Re: [PATCH 1/2 v3] drm/i915: intel_backlight scale() math WA

[Joe Konno <joe.konno@linux.intel.com>]

Reviewed-By: Joe Konno <joe.konno@intel.com>

On 09/29/2014 03:49 PM, U. Artie Eoff wrote:
> 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
>
> v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
>     -Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
>     -v1 and v2 originally authored by Joe Konno.
>
> Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f17ada3..f7da913 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *dev)
>  	}
>  }
>  
> +#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
> +({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
> +
>  /**
>   * scale - scale values from one range to another
>   *
> @@ -419,9 +422,8 @@ static uint32_t scale(uint32_t source_val,
>  	source_val = clamp(source_val, source_min, source_max);
>  
>  	/* 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_ULL((uint64_t)(source_val - source_min) *
> +			(target_max - target_min), source_max - source_min);
>  	target_val += target_min;
>  
>  	return target_val;