Re: [PATCH 2/2] drm/i915/dp_mst: Fix configuring TUs for a disconnected stream

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

On Thu, Sep 10, 2026 at 12:50:16PM +0300, Luca Coelho wrote:
> On Mon, 2026-09-07 at 20:44 +0300, Imre Deak wrote:
> > During an atomic commit after all the MST stream CRTC state is computed
> > the driver ensures that the sum of TUs of all the streams on a given MST
> > topology link is within limits (63 for 8b10 and 64 for 128b132b). For a
> > disconnected stream the DRM MST core's BW verification doesn't ensure
> > this, because the topology state it uses for this is destroyed as soon
> > as the stream (i.e. MST connector/port) is disconnected. The driver
> > should keep the link state valid even for such disconnected streams, as
> > userspace may disable them one-by-one only in a deferred way. Ensure the
> > link's sum of TUs stays within limits in this case by simply reusing the
> > maximum link BPP limit from the stream's (i.e. CRTC's) old state.
> > 
> > The disconnection can happen either via the whole topology getting
> > disconnected or via only the given stream's port getting disconnected.
> > Check for both of these conditions separately, as a connector gets
> > unregistered after a link disconnect event only in a deferred way.
> > 
> > Cc: stable@vger.kernel.org # v6.10+
> > Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16073
> > Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16384
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c  | 21 ++++++++++++++++++++
> >  drivers/gpu/drm/i915/display/intel_dp_mst.h  |  2 ++
> >  drivers/gpu/drm/i915/display/intel_link_bw.c |  3 ++-
> >  3 files changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index 20766b6c0bcfa..0c362784afe4a 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -2172,6 +2172,27 @@ bool intel_dp_mst_crtc_needs_modeset(struct intel_atomic_state *state,
> >  	return false;
> >  }
> >  
> > +bool intel_dp_mst_stream_disconnected(struct intel_atomic_state *state,
> > +				      const struct intel_crtc *crtc)
> > +{
> > +	struct intel_connector *connector;
> > +
> > +	connector = get_connector_in_state_for_crtc(state, crtc);
> > +	if (!connector)
> > +		return false;
> > +
> > +	if (!connector->mst.dp)
> > +		return false;
> > +
> > +	if (!connector->mst.dp->mst.mgr.mst_state)
> > +		return true;
> > +
> > +	if (drm_connector_is_unregistered(&connector->base))
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> >  /**
> >   * intel_dp_mst_prepare_probe - Prepare an MST link for topology probing
> >   * @intel_dp: DP port object
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.h b/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > index ab09b487c6bb5..8ce89242c05c9 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.h
> > @@ -28,6 +28,8 @@ 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_stream_disconnected(struct intel_atomic_state *state,
> > +				      const struct intel_crtc *crtc);
> >  void intel_dp_mst_prepare_probe(struct intel_dp *intel_dp);
> >  bool intel_dp_mst_verify_dpcd_state(struct intel_dp *intel_dp);
> >  
> > diff --git a/drivers/gpu/drm/i915/display/intel_link_bw.c b/drivers/gpu/drm/i915/display/intel_link_bw.c
> > index b47474a3e9fec..e71e76d6fd3e0 100644
> > --- a/drivers/gpu/drm/i915/display/intel_link_bw.c
> > +++ b/drivers/gpu/drm/i915/display/intel_link_bw.c
> > @@ -64,7 +64,8 @@ void intel_link_bw_init_limits(struct intel_atomic_state *state,
> >  			intel_atomic_get_new_crtc_state(state, crtc);
> >  		int forced_bpp_x16 = get_forced_link_bpp_x16(state, crtc);
> >  
> > -		if (state->base.duplicated && crtc_state) {
> > +		if ((state->base.duplicated && crtc_state) ||
> > +		    intel_dp_mst_stream_disconnected(state, crtc)) {
> >  			limits->max_bpp_x16[pipe] = crtc_state->max_link_bpp_x16;
> >  			if (intel_dsc_enabled_on_link(crtc_state))
> >  				limits->link_dsc_pipes |= BIT(pipe);
> 
> I think Sashiko's NULL-dereference commment is valid.  Can you check
> it?

At least I can't see how a connector can get into the atomic state
without the crtc state for the CRTC associated with the connector.
Sashiko didn't provide the exact scenario it had in mind.

> 
> --
> Cheers,
> Luca.