Re: [PATCH v4 7/8] drm/i915/skl+: change WM calc to fixed point 16.16

[Paulo Zanoni <paulo.r.zanoni@intel.com>]

Em Qui, 2016-10-13 às 16:28 +0530, Kumar, Mahesh escreveu:
> From: Mahesh Kumar <mahesh1.kumar@intel.com>
> 
> This patch changes Watermak calculation to fixed point calculation.
> Problem with current calculation is during plane_blocks_per_line
> calculation we divide intermediate blocks with min_scanlines and
> takes floor of the result because of integer operation.
> hence we end-up assigning less blocks than required. Which leads to
> flickers.
> 

There are still variables that got auto-converted to 16.16 and need to
be adjusted because later they are mixed with non-16.16 in non-safe
ways. The fact that's it's hard to identify these things really worries
me.


> Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_pm.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c
> b/drivers/gpu/drm/i915/intel_pm.c
> index 0eaaadc..4263212 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -3527,16 +3527,19 @@ static uint32_t skl_pipe_pixel_rate(const
> struct intel_crtc_state *config)
>   * for the read latency) and cpp should always be <= 8, so that
>   * should allow pixel_rate up to ~2 GHz which seems sufficient since
> max
>   * 2xcdclk is 1350 MHz and the pixel rate should never exceed that.
> + * Both Method1 & Method2 returns fixedpoint 16.16 output
>  */
>  static uint32_t skl_wm_method1(uint32_t pixel_rate, uint8_t cpp,
> uint32_t latency)
>  {
> -	uint32_t wm_intermediate_val, ret;
> +	uint64_t wm_intermediate_val;
> +	uint32_t ret;
>  
>  	if (latency == 0)
>  		return UINT_MAX;
>  
> -	wm_intermediate_val = latency * pixel_rate * cpp / 512;
> -	ret = DIV_ROUND_UP(wm_intermediate_val, 1000);
> +	wm_intermediate_val = latency * pixel_rate * cpp;
> +	wm_intermediate_val <<= 16;
> +	ret = DIV_ROUND_UP_ULL(wm_intermediate_val, 1000 * 512);
>  
>  	return ret;
>  }
> @@ -3658,12 +3661,15 @@ static int skl_compute_plane_wm(const struct
> drm_i915_private *dev_priv,
>  	if (y_tiled) {
>  		plane_blocks_per_line =
>  		      DIV_ROUND_UP(plane_bytes_per_line *
> y_min_scanlines, 512);
> -		plane_blocks_per_line /= y_min_scanlines;
> +		plane_blocks_per_line = (plane_blocks_per_line <<
> 16) /
> +								y_mi
> n_scanlines;
>  	} else if (x_tiled) {
>  		plane_blocks_per_line =
> DIV_ROUND_UP(plane_bytes_per_line, 512);
> +		plane_blocks_per_line <<= 16;
>  	} else {
>  		plane_blocks_per_line =
> DIV_ROUND_UP(plane_bytes_per_line, 512)
>  					+ 1;
> +		plane_blocks_per_line <<= 16;
>  	}
>  
>  	method1 = skl_wm_method1(plane_pixel_rate, cpp, latency);
> @@ -3690,7 +3696,7 @@ static int skl_compute_plane_wm(const struct
> drm_i915_private *dev_priv,
>  			selected_result = method1;
>  	}
>  
> -	res_blocks = selected_result + 1;
> +	res_blocks = DIV_ROUND_UP(selected_result, 1 << 16) + 1;
>  	res_lines = DIV_ROUND_UP(selected_result,
> plane_blocks_per_line);
>  
>  	if (level >= 1 && level <= 7) {
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