[Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA

[Joe Konno]

From: Joe Konno <joe.konno at intel.com>

Improper integer division-- truncated rather than rounded-- 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

Signed-off-by: Joe Konno <joe.konno at 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 += target_min;
 
 	return target_val;
-- 
2.1.0