Re: [PATCH 01/10] drm/fourcc: Add warning for bad bpp

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi Vishal,

On 04/12/2024 18:26, Sagar, Vishal wrote:
> [AMD Official Use Only - AMD Internal Distribution Only]
> 
> Hi Tomi,
> 
> Thanks for the patch.
> 
>> -----Original Message-----
>> From: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> Sent: Wednesday, December 4, 2024 10:31 AM
>> To: Sagar, Vishal <vishal.sagar@amd.com>; Klymenko, Anatoliy
>> <Anatoliy.Klymenko@amd.com>; Maarten Lankhorst
>> <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>;
>> Thomas Zimmermann <tzimmermann@suse.de>; David Airlie
>> <airlied@gmail.com>; Simona Vetter <simona@ffwll.ch>; Laurent Pinchart
>> <laurent.pinchart@ideasonboard.com>; Simek, Michal <michal.simek@amd.com>
>> Cc: dri-devel@lists.freedesktop.org; linux-kernel@vger.kernel.org; linux-arm-
>> kernel@lists.infradead.org; Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> Subject: [PATCH 01/10] drm/fourcc: Add warning for bad bpp
>>
>> drm_format_info_bpp() cannot be used for formats which do not have an
>> integer bits-per-pixel. Handle wrong calls by printing a warning and
>> returning 0.
> 
> It would be good to add an example of pixel format that may cause this issue.

Indeed, the description is perhaps a bit lacking. I'll change it to:

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.

  Tomi

>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.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 193cf8ed7912..e84c4ed6928c 100644
>> --- a/drivers/gpu/drm/drm_fourcc.c
>> +++ b/drivers/gpu/drm/drm_fourcc.c
>> @@ -429,6 +429,13 @@ 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))) {
>> +             pr_warn("unable to return an integer bpp\n");
>> +             return 0;
>> +     }
>> +
>>        return info->char_per_block[plane] * 8 /
>>               (drm_format_info_block_width(info, plane) *
>>                drm_format_info_block_height(info, plane));
>>
>> --
>> 2.43.0
> 
> Regards
> Vishal Sagar