From cac6a5ae0118832936eb162ec4cedb30f2422bcc Mon Sep 17 00:00:00 2001 From: Jani Nikula Date: Fri, 23 Aug 2013 10:50:39 +0300 Subject: [PATCH] drm/i915: try not to lose backlight CBLV precision 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 Signed-off-by: Jani Nikula Reviewed-by: Aaron Lu Cc: stable@vger.kernel.org Signed-off-by: Daniel Vetter --- 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 cfb8fb68f09c..119771ff46ab 100644 --- a/drivers/gpu/drm/i915/intel_opregion.c +++ b/drivers/gpu/drm/i915/intel_opregion.c @@ -173,7 +173,7 @@ static u32 asle_set_backlight(struct drm_device *dev, u32 bclp) return ASLE_BACKLIGHT_FAILED; intel_panel_set_backlight(dev, bclp, 255); - iowrite32((bclp*0x64)/0xff | ASLE_CBLV_VALID, &asle->cblv); + iowrite32(DIV_ROUND_UP(bclp * 100, 255) | ASLE_CBLV_VALID, &asle->cblv); return 0; } -- 2.20.1