[RFC PATCH] drm: allow encoder mode_set even when connectors change for crtc

[Abhinav Kumar]

Hi Maxime

On 9/9/2024 6:37 AM, Maxime Ripard wrote:
> Hi,
> 
> On Thu, Sep 05, 2024 at 03:11:24PM GMT, Abhinav Kumar wrote:
>> In certain use-cases, a CRTC could switch between two encoders
>> and because the mode being programmed on the CRTC remains
>> the same during this switch, the CRTC's mode_changed remains false.
>> In such cases, the encoder's mode_set also gets skipped.
>>
>> Skipping mode_set on the encoder for such cases could cause an issue
>> because even though the same CRTC mode was being used, the encoder
>> type could have changed like the CRTC could have switched from a
>> real time encoder to a writeback encoder OR vice-versa.
>>
>> Allow encoder's mode_set to happen even when connectors changed on a
>> CRTC and not just when the mode changed.
>>
>> Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
> 
> The patch and rationale looks sane to me, but we should really add kunit
> tests for that scenarii.
> 

Thanks for the review.

We have a IGT for recreating this scenario and thats how this issue was 
captured

kms_writeback --run-subtest writeback-check-output -c <primary display mode>

We had added an option ( 'c' - custom mode) a couple of yrs ago to allow 
writeback to be tested using any mode the user passes in 
(https://lore.kernel.org/r/all/YuJhGkkxah9U6FGx@platvala-desk.ger.corp.intel.com/T/)

If we pass in the same resolution as the primary RT display, this 
scenario always happens as the CRTC switches between RT encoder and WB 
encoder. Hope that addresses some of the concern.

Regarding KUnit tests, I have a couple of questions:

1) This is more of a run-time scenario where CRTC switch happens, does 
this qualify for a KUnit or perhaps I am missing something.

2) Is there any existing KUnit test file under drm/tests for validating 
drm_atomic_helper_commit_modeset_disables() / 
drm_atomic_helper_commit_modeset_enables() path because this will fall 
under that bucket. I didnt find any matching case where we can extend this.

Thanks

Abhinav

> Maxime