Re: [PATCH] drm/i915: don't warn if backlight unexpectedly enabled

[Scot Doyle <lkml14@scotdoyle.com>]

On Tue, 26 Aug 2014, Daniel Vetter wrote:
> On Thu, Aug 21, 2014 at 07:12:59AM +0000, Scot Doyle wrote:
>> When we enter intel_modeset_setup_hw_state during resume
>> - BLC_PWM_CPU_CTL2 == BLM_PWM_ENABLE
>> - the physical backlight is off
>
> Hm, this is actually interesting - we have some other evidence that the
> best way to shut off the backlight is actually to just set the pwm duty
> cycle to 0. Can you please check that this is the case for your system?

/sys/class/backlight/intel_backlight/brightness
0 -> backlight not visible
1 -> backlight visible
937 -> max backlight

Setting /sys/class/backlight/intel_backlight/brightness to 0 updates 
BLC_PWM_CPU_CTL, but BLC_PWM_CPU_CTL2 remains 0xe0000000.


> Maybe we just need to extend the check to look for !PWM_ENABLE ||
> duty_cycle == 0.

The following measurements hold true no matter the duty cycle before 
suspend:

When entering hsw_enable_pc8 during suspend
- the physical backlight is off
- BLC_PWM_CPU_CTL == 0x3a900000 (BACKLIGHT_DUTY_CYCLE_MASK == ffff)
- BLC_PWM_CPU_CTL2 == 0x60000000 (BLM_PWM_ENABLE)

When exiting hsw_disable_pc8 during resume
- the physical backlight is off
- BLC_PWM_CPU_CTL == 0x200
- BLC_PWM_CPU_CTL2 == 0x80000000 (BLM_PWM_ENABLE | BLM_TRANSCODER_EDP)

When entering pch_enable_backlight during resume
- the physical backlight is off
- BLC_PWM_CPU_CTL == 0x200
- BLC_PWM_CPU_CTL2 == 0x80000000 (BLM_PWM_ENABLE)

When exiting pch_enable_backlight during resume
- the physical backlight is off
- BLC_PWM_CPU_CTL == duty cycle prior to suspend
- BLC_PWM_CPU_CTL2 == 0xe0000000 (BLM_PWM_ENABLE | BLM_TRANSCODER_EDP)


So the BIOS is setting BLC_PWM_CPU_CTL=0x200 and BLC_PWM_CPU_CTL2=0x80000000 ?