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

[Jani Nikula]

On Fri, 02 Feb 2024, Ville Syrjälä <ville.syrjala at 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?

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 at intel.com>
>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
>> Signed-off-by: Jani Nikula <jani.nikula at 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