Re: [Intel-gfx] [PATCH 03/15] drm/i915: Fix plane relative_data_rate calculation

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

On Tue, Jan 18, 2022 at 11:23:42AM +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> We are currently computing the relative data rates as
> src_size * scale_factor where scale_factor is src_size / dst_size.
> Thus relative data rate is src_size * src_size / dst_size,
> which is just utter nonsense. What we really seem to want is
> just a reasonable estimate on how much data will be fetched
> which is just src_size. So let's do that instead.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Omg, how could it stay like this all the time?

I actually had similar question, but thought that there just might
be again some magical/empirical heuristics behind this.
Need to challenge more anything, which can't be explained with
BSpec formulas or by common sense...

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>

> ---
>  drivers/gpu/drm/i915/intel_pm.c | 63 ++-------------------------------
>  1 file changed, 2 insertions(+), 61 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 9a9d4acb2988..e8fb56f288b4 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -4373,55 +4373,6 @@ void skl_pipe_ddb_get_hw_state(struct intel_crtc *crtc,
>  	intel_display_power_put(dev_priv, power_domain, wakeref);
>  }
>  
> -/*
> - * Determines the downscale amount of a plane for the purposes of watermark calculations.
> - * The bspec defines downscale amount as:
> - *
> - * """
> - * Horizontal down scale amount = maximum[1, Horizontal source size /
> - *                                           Horizontal destination size]
> - * Vertical down scale amount = maximum[1, Vertical source size /
> - *                                         Vertical destination size]
> - * Total down scale amount = Horizontal down scale amount *
> - *                           Vertical down scale amount
> - * """
> - *
> - * Return value is provided in 16.16 fixed point form to retain fractional part.
> - * Caller should take care of dividing & rounding off the value.
> - */
> -static uint_fixed_16_16_t
> -skl_plane_downscale_amount(const struct intel_crtc_state *crtc_state,
> -			   const struct intel_plane_state *plane_state)
> -{
> -	struct drm_i915_private *dev_priv = to_i915(crtc_state->uapi.crtc->dev);
> -	u32 src_w, src_h, dst_w, dst_h;
> -	uint_fixed_16_16_t fp_w_ratio, fp_h_ratio;
> -	uint_fixed_16_16_t downscale_h, downscale_w;
> -
> -	if (drm_WARN_ON(&dev_priv->drm,
> -			!intel_wm_plane_visible(crtc_state, plane_state)))
> -		return u32_to_fixed16(0);
> -
> -	/*
> -	 * Src coordinates are already rotated by 270 degrees for
> -	 * the 90/270 degree plane rotation cases (to match the
> -	 * GTT mapping), hence no need to account for rotation here.
> -	 *
> -	 * n.b., src is 16.16 fixed point, dst is whole integer.
> -	 */
> -	src_w = drm_rect_width(&plane_state->uapi.src) >> 16;
> -	src_h = drm_rect_height(&plane_state->uapi.src) >> 16;
> -	dst_w = drm_rect_width(&plane_state->uapi.dst);
> -	dst_h = drm_rect_height(&plane_state->uapi.dst);
> -
> -	fp_w_ratio = div_fixed16(src_w, dst_w);
> -	fp_h_ratio = div_fixed16(src_h, dst_h);
> -	downscale_w = max_fixed16(fp_w_ratio, u32_to_fixed16(1));
> -	downscale_h = max_fixed16(fp_h_ratio, u32_to_fixed16(1));
> -
> -	return mul_fixed16(downscale_w, downscale_h);
> -}
> -
>  struct dbuf_slice_conf_entry {
>  	u8 active_pipes;
>  	u8 dbuf_mask[I915_MAX_PIPES];
> @@ -4932,10 +4883,7 @@ skl_plane_relative_data_rate(const struct intel_crtc_state *crtc_state,
>  {
>  	struct intel_plane *plane = to_intel_plane(plane_state->uapi.plane);
>  	const struct drm_framebuffer *fb = plane_state->hw.fb;
> -	u32 data_rate;
> -	u32 width = 0, height = 0;
> -	uint_fixed_16_16_t down_scale_amount;
> -	u64 rate;
> +	int width, height;
>  
>  	if (!plane_state->uapi.visible)
>  		return 0;
> @@ -4961,14 +4909,7 @@ skl_plane_relative_data_rate(const struct intel_crtc_state *crtc_state,
>  		height /= 2;
>  	}
>  
> -	data_rate = width * height;
> -
> -	down_scale_amount = skl_plane_downscale_amount(crtc_state, plane_state);
> -
> -	rate = mul_round_up_u32_fixed16(data_rate, down_scale_amount);
> -
> -	rate *= fb->format->cpp[color_plane];
> -	return rate;
> +	return width * height * fb->format->cpp[color_plane];
>  }
>  
>  static u64
> -- 
> 2.32.0
>