[Intel-gfx] How to work around zero-initialized BLC_PWM_PCH_CTL2?

[Takashi Iwai]

Hi,

a new laptop model we've been struggling has some strange hardware
configuration.  BIOS turns off backlight and skips its initialization
when the machine is booted with the lid closed.  This leaves
BLC_PWM_PCH_CTL2 and other registers uninitialized.  Because a proper
max brightness value can't be obtained from this register, i915 driver
doesn't create the own backlight control any more.  It results in the
permanent blank screen even after the lid is opened.

Actually, the only missing piece is the initial BLC_PWM_PCH_CTL2
value.  If I overwrite it via intel_reg_write before loading i915
module, everything works fine.

Now I wonder whether we can get this max brightness value from
somewhere else.  Is it defined in VBT or anywhere else persistent?


thanks,

Takashi