Re: [PATCH v2] drm/i915/edp: don't write to DP_LINK_BW_SET when using rate select

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

On Mon, 11 Dec 2023, Jani Nikula <jani.nikula@intel.com> wrote:
> On Tue, 05 Dec 2023, Jani Nikula <jani.nikula@intel.com> wrote:
>> The eDP 1.5 spec adds a clarification for eDP 1.4x:
>>
>>> For eDP v1.4x, if the Source device chooses the Main-Link rate by way
>>> of DPCD 00100h, the Sink device shall ignore DPCD 00115h[2:0].
>>
>> We write 0 to DP_LINK_BW_SET (DPCD 100h) even when using
>> DP_LINK_RATE_SET (DPCD 114h). Stop doing that, as it can cause the panel
>> to ignore the rate set method.
>>
>> Moreover, 0 is a reserved value for DP_LINK_BW_SET, and should not be
>> used.
>>
>> v2: Improve the comments (Ville)
>>
>> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9081
>> Tested-by: Animesh Manna <animesh.manna@intel.com>
>> Reviewed-by: Uma Shankar <uma.shankar@intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>
> Pushed to drm-intel-next, thanks for the review and testing.

Also picked up to drm-intel-fixes with cc: stable that I forgot to add.

BR,
Jani.


>
> BR,
> Jani.
>
>> ---
>>  .../drm/i915/display/intel_dp_link_training.c | 31 +++++++++++++------
>>  1 file changed, 21 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> index dbc1b66c8ee4..1abfafbbfa75 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> @@ -650,19 +650,30 @@ intel_dp_update_link_bw_set(struct intel_dp *intel_dp,
>>  			    const struct intel_crtc_state *crtc_state,
>>  			    u8 link_bw, u8 rate_select)
>>  {
>> -	u8 link_config[2];
>> +	u8 lane_count = crtc_state->lane_count;
>>  
>> -	/* Write the link configuration data */
>> -	link_config[0] = link_bw;
>> -	link_config[1] = crtc_state->lane_count;
>>  	if (crtc_state->enhanced_framing)
>> -		link_config[1] |= DP_LANE_COUNT_ENHANCED_FRAME_EN;
>> -	drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_BW_SET, link_config, 2);
>> +		lane_count |= DP_LANE_COUNT_ENHANCED_FRAME_EN;
>> +
>> +	if (link_bw) {
>> +		/* DP and eDP v1.3 and earlier link bw set method. */
>> +		u8 link_config[] = { link_bw, lane_count };
>>  
>> -	/* eDP 1.4 rate select method. */
>> -	if (!link_bw)
>> -		drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_RATE_SET,
>> -				  &rate_select, 1);
>> +		drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_BW_SET, link_config,
>> +				  ARRAY_SIZE(link_config));
>> +	} else {
>> +		/*
>> +		 * eDP v1.4 and later link rate set method.
>> +		 *
>> +		 * eDP v1.4x sinks shall ignore DP_LINK_RATE_SET if
>> +		 * DP_LINK_BW_SET is set. Avoid writing DP_LINK_BW_SET.
>> +		 *
>> +		 * eDP v1.5 sinks allow choosing either, and the last choice
>> +		 * shall be active.
>> +		 */
>> +		drm_dp_dpcd_writeb(&intel_dp->aux, DP_LANE_COUNT_SET, lane_count);
>> +		drm_dp_dpcd_writeb(&intel_dp->aux, DP_LINK_RATE_SET, rate_select);
>> +	}
>>  }
>>  
>>  /*

-- 
Jani Nikula, Intel