[Bug 782923] gl: Add Mesa3D GBM backend

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=782923

Daniel Stone <daniel at fooishbar.org> changed:

           What    |Removed                     |Added
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                 CC|                            |daniel at fooishbar.org

--- Comment #22 from Daniel Stone <daniel at fooishbar.org> ---
Sorry for chiming in so late here.

Firstly, get_{depth,bpp}_from_format are wrong. There is a very very small and
fixed depth/bpp -> format mapping inside the kernel, which is pretty much:
  * 8/8 -> C8
  * 15/16 -> XRGB1555
  * 16/16 -> RGB565
  * 24/24 -> RGB888
  * 24/32 -> XRGB8888
  * 30/32 -> XRGB2101010
  * 32/32 -> ARGB8888

Anything else will silently be assumed to be XRGB8888.

So you should delete all but those formats, but _also_ preferentially use
drmModeAddFB2, for which you can just pass a format directly. (GBM_FORMAT_* ==
DRM_FORMAT_*, so no need to add a mapping; you can rely on this and it is both
API and ABI guaranteed.) Then you can just use drmModeAddFB as a fallback for
the above formats only.

You can also delete the GBM_BO_FORMAT_* handling inside those two functions, as
gbm_bo_get_format() only ever returns GBM_FORMAT_* enums. Very intuitive, I
know.

I don't quite follow what the comment in
gst_gl_window_gbm_egl_set_window_handle() means (wrt external objects and DRM
pageflips), but I'm happy to try to answer if you can elaborate.

As a final nitpick, GBM is not Mesa-specific: it's also implemented by the
Imagination, ARM, and Vivante proprietary drivers. Those tend to advertise
EGL_KHR_platform_gbm rather than EGL_MESA_platform_gbm in the extension string.

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