linux-kernel - Re: [PATCH] drm: xlnx: zynqmp_dp: Support DRM_FORMAT

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Message-ID: <6b52a77f-47fb-4da3-a750-d5344b1f3416@topic.nl>
Date: Mon, 30 Jun 2025 16:47:28 +0200
From: Mike Looijmans <mike.looijmans@...ic.nl>
To: Maxime Ripard <mripard@...nel.org>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC: dri-devel@...ts.freedesktop.org, David Airlie <airlied@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Michal Simek <michal.simek@....com>, Simona Vetter <simona@...ll.ch>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: xlnx: zynqmp_dp: Support DRM_FORMAT_XRGB8888

On 30-06-2025 11:29, Maxime Ripard wrote:
> On Mon, Jun 30, 2025 at 12:11:56PM +0300, Laurent Pinchart wrote:
>> On Mon, Jun 30, 2025 at 10:27:55AM +0200, Maxime Ripard wrote:
>>> On Mon, Jun 30, 2025 at 10:03:16AM +0200, Mike Looijmans wrote:
>>>> On 27-06-2025 20:19, Laurent Pinchart wrote:
>>>>> 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@...ic.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)
>>> Given that we consider XRGB8888 mandatory,
>> How about platforms that can't support it at all ?
> We emulate it.
>
>>> this patch is a good thing to
>>> have anyway, even if suboptimal, or broken in some scenario we can
>>> always fix later.
>> It needs to at least be updated to disallow XRGB8888 usage when the
>> video plan is enabled, or when global alpha is set to a non-opaque
>> value.
> Yeah, that's reasonable

And feasible too I think. Basically only allow XRGB8888 when things are 
either totally transparent or totally opaque. I'm only concerned it 
might to lead to strange behavior, depending on which layer you enable 
first.


>
> Maxime


-- 
Mike Looijmans