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

[Scot Doyle]

On Tue, 26 Aug 2014, Daniel Vetter wrote:
> On Tue, Aug 26, 2014 at 07:33:25PM +0200, Daniel Vetter wrote:
>> 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.
>
> Out of curiosity: What are the PCH copies of the backlight registers doing
> after resume?

Thanks Daniel, here's the info:

When entering hsw_enable_pc8 during suspend
- the physical backlight is off
- BLC_PWM_CPU_CTL  == 0x 3a90000 (BACKLIGHT_DUTY_CYCLE_MASK == ffff)
- BLC_PWM_CPU_CTL2 == 0x60000000
- BLC_PWM_PCH_CTL  == 0x       0
- BLC_PWM_PCH_CTL2 == 0x 3a90000

When exiting hsw_disable_pc8 during resume
- the physical backlight is off
- BLC_PWM_CPU_CTL  == 0x     200
- BLC_PWM_CPU_CTL2 == 0x80000000 (BLM_PWM_ENABLE)
- BLC_PWM_PCH_CTL  == 0x80000000
- BLC_PWM_PCH_CTL2 == 0x 4000000

When entering pch_enable_backlight during resume
- the physical backlight is off
- BLC_PWM_CPU_CTL  == 0x     200
- BLC_PWM_CPU_CTL2 == 0x80000000
- BLC_PWM_PCH_CTL  == 0x80000000
- BLC_PWM_PCH_CTL2 == 0x 4000000

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)
- BLC_PWM_PCH_CTL  == 0x80000000
- BLC_PWM_PCH_CTL2 == 0x 3a90000