| Message ID | 1412090076-4873-1-git-send-email-ullysses.a.eoff@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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@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;
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;
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@intel.com> --- drivers/gpu/drm/i915/intel_panel.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)