Re: [PATCH v3] drm/msm/dp: check hpd_state before push idle pattern at dp_bridge_disable()

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

Hi Stephen

On 8/15/2022 10:08 AM, Stephen Boyd wrote:
> Quoting Kuogee Hsieh (2022-08-11 08:20:01)
>>
>> On 8/10/2022 6:00 PM, Abhinav Kumar wrote:
>>>
>>> Even then, you do have a valid point. DRM framework should not have
>>> caused the disable path to happen without an enable.
>>>
>>> I went through the stack mentioned in the issue.
>>>
>>> Lets see this part of the stack:
>>>
>>> dp_ctrl_push_idle+0x40/0x88
>>>   dp_bridge_disable+0x24/0x30
>>>   drm_atomic_bridge_chain_disable+0x90/0xbc
>>>   drm_atomic_helper_commit_modeset_disables+0x198/0x444
>>>   msm_atomic_commit_tail+0x1d0/0x374
>>>
>>> In drm_atomic_helper_commit_modeset_disables(), we call
>>> disable_outputs().
>>>
>>> AFAICT, this is the only place which has a protection to not call the
>>> disable() flow if it was not enabled here:
>>>
>>> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/drm_atomic_helper.c#L1063
>>>
>>>
>>> But that function is only checking crtc_state->active. Thats set by
>>> the usermode:
>>>
>>> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/drm_atomic_uapi.c#L407
>>>
>>>
>>> Now, if usermode sets that to true and then crashed it can bypass this
>>> check and we will crash in the location kuogee is trying to fix.
> 
> That seems bad, no? We don't want userspace to be able to crash and then
> be able to call the disable path when enable never succeeded.
> 
>>>
>>>  From the issue mentioned in
>>> https://gitlab.freedesktop.org/drm/msm/-/issues/17, the reporter did
>>> mention the usermode crashed.
>>>
>>> So this is my tentative analysis of whats happening here.
>>>
>>> Ideally yes, we should have been protected by the location mentioned
>>> above in disable_outputs() but looks to me due to the above hypothesis
>>> its getting bypassed.
> 
> Can you fix the problem there? Not fixing it means that every driver out
> there has to develop the same "fix", when it could be fixed in the core
> one time.
> 

As per discussion on IRC with Rob, we have pushed another fix for this 
issue 
https://lore.kernel.org/all/1660759314-28088-1-git-send-email-quic_khsieh@quicinc.com/.

So, we can drop this one in favor of the other.

Thanks

Abhinav
> Ideally drivers are simple. They configure the hardware for what the
> function pointer is asking for. State management and things like that
> should be pushed into the core framework so that we don't have to
> duplicate that multiple times.
> 
>>>
>>> Thanks
>>>
>>> Abhinav
>>>
>>>
>> Ii sound likes that there is a hole either at user space or drm.
>>
>> But that should not cause dp_bridge_disable() at dp driver to crash.
> 
> Agreed.
> 
>>
>> Therefore it is properly to check hdp_state condition at
>> dp_bridge_disable() to prevent it from crashing.
>>
> 
> Disagree. Userspace shouldn't be able to get drm into a wedged state.