Fwd: Question about handling RGBA/BGRA in etnaviv driver

[Wladimir]

Sorry for getting the list wrong again. Please reply to mesa-dev not
dri-dev .

---------- Forwarded message ----------
From: "Wladimir J. van der Laan" <laanwj at gmail.com>
Date: Feb 3, 2017 10:56
Subject: Question about handling RGBA/BGRA in etnaviv driver
To: <dri-devel at lists.freedesktop.org>, <etnaviv at lists.freedesktop.org>
Cc: "Chris Healy" <cphealy at gmail.com>

Hello,

With the Etnaviv driver we're running into an issue: The GPU can only
render *to*
BGRA formats. It can however render *from* BGRA as well as RGBA textures.

I know that the OpenGL ES standard allows drivers to choose what order is
most
appropriate when being asked for "GL_RGBA" textures. So back when etnaviv
supported
only BGRA, Mesa automatically picked that and everything was okay.

However a recent patch added support for RGBA formats in etnaviv_format.c.

Now, Mesa creates a real GL_RGBA texture when this is requested. This is all
and well for rendering, however for anything using FBO to render to textures
this is a problem.

Qt, for example, is assuming it can attach the GL_RGBA texture to a FBO.
This
fails now that GL_RGBA textures are really GL_RGBA, and it doesn't handle
that
error to fall back to something else so rendering issues ensue.

I'm not sure how to handle this:

- The quick fix would be to revert the RGBA formats patch, but the hardware
  supports rendering *from* RGBA textures fine so this would be throwing
away a
  feature.

- Another way would be to try to fix Qt to cope with this, and try e.g.
GL_BGRA_EXT
  when it wants to render to a texture. Burdening the client code with this
seems
  unintuitive to me.

- Another hack would be to implement shader variants, and swap R/B in the
pixel
  shader to emulate rendering to RGBA.

Neither seems great. Does anyone have suggestions, do any of the other
(gallium) drivers have this problem?

Regards,
Wladimir
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