Re: [PATCH 2/4] drm/msm/dp: Return early from atomic_enable() if ST_DISCONNECT_PENDING

[OSS <dmitry.baryshkov@oss.qualcomm.com>]

On 2 June 2025 18:54:12 BST, Jessica Zhang <jessica.zhang@oss.qualcomm.com> wrote:
>
>
>On 5/30/2025 9:04 AM, Dmitry Baryshkov wrote:
>> On Fri, 30 May 2025 at 02:15, Jessica Zhang
>> <jessica.zhang@oss.qualcomm.com> wrote:
>>> 
>>> From: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>> 
>>> The checks in msm_dp_bridge_atomic_enable() 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.
>> 
>> What if the state changes between atomic_check() and atomic_enable()?
>> There are no locks, cable unplugging is async, so it's perfectly
>> possible.
>> 
>>> 
>>> 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.
>>> 
>>> Since it's still possible for the state machine to transition to
>>> ST_DISCONNECT_PENDING between atomic_check() and atomic_commit(), change
>>> this check to return early if hpd_state is ST_DISCONNECT_PENDING.
>> 
>> Can we really, please, drop the state machine? I had other plans for
>> the next week, but maybe I should just do it, so that by the end of
>> 6.17 cycle we can have a merged, stable and working solution? This
>> topic has been lingering for months without any actual change.
>
>FWIW, I'm currently working on the state machine rework by the middle of next week.
>
>I'm anticipating that the rework itself will take some time to get merged, so didn't want MST to get delayed more by this series.

Yes, that's fine. I really want HPD to be merged before MST. And if I wasn't explicit, the state machine must be gone. Link training should happen from atomic_enable, detect should be reporting whether there is an actual display plugged, etc.  Current code must be dropped.


>
>Thanks,
>
>Jessica Zhang
>
>> 
>>> 
>>> Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>> Signed-off-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
>>> ---
>>>   drivers/gpu/drm/msm/dp/dp_display.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
>>> index 1d7cda62d5fb..f2820f06f5dc 100644
>>> --- a/drivers/gpu/drm/msm/dp/dp_display.c
>>> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
>>> @@ -1512,7 +1512,7 @@ void msm_dp_bridge_atomic_enable(struct drm_bridge *drm_bridge,
>>>          }
>>> 
>>>          hpd_state = msm_dp_display->hpd_state;
>>> -       if (hpd_state != ST_DISPLAY_OFF && hpd_state != ST_MAINLINK_READY) {
>>> +       if (hpd_state == ST_DISCONNECT_PENDING) {
>> 
>> 
>> 
>>>                  mutex_unlock(&msm_dp_display->event_mutex);
>>>                  return;
>>>          }
>>> 
>>> --
>>> 2.49.0
>>> 
>> 
>> 
>

With best wishes, 
Dmitry