[Nouveau] [Bug 109330] GL_ALPHA_BITS set to non-zero with EGL_PLATFORM_GBM_MESA

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109330

--- Comment #4 from Ilia Mirkin <imirkin at alum.mit.edu> ---
OK, so the 3 GPUs listed by emersion are all nv50, while the GP107 would be
covered by the nvc0 driver backend. Unfortunately I have a nv42 plugged in ATM,
but I'll have a look when I'm back on something a bit more modern.

If someone with C/C++ development experience and the requisite hardware is
interested, feel free to join in #nouveau on irc.freenode.net and I can provide
advice for things to investigate.

Lastly, if someone can provide an apitrace of the application doing
glReadPixels and getting unexpected results, that would avoid the need to have
to reproduce the software setup. I believe it's possible to look at
glReadPixels results in qapitrace ... would have to double-check though.

One guess is that we're copying from a RGBX surface to a RGBA surface, and not
whacking alpha to 1. However that should generally work, so one would have to
identify the precise code-path followed whereby that does not work.

Another guess is that the result of the rendering really does produce an alpha
of 0, either due to an earlier bug (unrelated to glReadPixels) happening, or
due to something specific to the drm formats exposed by nouveau and/or order of
gl configs, which causes something funny to happen..

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