Re: [PATCH 1/5] drm/rect: Round above 1 << 16 upwards to correct scale calculation functions.

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

On Thu, May 03, 2018 at 01:22:13PM +0200, Maarten Lankhorst wrote:
> When calculating limits we want to be as pessimistic as possible,
> so we have to explicitly say whether we want to round up or down
> to accurately calculate whether we are below min_scale or above
> max_scale.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> ---
>  drivers/gpu/drm/drm_rect.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_rect.c b/drivers/gpu/drm/drm_rect.c
> index a3783ecea297..4735526297aa 100644
> --- a/drivers/gpu/drm/drm_rect.c
> +++ b/drivers/gpu/drm/drm_rect.c
> @@ -106,7 +106,10 @@ static int drm_calc_scale(int src, int dst)
>  	if (dst == 0)
>  		return 0;
>  
> -	scale = src / dst;
> +	if (src > (dst << 16))
> +		return DIV_ROUND_UP(src, dst);
> +	else
> +		scale = src / dst;
>  
>  	return scale;
>  }
> @@ -121,6 +124,9 @@ static int drm_calc_scale(int src, int dst)
>   * Calculate the horizontal scaling factor as
>   * (@src width) / (@dst width).
>   *
> + * If the scale is below 1 << 16, round down, if above up. This will

The "if above up" part doesn't read all that well to me. Maybe write it
out fully "If the scale is above 1 << 16, round up"?

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> + * calculate the scale with the most pessimistic limit calculation.
> + *
>   * RETURNS:
>   * The horizontal scaling factor, or errno of out of limits.
>   */
> @@ -152,6 +158,9 @@ EXPORT_SYMBOL(drm_rect_calc_hscale);
>   * Calculate the vertical scaling factor as
>   * (@src height) / (@dst height).
>   *
> + * If the scale is below 1 << 16, round down, if above up. This will
> + * calculate the scale with the most pessimistic limit calculation.
> + *
>   * RETURNS:
>   * The vertical scaling factor, or errno of out of limits.
>   */
> @@ -189,6 +198,9 @@ EXPORT_SYMBOL(drm_rect_calc_vscale);
>   * If the calculated scaling factor is above @max_vscale,
>   * decrease the height of rectangle @src to compensate.
>   *
> + * If the scale is below 1 << 16, round down, if above up. This will
> + * calculate the scale with the most pessimistic limit calculation.
> + *
>   * RETURNS:
>   * The horizontal scaling factor.
>   */
> @@ -239,6 +251,9 @@ EXPORT_SYMBOL(drm_rect_calc_hscale_relaxed);
>   * If the calculated scaling factor is above @max_vscale,
>   * decrease the height of rectangle @src to compensate.
>   *
> + * If the scale is below 1 << 16, round down, if above up. This will
> + * calculate the scale with the most pessimistic limit calculation.
> + *
>   * RETURNS:
>   * The vertical scaling factor.
>   */
> -- 
> 2.17.0

-- 
Ville Syrjälä
Intel
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