Re: [Intel-gfx] [PATCH] drm/i915: Fix FIFO underruns caused by missing cumulative bpp W/A

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

On Thu, Apr 07, 2022 at 11:42:35AM +0300, Stanislav Lisovskiy wrote:
> We had some FIFO underruns appearing on platforms like ADL,
> which could be fixed though by increasing CDCLK, however we were
> lacking explanation for that - we were not calculating CDCLK,
> also based on cumulative bpp W/A formula, mentioned in BSpec 64631.

We already have that in intel_bw_crtc_min_cdclk().

> 
> With that patch no FIFO underruns appear anymore.
> 
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_bw.c | 71 ++++++++++++++++++++++---
>  drivers/gpu/drm/i915/display/intel_bw.h |  2 +
>  2 files changed, 67 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_bw.c b/drivers/gpu/drm/i915/display/intel_bw.c
> index 37bd7b17f3d0..3a2aeeffee7c 100644
> --- a/drivers/gpu/drm/i915/display/intel_bw.c
> +++ b/drivers/gpu/drm/i915/display/intel_bw.c
> @@ -743,20 +743,51 @@ static void skl_plane_calc_dbuf_bw(struct intel_bw_state *bw_state,
>  	}
>  }
>  
> -static void skl_crtc_calc_dbuf_bw(struct intel_bw_state *bw_state,
> +static int intel_plane_bw_bpp(const struct drm_format_info *info)
> +{
> +	/*
> +	 * For the purposes of memory bandwidth calculations,
> +	 * planar formats are treated as if both planes had the
> +	 * same bpp (with no reduction for vertical
> +	 * subsampling). I.e we take as usual the worst case
> +	 * scenario.
> +	 */
> +	if (drm_format_info_is_yuv_semiplanar(info))
> +		return 2 * max(info->cpp[0], info->cpp[1]);
> +
> +	return info->cpp[0];
> +}
> +
> +static void skl_crtc_calc_dbuf_bw(struct intel_atomic_state *state,
> +				  struct intel_bw_state *bw_state,
>  				  const struct intel_crtc_state *crtc_state)
>  {
>  	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
>  	struct drm_i915_private *i915 = to_i915(crtc->base.dev);
>  	struct intel_dbuf_bw *crtc_bw = &bw_state->dbuf_bw[crtc->pipe];
> -	enum plane_id plane_id;
> +	struct intel_plane *plane;
>  
>  	memset(crtc_bw, 0, sizeof(*crtc_bw));
>  
>  	if (!crtc_state->hw.active)
>  		return;
>  
> -	for_each_plane_id_on_crtc(crtc, plane_id) {
> +	for_each_intel_plane_on_crtc(&i915->drm, crtc, plane) {
> +		const struct drm_framebuffer *fb;
> +		enum plane_id plane_id = plane->id;
> +		unsigned int plane_bpp = 0;
> +		struct intel_plane_state *plane_state =
> +			intel_atomic_get_new_plane_state(state, plane);
> +
> +		if (plane_state) {
> +			fb = plane_state->hw.fb;
> +
> +			if (plane_state->uapi.visible && fb)
> +				plane_bpp = intel_plane_bw_bpp(fb->format);
> +		}
> +
> +		crtc_bw->pipe_cumulative_bpp += plane_bpp;
> +
>  		/*
>  		 * We assume cursors are small enough
>  		 * to not cause bandwidth problems.
> @@ -773,6 +804,10 @@ static void skl_crtc_calc_dbuf_bw(struct intel_bw_state *bw_state,
>  					       &crtc_state->wm.skl.plane_ddb_y[plane_id],
>  					       crtc_state->data_rate[plane_id]);
>  	}
> +
> +	crtc_bw->bpp_cdclk = DIV_ROUND_UP_ULL(mul_u32_u32(crtc_state->pixel_rate,
> +					      crtc_bw->pipe_cumulative_bpp * 512),
> +					      10) / 1000;
>  }
>  
>  /* "Maximum Data Buffer Bandwidth" */
> @@ -782,11 +817,13 @@ intel_bw_dbuf_min_cdclk(struct drm_i915_private *i915,
>  {
>  	unsigned int total_max_bw = 0;
>  	enum dbuf_slice slice;
> +	enum pipe pipe;
> +	unsigned int bpp_cdclk = 0;
> +	unsigned int dbuf_bw_cdclk;
>  
>  	for_each_dbuf_slice(i915, slice) {
>  		int num_active_planes = 0;
>  		unsigned int max_bw = 0;
> -		enum pipe pipe;
>  
>  		/*
>  		 * The arbiter can only really guarantee an
> @@ -803,7 +840,29 @@ intel_bw_dbuf_min_cdclk(struct drm_i915_private *i915,
>  		total_max_bw = max(total_max_bw, max_bw);
>  	}
>  
> -	return DIV_ROUND_UP(total_max_bw, 64);
> +	for_each_pipe(i915, pipe) {
> +		const struct intel_dbuf_bw *crtc_bw = &bw_state->dbuf_bw[pipe];
> +		/*
> +		 * From BSpec 64631:
> +		 * Pipe cumulative bytes should be less or equal to
> +		 * CDCLK / (pixel_rate * scaling_factors * 51.2) thus
> +		 * CDCLK = pipe_cumulative_bpp * pixel_rate * scaling_factors * 51.2.
> +		 * Considering that intel_plane_pixel_rate already returns adjusted pixel rate,
> +		 * no scaling factors needed here.
> +		 */
> +		bpp_cdclk = max_t(unsigned int, crtc_bw->bpp_cdclk,
> +						bpp_cdclk);
> +	}
> +
> +	dbuf_bw_cdclk = DIV_ROUND_UP(total_max_bw, 64);
> +
> +	/*
> +	 * So now we have two CDCLK estimations:
> +	 * one is based on required DBuf BW and another is
> +	 * based on pipe cumulative bpp W/A(BSpec 64631)
> +	 * Traditionally take the more demanding into use(worst case)
> +	 */
> +	return max_t(unsigned int, dbuf_bw_cdclk, bpp_cdclk);
>  }
>  
>  int intel_bw_min_cdclk(struct drm_i915_private *i915,
> @@ -842,7 +901,7 @@ int intel_bw_calc_min_cdclk(struct intel_atomic_state *state,
>  
>  		old_bw_state = intel_atomic_get_old_bw_state(state);
>  
> -		skl_crtc_calc_dbuf_bw(new_bw_state, crtc_state);
> +		skl_crtc_calc_dbuf_bw(state, new_bw_state, crtc_state);
>  
>  		new_bw_state->min_cdclk[crtc->pipe] =
>  			intel_bw_crtc_min_cdclk(crtc_state);
> diff --git a/drivers/gpu/drm/i915/display/intel_bw.h b/drivers/gpu/drm/i915/display/intel_bw.h
> index cb7ee3a24a58..9e3a6ad03b19 100644
> --- a/drivers/gpu/drm/i915/display/intel_bw.h
> +++ b/drivers/gpu/drm/i915/display/intel_bw.h
> @@ -19,6 +19,8 @@ struct intel_crtc_state;
>  struct intel_dbuf_bw {
>  	unsigned int max_bw[I915_MAX_DBUF_SLICES];
>  	u8 active_planes[I915_MAX_DBUF_SLICES];
> +	unsigned int pipe_cumulative_bpp;
> +	unsigned int bpp_cdclk;
>  };
>  
>  struct intel_bw_state {
> -- 
> 2.24.1.485.gad05a3d8e5

-- 
Ville Syrjälä
Intel