Re: [PATCH v4 2/3] drm/edid: Add drm_edid_detect_panel_size()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 10 Sep 2026, Javier Martinez Canillas <javierm@redhat.com> wrote:
> Thomas Zimmermann <tzimmermann@suse.de> writes:
>
>> Add drm_edid_detect_panel_size() to extract the panel's preferred
>> display resolution from a given EDID. Required for setting up DRM's
>> panel orientation quirks in sysfb drivers.
>>
>> v4:
>> - fix test for EDID PTD (Jani)
>> v3:
>> - mention use case in documentation (Jani)
>> - use is_detailed_timing_descriptor() (Jani)
>> - rename helper to drm_edid_detect_panel_size()
>> v2:
>> - handle EDID without pixel timing descriptor (Sashiko)
>> - fix checks for width and height pointers
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Acked-by: Ard Biesheuvel <ardb@kernel.org>
>> ---
>>  drivers/gpu/drm/drm_edid.c | 47 ++++++++++++++++++++++++++++++++++++++
>>  include/drm/drm_edid.h     |  2 ++
>>  2 files changed, 49 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
>> index 9990a836d0b6..1331efb4876c 100644
>> --- a/drivers/gpu/drm/drm_edid.c
>> +++ b/drivers/gpu/drm/drm_edid.c
>> @@ -7852,3 +7852,50 @@ bool drm_edid_is_digital(const struct drm_edid *drm_edid)
>>  		drm_edid->edid->input & DRM_EDID_INPUT_DIGITAL;
>>  }
>>  EXPORT_SYMBOL(drm_edid_is_digital);
>> +
>> +/**
>> + * drm_edid_detect_panel_size - Get a panel's size from EDID
>> + * @drm_edid: EDID of the panel.
>> + * @width: Returns the panel's width in pixels per scanline, if given
>> + * @height: Returns the panel's height in scanlines, if given
>> + *
>> + * This function detects the preferred size of a panel from the given
>> + * EDID. There is no such information stored in the EDID block directly,
>> + * but the preferred mode often corresponds to the panel's native geometry.
>> + *
>> + * This helper should only be used during initialization before the
>> + * connector is available. For regular use, retrieve the available display
>> + * modes with the connector functions.
>> + *
>> + * Return: Zero on success, or a negative errno code otherwise.
>> + */
>> +int drm_edid_detect_panel_size(const struct drm_edid *drm_edid,
>> +			       unsigned int *width, unsigned int *height)
>> +{
>> +	const struct edid *edid = drm_edid->edid;
>> +	const struct detailed_timing *dt;
>> +	const struct detailed_pixel_timing *pt;
>> +
>> +	/*
>> +	 * Use display mode from the Preferred Timing Descriptor. For old
>> +	 * and obscure displays, we might need better heuristics.
>> +	 */
>> +
>> +	if (edid->revision < 4 || !(edid->features & DRM_EDID_FEATURE_PREFERRED_TIMING))
>> +		return -EINVAL; /* no Preferred Timing Descriptor */
>> +
>
> I saw that Jani and you discussed this logic in v3. I originally thought that
> your v3 was correct and this one was wrong, but I noticed now that this flag
> changed its semantic between 1.3 and 1.4.
>
> So I think that this latest version is correct indeed, 1.3 didn't include the
> native pixel format and 1.4 does not either if DRM_EDID_FEATURE_PREFERRED_TIMING
> flag is not set.

Yeah, and "pixel format" makes me think RGB, YCbCr, 4:4:4, 4:2:2, etc,
*not* the resolution! Had to separately find the section that says pixel
format means resolution here.

BR,
Jani.

>
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

-- 
Jani Nikula, Intel