably after the 22.2 release or so, we'll delete classic.

Why wait so long? Well, it just landed and we don't have a Cherryview story 
yet so I'm hesitant to make it the default too quickly. Even if it were the 
default in 21.2, it's already too late, likely, to hit the fall 2021 distro 
release cycle. If we flip it to the default before the end of the year, 
that'll get crocus into spring distros. This is good because 22.04 is an 
Ubuntu LTS release and I think they'd rather bump crocus versions to fix 
bugs than backport on top of i965. But that's really fort Ubuntu to decide. 
In any case, we won't see broad-spread usage and the flood of bug reports 
until next spring so we may want to wait until then to stay deleting code.

If we wanted to accelerate things, one option, once we're ready, would be 
to ask the person who manages the oibaf PPA to switch to crocus early. That 
may get some early adopters on board.

Thoughts?

--Jason

On April 9, 2021 22:09:14 Dylan Baker <dylan at pnwbakers.com> wrote:

> Quoting Dylan Baker (2021-03-22 15:15:30)
>> 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
>> 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
>
> Here is a merge request to remove classic:
> https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10153
>
> Dylan

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