Re: [PATCH] drm/virtio: add ARGB8888 support for primary planes

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 9/30/26 18:03, Thomas Zimmermann wrote:
> Hi
> 
> Am 30.09.26 um 15:27 schrieb Dorinda Bassey:
>> Add DRM_FORMAT_HOST_ARGB8888 to the list of supported formats for
>> virtio-gpu primary planes. Without this, glmark2-drm fails at
>> startup because it selects ARGB visual configs by default, which
>> don't match the XRGB-only plane capabilities. This causes
>> drmModeSetCrtc to fail with EINVAL.
> 
> But does the host also support ARGB on the primary plane? We usually
> don't export formats that are not available in (virtual) hardware.
> 
> Best regards
> Thomas
> 
>>
>> The virtio_gpu_translate_format() function already handles
>> ARGB8888 translation to VIRTIO_GPU_FORMAT_B8G8R8A8_UNORM,
>> so this just exposes the existing capability to userspace.
>>
>> Signed-off-by: Dorinda Bassey <dbassey@redhat.com>
>> ---
>>   drivers/gpu/drm/virtio/virtgpu_plane.c | 1 +
>>   1 file changed, 1 insertion(+)

For primary plane, alpha channel often doesn't matter as primary has
fixed z=0 order and a usual alpha blending mode would account only
overlay's alpha channel. Many real hw drivers expose alpha-channel
formats for primary plane for compatibility with userspace, even though
for them the alpha has no meaning for the primary plane.

QEMU is most popular VMM using virtio-gpu, but there are others. For a
stricter compatibility we can translate B8G8R8A8 to X8R8G8B8 for the
primary plane in the driver code, but I assume in practice it's very
unlike this will be necessary and we can always do it later on if will
be needed.

The change looks okay, will apply after testing.

-- 
Best regards,
Dmitry