[PATCH 088/104] drm/i915: try not to lose backlight CBLV precision

[Luis Henriques]

3.5.7.22 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jani Nikula <jani.nikula at intel.com>

commit cac6a5ae0118832936eb162ec4cedb30f2422bcc upstream.

ACPI has _BCM and _BQC methods to set and query the backlight
brightness, respectively. The ACPI opregion has variables BCLP and CBLV
to hold the requested and current backlight brightness, respectively.

The BCLP variable has range 0..255 while the others have range
0..100. This means the _BCM method has to scale the brightness for BCLP,
and the gfx driver has to scale the requested value back for CBLV. If
the _BQC method uses the CBLV variable (apparently some implementations
do, some don't) for current backlight level reporting, there's room for
rounding errors.

Use DIV_ROUND_UP for scaling back to CBLV to get back to the same values
that were passed to _BCM, presuming the _BCM simply uses bclp = (in *
255) / 100 for scaling to BCLP.

Reference: https://gist.github.com/aaronlu/6314920
Reported-by: Aaron Lu <aaron.lu at intel.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
Reviewed-by: Aaron Lu <aaron.lu at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
[ luis: backported to 3.5: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/gpu/drm/i915/intel_opregion.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_opregion.c b/drivers/gpu/drm/i915/intel_opregion.c
index e27c170..6d708d7 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -163,7 +163,7 @@ static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
 
 	max = intel_panel_get_max_backlight(dev);
 	intel_panel_set_backlight(dev, bclp * max / 255);
-	iowrite32((bclp*0x64)/0xff | ASLE_CBLV_VALID, &asle->cblv);
+	iowrite32(DIV_ROUND_UP(bclp * 100, 255) | ASLE_CBLV_VALID, &asle->cblv);
 
 	return 0;
 }
-- 
1.8.3.2