Many thanks, Chris. I'll incorporate your feedback into v2. On 09/23/2014 11:04 PM, Chris Wilson wrote: > On Tue, Sep 23, 2014 at 03:50:25PM -0700, Joe Konno wrote: >> From: Joe Konno <joe.ko...@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.ko...@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;
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