On Tue, 09 Jan 2024, "Murthy, Arun R" <arun.r.murthy@xxxxxxxxx> wrote: >> -----Original Message----- >> From: Nikula, Jani <jani.nikula@xxxxxxxxx> >> Sent: Monday, January 8, 2024 7:01 PM >> To: Murthy, Arun R <arun.r.murthy@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; >> Deak, Imre <imre.deak@xxxxxxxxx> >> Cc: Murthy, Arun R <arun.r.murthy@xxxxxxxxx> >> Subject: Re: [PATCH] drm/i915/display/dp: 128/132b DP-capable with SST >> >> On Wed, 03 Jan 2024, Arun R Murthy <arun.r.murthy@xxxxxxxxx> 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.murthy@xxxxxxxxx> >> > --- >> > 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. Per spec it depends on intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] & DP_CAP_ANSI_128B132B, why not use that here too? > >> 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 -- Jani Nikula, Intel
