[PATCH 2/4] drm/msm/dp: remove redundant ST_DISPLAY_OFF checks in msm_dp_bridge_atomic_enable()

[Abhinav Kumar]

On 12/3/2024 5:53 AM, Dmitry Baryshkov wrote:
> On Mon, Dec 02, 2024 at 04:39:01PM -0800, Abhinav Kumar wrote:
>> The checks in msm_dp_display_prepare() for making sure that we are in
>> ST_DISPLAY_OFF OR ST_MAINLINK_READY seem redundant.
>>
>> DRM fwk shall not issue any commits if state is not ST_MAINLINK_READY as
>> msm_dp's atomic_check callback returns a failure if state is not ST_MAINLINK_READY.
> 
> Can the state change between atomic_check() and atomic_commit()?
> 

Good question.

I cannot deny that such a possibility does exist.

 From what I can see in the state machine today, the only possibility I 
can think of here is if a user very quickly removes the cable as soon as 
they connect the cable like so fast that the connect was not yet 
processed before disconnect.

Similarly, if an irq_hpd fires after atomic_check but before 
atomic_enable(), and moreover if we hit the sink_count == 0 case in 
msm_dp_display_handle_port_status_changed() during this irq_hpd,

In both these cases, then we will transition to ST_DISCONNECT_PENDING state.

Without this change, we would have bailed out in the 
ST_DISCONNECT_PENDING case.

But other than this, I cannot atleast think of a case where a different 
state transition can happen between atomic_check() and atomic_commit() 
because for other transitions, I think we should be still okay.

But this is purely based on theoretical observation and hypothesis.

Is it better to add a check to bail out in the DISCONNECT_PENDING case?

OR document this as "To-do: Need to bail out if DISCONNECT_PENDING" 
because even if I add this check, I dont know if can make sure this can 
be validated as the check could never hit.


>>
>> For the ST_DISPLAY_OFF check, its mainly to guard against a scenario that
>> there is an atomic_enable() without a prior atomic_disable() which once again
>> should not really happen.
>>
>> To simplify the code, get rid of these checks.
>>
>> Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
>> ---
>>   drivers/gpu/drm/msm/dp/dp_display.c | 6 ------
>>   1 file changed, 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
>> index 992184cc17e4..614fff09e5f2 100644
>> --- a/drivers/gpu/drm/msm/dp/dp_display.c
>> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
>> @@ -1513,12 +1513,6 @@ void msm_dp_bridge_atomic_enable(struct drm_bridge *drm_bridge,
>>   		return;
>>   	}
>>   
>> -	state = msm_dp_display->hpd_state;
>> -	if (state != ST_DISPLAY_OFF && state != ST_MAINLINK_READY) {
>> -		mutex_unlock(&msm_dp_display->event_mutex);
>> -		return;
>> -	}
>> -
>>   	rc = msm_dp_display_set_mode(dp, &msm_dp_display->msm_dp_mode);
>>   	if (rc) {
>>   		DRM_ERROR("Failed to perform a mode set, rc=%d\n", rc);
>>
>> -- 
>> 2.34.1
>>
>