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

[Kuogee Hsieh]

On 8/10/2022 6:00 PM, Abhinav Kumar wrote:
> Hi Stephen
>
> On 8/10/2022 5:09 PM, Stephen Boyd wrote:
> > Quoting Kuogee Hsieh (2022-08-10 16:57:51)
> > > On 8/10/2022 3:22 PM, Stephen Boyd wrote:
> > > > Quoting Kuogee Hsieh (2022-08-10 12:25:51)
> > > > > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c 
> > > > > b/drivers/gpu/drm/msm/dp/dp_display.c
> > > > > index b36f8b6..678289a 100644
> > > > > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > > > > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > > > > @@ -1729,10 +1729,20 @@ void dp_bridge_disable(struct drm_bridge 
> > > > > *drm_bridge)
> > > > >           struct msm_dp_bridge *dp_bridge = to_dp_bridge(drm_bridge);
> > > > >           struct msm_dp *dp = dp_bridge->dp_display;
> > > > >           struct dp_display_private *dp_display;
> > > > > +       u32 state;
> > > > >
> > > > >           dp_display = container_of(dp, struct dp_display_private, 
> > > > > dp_display);
> > > > >
> > > > > +       mutex_lock(&dp_display->event_mutex);
> > > > > +
> > > > > +       state = dp_display->hpd_state;
> > > > > +       if (state != ST_DISCONNECT_PENDING && state != 
> > > > > ST_CONNECTED) {
> > > > It's concerning that we have to check this at all. Are we still
> > > > interjecting into the disable path when the cable is disconnected?
> > >
> > > yes,
> > >
> > > The problem is not from cable disconnected.
> > >
> > > There is a corner case that this function is called at drm shutdown
> > > (drm_release).
> > >
> > > At that time, mainlink is not enabled, hence dp_ctrl_push_idle() will
> > > cause system crash.
> >
> > The mainlink is only disabled when the cable is disconnected though?
> >
> > Let me put it this way, if we have to check that the state is
> > "connected" or "disconnected pending" in the disable path then there's
> > an issue where this driver is being called in unexpected ways. This
> > driver is fighting the drm core each time there's a state check. We
> > really need to get rid of the state tracking entirely, and make sure
> > that the drm core is calling into the driver at the right time, i.e.
> > bridge disable is only called when the mainlink is enabled, etc.
>
> So if link training failed, we do not send a uevent to usermode and 
> will bail out here:
>
>         rc = dp_ctrl_on_link(dp->ctrl);
>         if (rc) {
>                 DRM_ERROR("failed to complete DP link training\n");
>                 goto end;
>         }
>
> So this commit is not coming from usermode but from the drm_release() 
> path.
>
> 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.
>
> 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.
>
> 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.

Therefore it is properly to check hdp_state condition at 
dp_bridge_disable() to prevent it from crashing.