[Intel-gfx] [PATCH] drm/i915/dp: Increase cdclk when DP audio is enabled with 4 lanes and HBR2

[Jim Bride]

On Thu, Oct 13, 2016 at 11:04:19AM -0700, Dhinakaran Pandiyan wrote:
> According to BSpec, cdclk has to be not less than 432 MHz with DP audio
> enabled, port width x4, and link rate HBR2 (5.4 GHz)
> 
> Having a lower cdclk triggers pipe underruns, which then lead to displays
> continuously cycling off and on. This is essential for DP MST audio as the
> link is trained at HBR2 and 4 lanes by default.
> 
> This should fix https://bugs.freedesktop.org/show_bug.cgi?id=97907
> 
> Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 47 +++++++++++++++++++++++++++++++++---
>  1 file changed, 43 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index cfcb03f..6a05183 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6603,14 +6603,43 @@ static int valleyview_modeset_calc_cdclk(struct drm_atomic_state *state)
>  	return 0;
>  }
>  
> +static bool cdclk_min_for_dp_audio(struct drm_atomic_state *state)
> +{
> +
> +	struct drm_crtc_state *crtc_state;
> +	struct drm_crtc *crtc;
> +	int i;
> +
> +	/* BSpec says "Do not use DisplayPort with CDCLK less than 432 MHz,
> +	 * audio enabled, port width x4, and link rate HBR2 (5.4 GHz), or else
> +	 * there may be audio corruption or screen corruption."
> +	 */

It's probably worth mentioning that this limitation really only applies for
SKL (at least that I can see) and BXT A (which I don't think we even care
about anymore since it's not a production part.)  Were we seeing underruns
on non-SKL platforms that increasing CDCLK fixed?   In any event, I'd rather
see this only implemented for SKL (see BDW code below) unless we've empirically
noticed a similar problem on BDW (or the B-Spec says something about this and I
missed it.)

Jim


> +
> +	for_each_crtc_in_state(state, crtc, crtc_state, i) {
> +		struct intel_crtc_state *pipe_config =
> +			to_intel_crtc_state(crtc_state);
> +
> +		return (intel_crtc_has_dp_encoder(pipe_config) &&
> +			pipe_config->has_audio &&
> +			pipe_config->port_clock == 540000 &&
> +			pipe_config->lane_count == 4);
> +	}
> +
> +	return false;
> +}
> +
>  static int bxt_modeset_calc_cdclk(struct drm_atomic_state *state)
>  {
>  	int max_pixclk = ilk_max_pixel_rate(state);
> +	int cdclk, min_cdclk = 0;
>  	struct intel_atomic_state *intel_state =
>  		to_intel_atomic_state(state);
>  
> -	intel_state->cdclk = intel_state->dev_cdclk =
> -		bxt_calc_cdclk(max_pixclk);
> +	if (cdclk_min_for_dp_audio(state))
> +		min_cdclk = bxt_calc_cdclk(432000);
> +
> +	cdclk = bxt_calc_cdclk(max_pixclk);
> +	intel_state->cdclk = intel_state->dev_cdclk = max(min_cdclk, cdclk);
>  
>  	if (!intel_state->active_crtcs)
>  		intel_state->dev_cdclk = bxt_calc_cdclk(0);
> @@ -10374,7 +10403,10 @@ static int broadwell_modeset_calc_cdclk(struct drm_atomic_state *state)
>  	struct drm_i915_private *dev_priv = to_i915(state->dev);
>  	struct intel_atomic_state *intel_state = to_intel_atomic_state(state);
>  	int max_pixclk = ilk_max_pixel_rate(state);
> -	int cdclk;
> +	int cdclk, min_cdclk = 0;
> +
> +	if (cdclk_min_for_dp_audio(state))
> +		min_cdclk = broadwell_calc_cdclk(432000);
>  
>  	/*
>  	 * FIXME should also account for plane ratio
> @@ -10382,6 +10414,8 @@ static int broadwell_modeset_calc_cdclk(struct drm_atomic_state *state)
>  	 */
>  	cdclk = broadwell_calc_cdclk(max_pixclk);
>  
> +	cdclk =	max(min_cdclk, cdclk);
> +
>  	if (cdclk > dev_priv->max_cdclk_freq) {
>  		DRM_DEBUG_KMS("requested cdclk (%d kHz) exceeds max (%d kHz)\n",
>  			      cdclk, dev_priv->max_cdclk_freq);
> @@ -10411,7 +10445,10 @@ static int skl_modeset_calc_cdclk(struct drm_atomic_state *state)
>  	struct drm_i915_private *dev_priv = to_i915(state->dev);
>  	const int max_pixclk = ilk_max_pixel_rate(state);
>  	int vco = intel_state->cdclk_pll_vco;
> -	int cdclk;
> +	int cdclk, min_cdclk = 0;
> +
> +	if (cdclk_min_for_dp_audio(state))
> +		min_cdclk = skl_calc_cdclk(432000, vco);
>  
>  	/*
>  	 * FIXME should also account for plane ratio
> @@ -10419,6 +10456,8 @@ static int skl_modeset_calc_cdclk(struct drm_atomic_state *state)
>  	 */
>  	cdclk = skl_calc_cdclk(max_pixclk, vco);
>  
> +	cdclk = max(min_cdclk, cdclk);
> +
>  	/*
>  	 * FIXME move the cdclk caclulation to
>  	 * compute_config() so we can fail gracegully.
> -- 
> 2.7.4