On 08/23/2013 03:50 PM, Jani Nikula wrote: > 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@xxxxxxxxx> > Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx> Reviewed-by: Aaron Lu <aaron.lu@xxxxxxxxx> Thanks, Aaron > > --- > > All of https://gist.github.com/aaronlu/6314920 included here for > reference: > > A typical ASL code for the backlight control method _BCM with Intel > graphics card is as follows: > > Method (_BCM, 1, NotSerialized) > { > If (BRNC) > { > AINT (One, Arg0) > } > Else > { > ^^^LPCB.EC0.STBR () > } > } > _BCM method takes one param: the target brightness level in the range > of 0-100. The BRNC variable is set if bit2 of _DOS's param is set, > which we do for Win8 systems now, so AINT will be executed. > > And the simplified ASL code for AINT on backlight control is: > > Method (AINT, 2, Serialized) > { > If (LEqual (Arg0, One)) > { > Store (Divide (Multiply (Arg1, 0xFF), 0x64, ), BCLP) > Or (BCLP, 0x80000000, BCLP) > Store (0x02, ASLC) > } > } > > The ASLC/BCLP are variables declared in IGD operation region. BCLP is > used to store the target brightness level in the range of 0-255. Due to > the mismatch of the level range in _BCM and BCLP, a convert is done here > for BCLP. The setting of the ASLC variable will trigger interrupt of the > graphics card and the GPU driver will find out this is due to IGD operation > region and will handle the irq accordingly. In backlight case, it will > set backlight level in the GPU driver according to the value of BCLP. > > So the setting of backlight is actually done in GPU driver, even though > it is triggered through firmware's interface. A side note is, there are > ASL implementations that would trigger the SMI handler on backlight > control and that would also result in GPU driver's irq handler and GPU > driver will handle backlight setting then. > > So this is how to make use of IGD operation region to do the backlight > brightness level control. > > There is a problem related to _BQC though on some firmware implementation. > _BQC is a control method provided by firmware to tell which backlight > level the firmware thinks the device is in. The broken implementation is: > > Method (_BQC, 0, NotSerialized) // _BQC: Brightness Query Current > { > If (LGreaterEqual (MSOS (), OSW8)) > { > And (CBLV, 0x7FFFFFFF, Local0) > Return (Local0) > } > } > > CBLV is a variable in IGD operation region, used to represent the > current brightness level in the range of 0-100 and is updated by > GPU driver everytime it is asked to set the backlight level. > > Say user wants to set target level to 8, then 8 will be converted to > 20(8 * 255 / 100) for BCLP in AINT, then in GPU driver, 20 will be > converted again to 7(20 * 100 / 255) for CBLV, so _BQC will return 7 > afterwards though user actually sets 8 in _BCM. But this doesn't happen > for every level set through _BCM, for those values that do not lose > precisions during the conversion back and forth like 20 are not affected. > This needs to be remembered when enhancing the quirk logic of _BQC, > unless we can somehow fix the problem. > > Some firmware doesn't have this problem as they simply store the target > level user has requested in _BCM in a variable and then return that > variable in _BQC, but then we probably do not need to evaluate _BQC at > all since we also know what level the device should be in too in ACPI > video module. > > PS: The above example ASL code is taken from a ASUS NV5Z system. > --- > 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 cfb8fb6..119771f 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; > } > -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
