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

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Message-ID: <20250627181911.GF24912@pendragon.ideasonboard.com>
Date: Fri, 27 Jun 2025 21:19:11 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Mike Looijmans <mike.looijmans@...ic.nl>
Cc: dri-devel@...ts.freedesktop.org, David Airlie <airlied@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	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

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@...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.

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

-- 
Regards,

Laurent Pinchart