Re: [PATCH 2/2] drm/amd: Fetch the EDID from _DDC if available for eDP

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

On Mon, 29 Jan 2024, Mario Limonciello <mario.limonciello@amd.com> wrote:
> On 1/29/2024 03:39, Jani Nikula wrote:
>> On Fri, 26 Jan 2024, Mario Limonciello <mario.limonciello@amd.com> wrote:
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
>>> index 9caba10315a8..c7e1563a46d3 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
>>> @@ -278,6 +278,11 @@ static void amdgpu_connector_get_edid(struct drm_connector *connector)
>>>   	struct amdgpu_device *adev = drm_to_adev(dev);
>>>   	struct amdgpu_connector *amdgpu_connector = to_amdgpu_connector(connector);
>>>   
>>> +	if (amdgpu_connector->edid)
>>> +		return;
>>> +
>>> +	/* if the BIOS specifies the EDID via _DDC, prefer this */
>>> +	amdgpu_connector->edid = amdgpu_acpi_edid(adev, connector);
>> 
>> Imagine the EDID returned by acpi_video_get_edid() has edid->extensions
>> bigger than 4. Of course it should not, but you have no guarantees, and
>> it originates outside of the kernel.
>> 
>> The real fix is to have the function return a struct drm_edid which
>> tracks the allocation size separately. Unfortunately, it requires a
>> bunch of changes along the way. We've mostly done it in i915, and I've
>> sent a series to do this in drm/bridge [1].

Looking at it again, perhaps the ACPI code should just return a blob,
and the drm code should have a helper to wrap that around struct
drm_edid, so that the ACPI code does not have to depend on drm. Basic
idea remains.

>> Bottom line, we should stop using struct edid in drivers. They'll all
>> parse the info differently, and from what I've seen, often wrong.
>> 
>> 
>
> Thanks for the feedback.  In that case this specific change should 
> probably rebase on the Melissa's work 
> https://lore.kernel.org/amd-gfx/20240126163429.56714-1-mwen@igalia.com/ 
> after she takes into account the feedback.
>
> Let me ask you this though - do you think that after that's done should 
> we let all drivers get EDID from BIOS as a priority?  Or would you 
> prefer that this is unique to amdgpu?

If the reason for having this is that the panel EDID contains some
garbage, that's certainly not unique to amdgpu... :p

> Something like:
>
> 1) If user specifies on kernel command line and puts an EDID in 
> /lib/firmware use that.
> 2) If BIOS has EDID in _DDC and it's eDP panel, use that.

I think we should also look into this. We currently don't do this, and
it might help with some machines. However, gut feeling says it's
probably better to keep this as a per driver decision instead of trying
to bolt it into drm helpers.

BR,
Jani.


> 3) Get panel EDID.
>

-- 
Jani Nikula, Intel