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

[Manasi Navare <manasi.d.navare@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.

Yes for DSC for eDP we currently try fast and wide and if that doesnt
fit the requested mode then we enable DSC.
Now with the fast and narrow approach, again for power savings we can try
the fastest link rate with narrowest lane count and see if DSC can be enabled
there before increasing the lane count. But anyways that calls for a separate
optimization discussion.

Otherwise yes this method looks correct as per eDP 1.4 spec.

> 
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Manasi Navare <manasi.d.navare@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105267
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> 
> ---
> 
> 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)

Should we make the name of this function intel_dp_compute_link_config_fast_narrow() to make it
more intuitive to the optimization approach

And while at it change the other one also to _wide_slow()?
Just a suggestion but otherwise everything else LGTM.

Regards
Manasi

> +{
> +	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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