[Buildroot] How to force mesa3d to use llvmpipe?

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

On Sat, 23 Dec 2017 01:08:43 +0200, Adrian Perez de Castro <aperez@igalia.com> wrote:
> 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.)

One more detail: If Mesa does not realize by itself that it should use a
software-based rendering driver (which can happen when there are multiple
drivers built into Mesa), you can force it at runtime by setting the
?LIBGL_ALWAYS_SOFTWARE=1? environment variable. There is more about the
environment variables supported by Mesa here:

  https://www.mesa3d.org/envvars.html

> > > 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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