Re: [PATCH] drm: xlnx: zynqmp_dp: Support DRM_FORMAT_XRGB8888

[Mike Looijmans <mike.looijmans@topic.nl>]

On 27-06-2025 20:19, Laurent Pinchart wrote:
> Hi Mike,
>
> Thank you for the patch.
>
> On Fri, Jun 27, 2025 at 04:50:46PM +0200, Mike Looijmans wrote:
>> XRGB8888 is the default mode that Xorg will want to use. Add support
>> for this to the Zynqmp DisplayPort driver, so that applications can use
>> 32-bit framebuffers. This solves that the X server would fail to start
>> unless one provided an xorg.conf that sets DefaultDepth to 16.
>>
>> Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
>> ---
>>
>>   drivers/gpu/drm/xlnx/zynqmp_disp.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/xlnx/zynqmp_disp.c b/drivers/gpu/drm/xlnx/zynqmp_disp.c
>> index 80d1e499a18d..501428437000 100644
>> --- a/drivers/gpu/drm/xlnx/zynqmp_disp.c
>> +++ b/drivers/gpu/drm/xlnx/zynqmp_disp.c
>> @@ -312,6 +312,11 @@ static const struct zynqmp_disp_format avbuf_gfx_fmts[] = {
>>   		.buf_fmt	= ZYNQMP_DISP_AV_BUF_FMT_NL_GFX_RGBA8888,
>>   		.swap		= true,
>>   		.sf		= scaling_factors_888,
>> +	}, {
>> +		.drm_fmt	= DRM_FORMAT_XRGB8888,
>> +		.buf_fmt	= ZYNQMP_DISP_AV_BUF_FMT_NL_GFX_RGBA8888,
>> +		.swap		= true,
>> +		.sf		= scaling_factors_888,
> I'm afraid that's not enough. There's a crucial difference between
> DRM_FORMAT_ARGB8888 (already supported by this driver) and
> DRM_FORMAT_XRGB8888: for the latter, the 'X' component must be ignored.
> The graphics layer is blended on top of the video layer, and the blender
> uses both a global alpha parameter and the alpha channel of the graphics
> layer for 32-bit RGB formats. This will lead to incorrect operation when
> the 'X' component is not set to full opacity.

I spent a few hours digging in the source code and what I could find in 
the TRM and register maps, but there's not enough information in there 
to explain how the blender works. The obvious "XRGB" implementation 
would be to just disable the blender.

What I got from experimenting so far is that the alpha component is 
ignored anyway while the video path isn't active. So as long as one 
isn't using the video blending path, the ARGB and XRGB modes are identical.

Guess I'll need assistance from AMD/Xilinx to completely implement the 
XRGB modes.

(For our application, this patch is sufficient as it solves the issues 
like X11 not starting up, OpenGL not working and horrendously slow 
scaling performance)


>
>>   	}, {
>>   		.drm_fmt	= DRM_FORMAT_RGBA8888,
>>   		.buf_fmt	= ZYNQMP_DISP_AV_BUF_FMT_NL_GFX_ABGR8888,


-- 
Mike Looijmans
System Expert

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