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

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

On Fri, Dec 01, 2023 at 03:41:41PM +0200, Jani Nikula 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.

What a terrible way to specify this :( This means the device must 
hav some internal state to keep track of whethe BW_SET was ever
written.

> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9081
> Tested-by: Animesh Manna <animesh.manna@intel.com>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  .../drm/i915/display/intel_dp_link_training.c | 23 +++++++++++--------
>  1 file changed, 13 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..6336a39030a4 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 +650,22 @@ 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) {
> +		/* eDP 1.3 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 1.4 rate select method. */
> +		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);

Doesn't look there's anything in the spec that specifies when the device
is supposed to reset its internal state to stop ignoring DP_LINK_RATE_SET.
Do we know when this panel does it? When VDD is removed?

> +	}
>  }
>  
>  /*
> -- 
> 2.39.2

-- 
Ville Syrjälä
Intel