Re: [RFC] drm/i915/dp: optimize eDP 1.4+ link config fast and narrow

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Wed, May 09, 2018 at 10:13:21AM +0300, Jani Nikula wrote:
> We've opted to use the maximum link rate and lane count for eDP panels,
> because typically the maximum supported configuration reported by the
> panel has matched the native resolution requirements of the panel, and
> optimizing the link has lead to problems.
> 
> With eDP 1.4 rate select method and DSC features, this is decreasingly
> the case. There's a need to optimize the link parameters. Moreover,
> already eDP 1.3 states fast link with fewer lanes is preferred over the
> wide and slow. (Wide and slow should still be more reliable for longer
> cable lengths.)
> 
> Additionally, there have been reports of panels failing on arbitrary
> link configurations, although arguably all configurations they claim to
> support should work.
> 
> Optimize eDP 1.4+ link config fast and narrow.
> 
> Side note: The implementation has a near duplicate of the link config
> function, with just the two inner for loops turned inside out. Perhaps
> there'd be a way to make this, say, more table driven to reduce the
> duplication, but seems like that would lead to duplication in the table
> generation. We'll also have to see how the link config optimization for
> DSC turns out.
> 
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Manasi Navare <manasi.d.navare@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>

Cc: Matt Atwood <matthew.s.atwood@intel.com>

I believe Matt is interested on this and know who could test this for us.

> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105267
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

This matches my understand of the eDP 1.4 spec I believe this is the
way to go, so

Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

but probably better to get a proper review and wait for someone
to test...

> 
> ---
> 
> Untested. It's possible this helps the referenced bug. The downside is
> that this patch has a bunch of dependencies that are too much to
> backport to stable kernels. If the patch works, we may need to consider
> hacking together an uglier backport.
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 73 ++++++++++++++++++++++++++++++++++-------
>  1 file changed, 62 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index dde92e4af5d3..1ec62965ece3 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -1768,6 +1768,42 @@ intel_dp_compute_link_config_wide(struct intel_dp *intel_dp,
>  	return false;
>  }
>  
> +/* Optimize link config in order: max bpp, min lanes, min clock */
> +static bool
> +intel_dp_compute_link_config_fast(struct intel_dp *intel_dp,
> +				  struct intel_crtc_state *pipe_config,
> +				  const struct link_config_limits *limits)
> +{
> +	struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
> +	int bpp, clock, lane_count;
> +	int mode_rate, link_clock, link_avail;
> +
> +	for (bpp = limits->max_bpp; bpp >= limits->min_bpp; bpp -= 2 * 3) {
> +		mode_rate = intel_dp_link_required(adjusted_mode->crtc_clock,
> +						   bpp);
> +
> +		for (lane_count = limits->min_lane_count;
> +		     lane_count <= limits->max_lane_count;
> +		     lane_count <<= 1) {
> +			for (clock = limits->min_clock; clock <= limits->max_clock; clock++) {
> +				link_clock = intel_dp->common_rates[clock];
> +				link_avail = intel_dp_max_data_rate(link_clock,
> +								    lane_count);
> +
> +				if (mode_rate <= link_avail) {
> +					pipe_config->lane_count = lane_count;
> +					pipe_config->pipe_bpp = bpp;
> +					pipe_config->port_clock = link_clock;
> +
> +					return true;
> +				}
> +			}
> +		}
> +	}
> +
> +	return false;
> +}
> +
>  static bool
>  intel_dp_compute_link_config(struct intel_encoder *encoder,
>  			     struct intel_crtc_state *pipe_config)
> @@ -1792,13 +1828,15 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
>  	limits.min_bpp = 6 * 3;
>  	limits.max_bpp = intel_dp_compute_bpp(intel_dp, pipe_config);
>  
> -	if (intel_dp_is_edp(intel_dp)) {
> +	if (intel_dp_is_edp(intel_dp) && intel_dp->edp_dpcd[0] < DP_EDP_14) {
>  		/*
>  		 * Use the maximum clock and number of lanes the eDP panel
> -		 * advertizes being capable of. The panels are generally
> -		 * designed to support only a single clock and lane
> -		 * configuration, and typically these values correspond to the
> -		 * native resolution of the panel.
> +		 * advertizes being capable of. The eDP 1.3 and earlier panels
> +		 * are generally designed to support only a single clock and
> +		 * lane configuration, and typically these values correspond to
> +		 * the native resolution of the panel. With eDP 1.4 rate select
> +		 * and DSC, this is decreasingly the case, and we need to be
> +		 * able to select less than maximum link config.
>  		 */
>  		limits.min_lane_count = limits.max_lane_count;
>  		limits.min_clock = limits.max_clock;
> @@ -1812,12 +1850,25 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
>  		      intel_dp->common_rates[limits.max_clock],
>  		      limits.max_bpp, adjusted_mode->crtc_clock);
>  
> -	/*
> -	 * Optimize for slow and wide. This is the place to add alternative
> -	 * optimization policy.
> -	 */
> -	if (!intel_dp_compute_link_config_wide(intel_dp, pipe_config, &limits))
> -		return false;
> +	if (intel_dp_is_edp(intel_dp)) {
> +		/*
> +		 * Optimize for fast and narrow. eDP 1.3 section 3.3 and eDP 1.4
> +		 * section A.1: "It is recommended that the minimum number of
> +		 * lanes be used, using the minimum link rate allowed for that
> +		 * lane configuration."
> +		 *
> +		 * Note that we use the max clock and lane count for eDP 1.3 and
> +		 * earlier, and fast vs. wide is irrelevant.
> +		 */
> +		if (!intel_dp_compute_link_config_fast(intel_dp, pipe_config,
> +						       &limits))
> +			return false;
> +	} else {
> +		/* Optimize for slow and wide. */
> +		if (!intel_dp_compute_link_config_wide(intel_dp, pipe_config,
> +						       &limits))
> +			return false;
> +	}
>  
>  	DRM_DEBUG_KMS("DP lane count %d clock %d bpp %d\n",
>  		      pipe_config->lane_count, pipe_config->port_clock,
> -- 
> 2.11.0
> 
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