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SubjectRe: [Bug Report] drm/edid: drm_edid_override_connector_update returns a incorrect value
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On Thu, 07 Dec 2023, bbaa <bbaa@bbaa.fun> wrote:
> Hello everyone,
>
> drm_edid_override_connector_update seem return a incorrect value.
>
> drivers/gpu/drm/drm_edid.c (Linux 6.7-rc4)
> 2294 /**
> 2295 * drm_edid_override_connector_update - add modes from override/firmware EDID
> 2296 * @connector: connector we're probing
> 2297 *
> 2298 * Add modes from the override/firmware EDID, if available. Only to be used from
> 2299 * drm_helper_probe_single_connector_modes() as a fallback for when DDC probe
> 2300 * failed during drm_get_edid() and caused the override/firmware EDID to be
> 2301 * skipped.
> 2302 *
> 2303 * Return: The number of modes added or 0 if we couldn't find any.
> 2304 */

Thanks for the report. I've sent a patch to hopefully fix this [1].

[1] https://patchwork.freedesktop.org/patch/msgid/20231207093821.2654267-1-jani.nikula@intel.com

However, please read the documentation comment above: "Only to be used
from drm_helper_probe_single_connector_modes() ..."

The function is a fallback for a *very* specific and rare scenario.

> This will break the EDID override behavior on Nvidia graphics cards.
>
> NVIDIA/open-gpu-kernel-modules:
> kernel-open/nvidia-drm/nvidia-drm-connector.c:
>   103  #if defined(NV_DRM_CONNECTOR_HAS_OVERRIDE_EDID)   104      if
> (connector->override_edid) {   105  #else   106      if
> (drm_edid_override_connector_update(connector) > 0) {   107  #endif
>   108          const struct drm_property_blob *edid =
> connector->edid_blob_ptr;   109
> drm_edid_override_connector_update(connector) will return zero here.

That's an out-of-tree driver that doesn't follow the documentation
above. Drivers have no business calling the function.

All of the override/firmware EDID handling should be covered
transparently via the drm_edid_read*() and drm_get_edid() functions, and
the drivers shouldn't have to ever care about overrides, at all. Drivers
shouldn't really use connector->edid_blob_ptr directly either.

Please report and get that fixed downstream.


BR,
Jani.


--
Jani Nikula, Intel

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