Re: [PATCH] drm/i915: intel_backlight scale() math WA v2

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 09/24/2014 05:54 PM, Joe Konno wrote:
> From: Joe Konno <joe.konno@intel.com>
> 
> Improper truncated integer division 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
> 
> v2: (thanks to Chris Wilson) clarify commit message, use rounded division
> macro

I wonder why do scaling at all, why not simply shift hw_min - hw_max range
to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace
to (hw_max - hw_min) ?

Regards,

Hans


> 
> Signed-off-by: Joe Konno <joe.konno@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..dcdfbb3 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -421,7 +421,7 @@ 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 = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
>  	target_val += target_min;
>  
>  	return target_val;
>