Ah snap... I forgot the v4 in the subject line :-/
Nonetheless, this is it.
On Tue, 2014-09-30 at 08:14 -0700, 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
>
> OTC-Jira: VIZ-4395
>
> 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.
>
> v4: Add reference to issue report in commit message.
>
> Signed-off-by: U. Artie Eoff <ullysses.a.eoff@xxxxxxxxx>
> ---
> 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;
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