Re: [PATCH] drm/i915: Fix Limited Range Color Handling

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

On Fri, Dec 29, 2017 at 08:29:11PM +0530, Uma Shankar wrote:
> From: Johnson Lin <johnson.lin@intel.com>
> 
> Some panels support limited range output (16-235) compared
> to full range RGB values (0-255). Also userspace can control
> the RGB range using "Broadcast RGB" property. Currently the
> code to handle full range to limited range is broken. This
> patch fixes the same by properly scaling down all the full
> range co-efficients with limited range scaling factor.
> 
> v2: Fixed Ville's review comments.
> 
> Signed-off-by: Johnson Lin <johnson.lin@intel.com>
> Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_color.c |   29 +++++++++++++++--------------
>  1 file changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_color.c b/drivers/gpu/drm/i915/intel_color.c
> index aa66e95..55408e2 100644
> --- a/drivers/gpu/drm/i915/intel_color.c
> +++ b/drivers/gpu/drm/i915/intel_color.c
> @@ -84,26 +84,27 @@ static bool crtc_state_is_legacy_gamma(struct drm_crtc_state *state)
>  
>  /*
>   * When using limited range, multiply the matrix given by userspace by
> - * the matrix that we would use for the limited range. We do the
> - * multiplication in U2.30 format.
> + * the matrix that we would use for the limited range.
>   */
>  static void ctm_mult_by_limited(uint64_t *result, int64_t *input)

Since your touching pretty much every line anyway, maybe
s/uint64/u64/ etc. all over. 

'input' should really be const u64 *.

>  {
>  	int i;
>  
> -	for (i = 0; i < 9; i++)
> -		result[i] = 0;
> +	for (i = 0; i < 9; i++) {
> +		uint64_t user_coeff = input[i];
> +		uint32_t limited_coeff = CTM_COEFF_LIMITED_RANGE;
> +		uint32_t abs_coeff = (uint32_t)(clamp_val(

The cast is not needed.

Apart from those nits this lgtm.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> +						CTM_COEFF_ABS(user_coeff),
> +						0,
> +						CTM_COEFF_4_0 - 1) >> 2);
>  
> -	for (i = 0; i < 3; i++) {
> -		int64_t user_coeff = input[i * 3 + i];
> -		uint64_t limited_coeff = CTM_COEFF_LIMITED_RANGE >> 2;
> -		uint64_t abs_coeff = clamp_val(CTM_COEFF_ABS(user_coeff),
> -					       0,
> -					       CTM_COEFF_4_0 - 1) >> 2;
> -
> -		result[i * 3 + i] = (limited_coeff * abs_coeff) >> 27;
> -		if (CTM_COEFF_NEGATIVE(user_coeff))
> -			result[i * 3 + i] |= CTM_COEFF_SIGN;
> +		/*
> +		 * By scaling every co-efficient with limited range (16-235)
> +		 * vs full range (0-255) the final o/p will be scaled down to
> +		 * fit in the limited range supported by the panel.
> +		 */
> +		result[i] = mul_u32_u32(limited_coeff, abs_coeff) >> 30;
> +		result[i] |= user_coeff & CTM_COEFF_SIGN;
>  	}
>  }
>  
> -- 
> 1.7.9.5
> 
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-- 
Ville Syrjälä
Intel OTC
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