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;
--
1.9.3
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