[ 3.8.y.z extended stable ] Patch "drm/i915: try not to lose backlight CBLV precision" has been added to staging queue

Kamal Mostafa kamal at canonical.com
Fri Sep 20 00:36:27 UTC 2013

This is a note to let you know that I have just added a patch titled

    drm/i915: try not to lose backlight CBLV precision

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:


This patch is scheduled to be released in version

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see



>From e8a806768f5d8f38a2bb8c06f1531ae835a56ff9 Mon Sep 17 00:00:00 2001
From: Jani Nikula <jani.nikula at intel.com>
Date: Fri, 23 Aug 2013 10:50:39 +0300
Subject: drm/i915: try not to lose backlight CBLV precision

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>
[ kamal: backport to 3.8 (context) ]
Signed-off-by: Kamal Mostafa <kamal 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 7741c22..494fbbf 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -165,7 +165,7 @@ 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;

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