linux-kernel - Re: [PATCH v3 01/11] drm/fourcc: Add warning for bad bpp

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Message-ID: <20250213204140.GB22998@pendragon.ideasonboard.com>
Date: Thu, 13 Feb 2025 22:41:40 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Vishal Sagar <vishal.sagar@....com>,
	Anatoliy Klymenko <anatoliy.klymenko@....com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Michal Simek <michal.simek@....com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v3 01/11] drm/fourcc: Add warning for bad bpp

Hi Tomi,

Thank you for the patch.

On Wed, Feb 12, 2025 at 04:56:05PM +0200, Tomi Valkeinen wrote:
> drm_format_info_bpp() cannot be used for formats which do not have an
> integer bits-per-pixel in a pixel block.
> 
> E.g. DRM_FORMAT_XV15's (not yet in upstream) plane 0 has three 10-bit
> pixels (Y components), and two padding bits, in a 4 byte block. That is
> 10.666... bits per pixel when considering the whole 4 byte block, which
> is what drm_format_info_bpp() does. Thus a driver that supports such
> formats cannot use drm_format_info_bpp(),
> 
> It is a driver bug if this happens, but so handle wrong calls by
> printing a warning and returning 0.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> ---
>  drivers/gpu/drm/drm_fourcc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> index 3a94ca211f9c..1e9afbf6ef99 100644
> --- a/drivers/gpu/drm/drm_fourcc.c
> +++ b/drivers/gpu/drm/drm_fourcc.c
> @@ -457,6 +457,13 @@ unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane)
>  	if (!info || plane < 0 || plane >= info->num_planes)
>  		return 0;
>  
> +	if (info->char_per_block[plane] * 8 %
> +	    (drm_format_info_block_width(info, plane) *
> +	     drm_format_info_block_height(info, plane))) {

Can you cache 

	    drm_format_info_block_width(info, plane) *
	    drm_format_info_block_height(info, plane)

in a local variable to avoid computing it twice ?

> +		pr_warn("unable to return an integer bpp\n");

It would be nice to print the warning in the context of the drm device,
but we don't have the required structure here.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

> +		return 0;
> +	}
> +
>  	return info->char_per_block[plane] * 8 /
>  	       (drm_format_info_block_width(info, plane) *
>  		drm_format_info_block_height(info, plane));

-- 
Regards,

Laurent Pinchart