> -----Original Message----- > From: Nikula, Jani <jani.nik...@intel.com> > Sent: Monday, January 8, 2024 7:01 PM > To: Murthy, Arun R <arun.r.mur...@intel.com>; intel-gfx@lists.freedesktop.org; > Deak, Imre <imre.d...@intel.com> > Cc: Murthy, Arun R <arun.r.mur...@intel.com> > Subject: Re: [PATCH] drm/i915/display/dp: 128/132b DP-capable with SST > > On Wed, 03 Jan 2024, Arun R Murthy <arun.r.mur...@intel.com> wrote: > > With a value of '0' read from MSTM_CAP register MST to be enabled. > > DP2.1 SCR updates the spec for 128/132b DP capable supporting only one > > stream and not supporting single stream sideband MSG. > > The underlying protocol will be MST to enable use of MTP. > > > > Signed-off-by: Arun R Murthy <arun.r.mur...@intel.com> > > --- > > drivers/gpu/drm/i915/display/intel_dp.c | 9 +++++++-- > > 1 file changed, 7 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c > > b/drivers/gpu/drm/i915/display/intel_dp.c > > index 9ff0cbd9c0df..40d3280f8d98 100644 > > --- a/drivers/gpu/drm/i915/display/intel_dp.c > > +++ b/drivers/gpu/drm/i915/display/intel_dp.c > > @@ -4038,8 +4038,13 @@ intel_dp_configure_mst(struct intel_dp *intel_dp) > > if (!intel_dp_mst_source_support(intel_dp)) > > return; > > > > - intel_dp->is_mst = sink_can_mst && > > - i915->display.params.enable_dp_mst; > > + /* > > + * Even if dpcd reg MSTM_CAP is 0, if the sink supports UHBR rates > then > > + * DP2.1 can be enabled with underlying protocol using MST for MTP > > + */ > > + intel_dp->is_mst = (sink_can_mst || > > + > drm_dp_is_uhbr_rate(intel_dp_max_common_rate(intel_dp))) > > + && i915->display.params.enable_dp_mst; > > We use drm_dp_is_uhbr_rate() in intel_dp_is_uhbr() to determine whether the > link rate in the *crtc state* is uhbr, and by proxy whether the link in the > *crtc > state* is 128b/132b. > Yes! If the link rate in crtc_state is not uhbr then we dont enable 128/132b. Also the return from drm_dp_read_mst_cap() return 0 or 1 and if not mst then 128/132b is not enabled. We need to deviate here and a value of bit[0]=0, bit[1]=0 in MSTM_CAP register is 128/132b with SST. Hence this hack is added to see if the return from MSTM_CAP is 0x00 and if uhbr rates are supported then enable 128/132b.
> There, we've already decided to use uhbr and 128b/132b. > > This one here is different, and I think it's taking us to the wrong > direction. For > example, it should be possible to downgrade the link from uhbr to non-uhbr on > link failures. We don't have that yet. But this makes untangling that even > harder. Yes upon having the fallback, I think we will have to handle fallback in this case separately. Change might be required in drm core, drm_dp_read_mst_cap() should consider the DP2.1 changes to accommodate this 0x00 value. Will be floating the fallback patches very soon. Thanks and Regards, Arun R Murthy -------------------- > > > BR, > Jani. > > > > > > drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr, > > intel_dp->is_mst); > > -- > Jani Nikula, Intel
