Re: [PATCH v3 2/3] drm/edid: Add drm_edid_detect_panel_size()

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 02.09.26 um 10:45 schrieb Jani Nikula:
> On Tue, 01 Sep 2026, Thomas Zimmermann <tzimmermann@suse.de> wrote:
>> 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.
>>
>> 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..d5d9e77dc91b 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 */
> Okay, so I should've asked this n versions ago, but should that be
> s/&&/||/ instead?

You're right. And I could have sworn that I took this logic from 
somewhere in this file, but that doesn't seem to be the case. Apologies 
for such a stupid mistake.

Best regards
Thomas

>
>> +
>> +	dt = &edid->detailed_timings[0];
>> +
>> +	if (!is_detailed_timing_descriptor(dt))
>> +		return -EINVAL;
>> +
>> +	pt = &dt->data.pixel_data;
>> +
>> +	if (width)
>> +		*width = (pt->hactive_hblank_hi & 0xf0) << 4 | pt->hactive_lo;
>> +	if (height)
>> +		*height = (pt->vactive_vblank_hi & 0xf0) << 4 | pt->vactive_lo;
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL(drm_edid_detect_panel_size);
>> diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
>> index 04f7a7f1f108..a2617aa34edf 100644
>> --- a/include/drm/drm_edid.h
>> +++ b/include/drm/drm_edid.h
>> @@ -490,5 +490,7 @@ u32 drm_edid_get_panel_id(const struct drm_edid *drm_edid);
>>   bool drm_edid_match(const struct drm_edid *drm_edid,
>>   		    const struct drm_edid_ident *ident);
>>   bool drm_edid_has_quirk(struct drm_connector *connector, enum drm_edid_quirk quirk);
>> +int drm_edid_detect_panel_size(const struct drm_edid *drm_edid,
>> +			       unsigned int *width, unsigned int *height);
>>   
>>   #endif /* __DRM_EDID_H__ */

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, (HRB 36809, AG Nürnberg)