[Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA v2

[Jani Nikula]

On Wed, 24 Sep 2014, Joe Konno <joe.konno at linux.intel.com> wrote:
> From: Joe Konno <joe.konno at 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
>
> Signed-off-by: Joe Konno <joe.konno at 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);

This fails to build with CONFIG_X86_32=y:

drivers/built-in.o: In function `scale':
intel_panel.c:(.text+0x12ab38): undefined reference to `__udivdi3'
make: *** [vmlinux] Error 1


BR,
Jani.


>  	target_val += target_min;
>  
>  	return target_val;
> -- 
> 2.1.0
>
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-- 
Jani Nikula, Intel Open Source Technology Center