Re: [PATCH] drm/i915/dp_mst: Fix MST state after a sink reset

[Imre Deak <imre.deak@intel.com>]

On Wed, Aug 21, 2024 at 05:19:11PM +0300, Jani Nikula wrote:
> On Wed, 24 Jul 2024, Imre Deak <imre.deak@intel.com> wrote:
> > In some cases the sink can reset itself after it was configured into MST
> > mode, without the driver noticing the disconnected state. For instance
> > the reset may happen in the middle of a modeset, or the (long) HPD pulse
> > generated may be not long enough for the encoder detect handler to
> > observe the HPD's deasserted state. In this case the sink's DPCD
> > register programmed to enable MST will be reset, while the driver still
> > assumes MST is still enabled. Detect this condition, which will tear
> > down and recreate/re-enable the MST topology.
> >
> > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11195
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c     |  3 +++
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 30 +++++++++++++++++++++
> >  drivers/gpu/drm/i915/display/intel_dp_mst.h |  1 +
> >  3 files changed, 34 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > index 1e43e32e05199..c621f6daf8235 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -5878,6 +5878,9 @@ intel_dp_detect(struct drm_connector *connector,
> >  	else
> >  		status = connector_status_disconnected;
> >  
> > +	if (!intel_dp_mst_verify_dpcd_state(intel_dp))
> > +		status = connector_status_disconnected;
> > +
> 
> So I still don't understand. We've got a detect which we've determined
> is connected. But then we disconnect because the sink state has changed.
> 
> How do we get another connect?

The detect in this case will be retried with a 1 sec delay, since the
mode changes back to SST and the connector state stays disconnected.

> BR,
> Jani.
> 
> 
> >  	if (status == connector_status_disconnected) {
> >  		memset(&intel_dp->compliance, 0, sizeof(intel_dp->compliance));
> >  		memset(intel_connector->dp.dsc_dpcd, 0, sizeof(intel_connector->dp.dsc_dpcd));
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index 27ce5c3f5951e..89b147e37b400 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -1998,3 +1998,33 @@ bool intel_dp_mst_crtc_needs_modeset(struct intel_atomic_state *state,
> >  
> >  	return false;
> >  }
> > +
> > +/**
> > + * intel_dp_mst_verify_dpcd_state - verify the MST SW enabled state wrt. the DPCD
> > + * @intel_dp: DP port object
> > + *
> > + * Verify if @intel_dp's MST enabled SW state matches the corresponding DPCD
> > + * state. A long HPD pulse -not long enough to be detected as a disconnected
> > + * state - could've reset the DPCD state, which requires tearing
> > + * down/recreating the MST topology.
> > + *
> > + * Returns %true if the SW MST enabled and DPCD states match, %false
> > + * otherwise.
> > + */
> > +bool intel_dp_mst_verify_dpcd_state(struct intel_dp *intel_dp)
> > +{
> > +	int ret;
> > +	u8 val;
> > +
> > +	if (!intel_dp->is_mst)
> > +		return true;
> > +
> > +	ret = drm_dp_dpcd_readb(intel_dp->mst_mgr.aux, DP_MSTM_CTRL, &val);
> > +	if (ret < 0)
> > +		return false;
> > +
> > +	if (val != (DP_MST_EN | DP_UP_REQ_EN | DP_UPSTREAM_IS_SRC))
> > +		return false;
> > +
> > +	return true;
> > +}
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.h b/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > index 8ca1d599091c6..9e4c7679f1c3a 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > @@ -27,5 +27,6 @@ int intel_dp_mst_atomic_check_link(struct intel_atomic_state *state,
> >  				   struct intel_link_bw_limits *limits);
> >  bool intel_dp_mst_crtc_needs_modeset(struct intel_atomic_state *state,
> >  				     struct intel_crtc *crtc);
> > +bool intel_dp_mst_verify_dpcd_state(struct intel_dp *intel_dp);
> >  
> >  #endif /* __INTEL_DP_MST_H__ */
> 
> -- 
> Jani Nikula, Intel