[Intel-gfx] [PATCH] drm/i915: don't warn if backlight unexpectedly enabled

[Daniel Vetter]

On Tue, Aug 26, 2014 at 04:15:53PM +0000, Scot Doyle wrote:
> 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 ?

Indeed the bios seems to just but gunk into that register. And if we add
in all the knobs there's piles of them (you have semi-duplicated backlight
registers on hsw on the PCH), so I guess it doesn't make sense to combine
them all and warn if something goes awry, at least not in a -fixes patch.
So Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch> on your original
patch.

Jani can decide whether he wants to save this WARN_ON (imo it's useful to
have such sanity-checks) in -next by taking all the various bits and duty
cycles into account. But maybe just on the latest platforms, that still
should give is good coverage, but with a lot less fuss.

Thanks for tracking this all down.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch