Re: [PATCH v2 2/6] drm/i915/mst: improve debug logging of DP MST mode detect

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

On Thu, Feb 15, 2024 at 01:46:48PM +0200, Jani Nikula wrote:
> On Wed, 14 Feb 2024, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > On Tue, Feb 13, 2024 at 01:30:57PM +0200, Jani Nikula wrote:
> >> Rename intel_dp_can_mst() to intel_dp_mst_detect(), and move all DP MST
> >> detect debug logging there. Debug log the sink's MST capability,
> >> including single-stream sideband messaging support, and the decision
> >> whether to enable MST mode or not. Do this regardless of whether we're
> >> actually enabling MST or not.
> >> 
> >> 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 | 45 +++++++++++++++++--------
> >>  1 file changed, 31 insertions(+), 14 deletions(-)
> >> 
> >> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> >> index a1c304f451bd..944f566525dd 100644
> >> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> >> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> >> @@ -4007,31 +4007,48 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> >>  					   intel_dp->downstream_ports) == 0;
> >>  }
> >>  
> >> +static const char *intel_dp_mst_mode_str(enum drm_dp_mst_mode mst_mode)
> >> +{
> >> +	if (mst_mode == DRM_DP_SST_SIDEBAND_MSG)
> >> +		return "single-stream sideband messaging";
> >> +	else
> >> +		return str_yes_no(mst_mode == DRM_DP_MST);
> >
> > I wonder if this should also just say "sst"/"mst"/"sst sideband" etc.
> > Shrug.
> >
> > Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> I realize there's an issue here.
> 
> intel_dp_detect_dpcd() bails out early for !drm_dp_is_branch(), before
> the intel_dp_can_mst() call. (Renamed intel_dp_mst_detect() here.)
> 
> We'll still happily call intel_dp_configure_mst() later also for
> !branch.
> 
> We'll need to call intel_dp_mst_detect() earlier and/or somehow combine
> these together. I don't think branch devices need to support MST, but I
> think MST devices need to support branching. And single-stream sideband
> support does not mean branching.
> 
> The intention of this patch was to improve MST debug logging, but the
> end result is that it reduces it! Auch.
> 
> I wonder if we should branch (eh) the detect earlier for eDP, SST and
> MST/branch paths. Just to make it easier for our poor brains to follow.

Hmm. The sink count check is another case as well. If the device
has a local sink, or somehting connected to its DFP(s) it should
declare sink count >= 1.

-- 
Ville Syrjälä
Intel