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

[Mario Limonciello <mario.limonciello@amd.com>]

On 1/29/2024 10:46, Jani Nikula wrote:
> 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.

I'd ideally like to split this stuff and Melissa's rework to be 
independent if possible.  I'll see if that's actually feasible.

> 
>>> 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

OK; maybe a helper in DRM that wraps the ACPI code then and amdgpu will 
use the helper for this series.

I'm also thinking it makes sense to have a new /proc/cmdline setup 
option to ignore the BIOS for EDID.  I'm hoping that since Windows uses 
_DDC that BIOS will be higher quality; but you know; BIOS =)

> 
>> 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.

OK; I'll wire up the helper and if you want to use in the future you can 
too then.

> 
> BR,
> Jani.
> 
> 
>> 3) Get panel EDID.
>>
>