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

[Jani Nikula <jani.nikula@intel.com>]

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.


BR,
Jani.


>
>> +}
>> +
>>  static bool
>> -intel_dp_can_mst(struct intel_dp *intel_dp)
>> +intel_dp_mst_detect(struct intel_dp *intel_dp)
>>  {
>>  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>> +	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
>> +	enum drm_dp_mst_mode sink_mst_mode;
>> +	enum drm_dp_mst_mode mst_detect;
>> +
>> +	sink_mst_mode = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd);
>> +
>> +	if (i915->display.params.enable_dp_mst &&
>> +	    intel_dp_mst_source_support(intel_dp) &&
>> +	    sink_mst_mode == DRM_DP_MST)
>> +		mst_detect = DRM_DP_MST;
>> +	else
>> +		mst_detect = DRM_DP_SST;
>>  
>> -	return i915->display.params.enable_dp_mst &&
>> -		intel_dp_mst_source_support(intel_dp) &&
>> -		drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd) == DRM_DP_MST;
>> +	drm_dbg_kms(&i915->drm,
>> +		    "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: %s -> enable: %s\n",
>> +		    encoder->base.base.id, encoder->base.name,
>> +		    str_yes_no(intel_dp_mst_source_support(intel_dp)),
>> +		    intel_dp_mst_mode_str(sink_mst_mode),
>> +		    str_yes_no(i915->display.params.enable_dp_mst),
>> +		    intel_dp_mst_mode_str(mst_detect));
>> +
>> +	return mst_detect != DRM_DP_SST;
>>  }
>>  
>>  static void
>>  intel_dp_configure_mst(struct intel_dp *intel_dp)
>>  {
>>  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>> -	struct intel_encoder *encoder =
>> -		&dp_to_dig_port(intel_dp)->base;
>>  	bool sink_can_mst = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd) == DRM_DP_MST;
>>  
>> -	drm_dbg_kms(&i915->drm,
>> -		    "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: %s\n",
>> -		    encoder->base.base.id, encoder->base.name,
>> -		    str_yes_no(intel_dp_mst_source_support(intel_dp)),
>> -		    str_yes_no(sink_can_mst),
>> -		    str_yes_no(i915->display.params.enable_dp_mst));
>> -
>>  	if (!intel_dp_mst_source_support(intel_dp))
>>  		return;
>>  
>> @@ -5390,7 +5407,7 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
>>  		connector_status_connected : connector_status_disconnected;
>>  	}
>>  
>> -	if (intel_dp_can_mst(intel_dp))
>> +	if (intel_dp_mst_detect(intel_dp))
>>  		return connector_status_connected;
>>  
>>  	/* If no HPD, poke DDC gently */
>> -- 
>> 2.39.2

-- 
Jani Nikula, Intel