[Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA

[Eoff, Ullysses A]

On Tue, 2014-09-23 at 15:50 -0700, Joe Konno wrote:
> From: Joe Konno <joe.konno at intel.com>
> 
> Improper integer division-- truncated rather than rounded-- in the
> scale() function causes actual_brightness != brightness. This (partial)
> work-around should be sufficient for a majority of use-cases, but it is
> by no means a complete solution.
> 
> TODO: Determine how best to scale "user" values to "hw" values, and
> vice-versa, when the ranges are of different sizes. That would be a
> buggy scenario even with this work-around.
> 
> The issue was introduced in the following (v3.17-rc1) commit:
> 
>     6dda730 drm/i915: respect the VBT minimum backlight brightness
> 

Reviewed-by: U. Artie Eoff <ullysses.a.eoff at intel.com>

> Signed-off-by: Joe Konno <joe.konno at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..c40f837 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val,
>  	/* avoid overflows */
>  	target_val = (uint64_t)(source_val - source_min) *
>  		(target_max - target_min);
> -	do_div(target_val, source_max - source_min);
> +	target_val = (target_val + ((source_max - source_min) / 2)) /
> +		(source_max - source_min);
>  	target_val += target_min;
>  
>  	return target_val;