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

Chris Wilson
2014-09-24T06:04:07Z

On Tue, Sep 23, 2014 at 03:50:25PM -0700, Joe Konno wrote:
> From: Joe Konno <joe.konno@intel.com>
>
> Improper integer division-- truncated rather than rounded-- in the

That's confusing. It reads that you are converting to using truncated,
rather than to rounding.

> 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
>
> Signed-off-by: Joe Konno <joe.konno@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 = DIV_ROUND_CLOSEST(target_val, source_max - source_min);

> target_val += target_min;
>
> return target_val;

--
Chris Wilson, Intel Open Source Technology Centre