[Buildroot] How to force mesa3d to use llvmpipe?

[Adrian Perez de Castro <aperez@igalia.com>]

Hello Nimai,

On Fri, 22 Dec 2017 17:10:49 -0500, Nimai Mahajan <nimaim@gmail.com> wrote:
> On Fri, Dec 22, 2017 at 4:52 PM, Romain Naour <romain.naour@gmail.com> wrote:
> 
> > Hi Nimai,
> >
> > Le 22/12/2017 ? 22:22, nimaim a ?crit :
> > > Looks like BR may still not have llvmpipe support. Curious if this patch got
> > > anywhere?
> > >  http://lists.busybox.net/pipermail/buildroot/2017-July/195823.html

I have made those patches as part of a proof-of-concept for a project. The
people involved have decided to switch over to a GPU-accelerated solution, and
that means that I have not been devoting time to polish the patch set for
inclusion upstream. While I think it would be nice to have LLVM packages
merged, so far I have been working on Buildroot using my laptop and it has
been prohibitive to me to spend CPU time on side quests which don't help my
daily work ? sorry about that!

> > This patch series is still present in the patchwork [1] but this work need
> > to be updated with the latest llvm/clang release (5.0.1).

After Christmas I expect to have a powerful build machine and plenty of disk
space, and then it will be more feasible for me to try do some additional
contributions to Buildroot. Let's see then whether I can get it updated.

> > However, you can still test this series and you should be able to use
> > llvmpipe.
> 
> Thanks! Sounds good, I will try to merge Adrian's llvm package and mesa3d
> patches locally and rebuild mesa and see if I can get Qt to work with the
> llvm OpenGL driver. By any chance, do you know if llvm v4.0.0 will work
> with mesa 17.2.x or 17.3.x or will it have to be bumped to v5? Also, not
> sure how familiar with Qt you are, but do you know if Qt will require
> special hooks to be able to use this with xcb?

Any Mesa 17.x should work fine with LLVM 4.x; it should not be needed to
update the build recipes to LLVM 5. Quoting from the Mesa website:

  ?Unless otherwise stated, LLVM version 3.4 is recommended; 3.3 or
   later is required.?

There is also this note regarding PowerPC support:

  ?For ppc64le processors, use of the Altivec feature (the Vector Facility) is
  recommended if supported; use of the VSX feature (the Vector-Scalar
  Facility) is recommended if supported AND Mesa is built with LLVM version
  4.0 or later.?

(See https://www.mesa3d.org/llvmpipe.html for more details.)
 
> > I have not announced it yet on the mailing list that there will be a
> > student at Smile working on this subject. He will present his internship
> > topic during the next Buildroot meeting [2].
> 

Nice! I am considering going to FOSDEM, if I do so it could be nice to meet
and maybe talk a bit about the topic  :-)

> > Enabling llvmpipe support in mesa is not really difficult but packaging
> > llvm/clang is a more difficult and time consuming task.
> 
> Great! Agreed, something that definitely needs some work. Glad to see
> someone will be working it.

Indeed. It would be great to see full support for LLVM in Buildroot, as it
would be a building block for many other things:

 - Enabling ?llvmpipe? the and ?swr? rasterizers in Mesa3D. Also, the ?amdgpu?
   can use (needs?) LLVM for some of the GPUs.

 - As said, it's a requisite of Clang. And having Clang could open the
   door to use Clang-based cross-compiler toolchains.

 - Rust uses LLVM for code generation. There has been patches recently flying
   by the mailing list which use prebuilt binaries (and that's already a great
   first step), but it would be nice that Buildroot compiles its own Rust
   toolchain, like it does for C/C++.

 - Probably others.

> > You're welcome to work on this series and give some feedback.
> >
> 
> I'm merely a small time dev and still learning (as is probably evident by
> all my questions :)), especially with embedded graphics, but will be happy
> to test and contribute any way I can. Hopefully I can get this in a decent
> enough state. Quite surprised that this hasn't made it to BR yet.

We all had to learn at some point, that's all right! Well, I would even say
that there's always one more thing to learn ;-)

Best regards,


--
 Adri?n ?
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