[PATCH] drm/i915/display/dp: 128/132b DP-capable with SST

[Murthy, Arun R]

> -----Original Message-----
> From: Nikula, Jani <jani.nikula at intel.com>
> Sent: Tuesday, January 9, 2024 2:59 PM
> To: Murthy, Arun R <arun.r.murthy at intel.com>; intel-gfx at lists.freedesktop.org;
> Deak, Imre <imre.deak at intel.com>
> Subject: RE: [PATCH] drm/i915/display/dp: 128/132b DP-capable with SST
> 
> On Tue, 09 Jan 2024, Jani Nikula <jani.nikula at intel.com> wrote:
> > On Tue, 09 Jan 2024, "Murthy, Arun R" <arun.r.murthy at intel.com> wrote:
> >>> -----Original Message-----
> >>> From: Nikula, Jani <jani.nikula at intel.com>
> >>> Sent: Monday, January 8, 2024 7:01 PM
> >>> To: Murthy, Arun R <arun.r.murthy at intel.com>;
> >>> intel-gfx at lists.freedesktop.org; Deak, Imre <imre.deak at intel.com>
> >>> Cc: Murthy, Arun R <arun.r.murthy at 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.murthy at 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.murthy at 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.
> >
> > Per spec it depends on intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] &
> > DP_CAP_ANSI_128B132B, why not use that here too?
> 
> Also, shouldn't we ensure we don't try to do more than one stream?
> 
Yes, this will be taken care of in a separate patch, tracked as part of separate JIRA. VLK-55112.

Thanks and Regards,
Arun R Murthy
--------------------
> >
> >>
> >>> 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