Re: [RFC 4/4] drm/i915/mst: enable MST mode for 128b/132b single-stream sideband

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Feb 12, 2024 at 05:30:40PM +0200, Jani Nikula wrote:
> On Fri, 02 Feb 2024, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > On Fri, Feb 02, 2024 at 04:05:34PM +0200, Jani Nikula wrote:
> >> If the sink supports 128b/132b and single-stream sideband messaging,
> >> enable MST mode.
> >> 
> >> With this, the topology manager will still write DP_MSTM_CTRL, which
> >> should be ignored by the sink. In the future,
> >> drm_dp_mst_topology_mgr_set_mst() bool mst_state parameter should
> >> probably be turned into an enum drm_dp_mst_mode mst_mode parameter.
> >
> > Rather I'd say the topology manager should stop concerning itself
> > with the MST enable bit and just frob the sideband enable bit.
> > The MST enable bit should be configured at modeset time to
> > reflect whether we're about to transmit in MST or SST mode.
> 
> Are you suggesting the driver should write the MST vs. SST mode in
> DP_MSTM_CTRL?

Either that or there needs to be a function provided by the
topology manager that gets called at modeset time to configure it.
I don't recall when exactly it should be configured according the
DP spec.

> 
> I worry a bit about the rmw on DPCD regs. The topology manager only does
> writes.
> 
> BR,
> Jani.
> 
> 
> 
> 
> 
> >
> >> 
> >> Cc: Arun R Murthy <arun.r.murthy@intel.com>
> >> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/display/intel_dp.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> >> index 4dd9c50226d1..16130e87dc23 100644
> >> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> >> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> >> @@ -4020,7 +4020,9 @@ static bool intel_dp_mst_detect(struct intel_dp *intel_dp)
> >>  
> >>  	intel_dp->is_mst = i915->display.params.enable_dp_mst &&
> >>  		intel_dp_mst_source_support(intel_dp) &&
> >> -		sink_mst_mode == DP_MST_CAPABLE;
> >> +		(sink_mst_mode == DP_MST_CAPABLE ||
> >> +		 (sink_mst_mode == DP_MST_SIDEBAND_MSG &&
> >> +		  intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] & DP_CAP_ANSI_128B132B));
> >>  
> >>  	drm_dbg_kms(&i915->drm,
> >>  		    "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: %s -> enable: %s\n",
> >> -- 
> >> 2.39.2
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel