Intel backlight info

[Jesse Barnes]

Seems there have been a few bugs and questions about the backlight support in 
the latest Intel driver, so I figured it would be a good idea to send a note 
out to the list describing how things work these days.

Intel chips natively support a couple of different backlight control methods:  
the backlight duty cycle field of BLC_PWM_CTL[2] and optionally the "legacy" 
backlight control register, or LBB, located in PCI config space.  If the LBB 
is in use, it's supposed to determine the backlight brightness in conjunction 
with the backlight duty cycle.

System firmware code is responsible for setting the BLC_PWM_CTL[2] legacy bit 
if the platform has the LBB register wired up in addition to the duty cycle 
field.  Some get this right and things work as expected.  However in many 
cases there are a few additional wrinkles.

Some machines include an external backlight controller.  This controller may 
be wired up to BLC_PWM_CTL or LBB, both or neither.  In the latter case, the 
current Intel driver has no way of directly manipulating the backlight level 
unless your kernel happens to provide a driver that can talk to said 
backlight controller.

And then there are hotkeys.  In many cases, the hotkeys on your laptop will 
adjust the BLC_PWM_CTL and/or LBB values directly, without notifying the 
driver.  In the past, this meant the driver could get confused about the 
current level and you'd end up seeing "jumps" in backlight values when you 
used the hotkeys alongside xbacklight or a desktop utility for example.  
The "get property" stuff I pushed last week was meant to address this, since 
it allows xbacklight & xrandr to query the current value directly rather than 
returning the last value sent to the server.  And then again, your machine 
may have an external controller that the hotkeys control, which again means 
no software backlight control unless you have a kernel driver.

So, the bottom line is that you should be using a kernel driver if at all 
possible.  However, given the design of the /sys/class/backlight interface 
and bugs in old kernels, we can't just open /sys/class/backlight, walk the 
directory, and pick an interface.  So we have a list in the driver if 
supported interfaces, which at this point includes ACPI, thinkpad, and 
fujitsu.  If your machine has a /sys/class/backlight interface available that 
we don't support, please file a bug and we'll test it and add it to the list.

If you don't have a kernel driver for your platform and are seeing issues with 
the backlight brightness jumping around, please try the attached patch.  I'm 
hoping it'll provide a better default for machines that use both the 
BLC_PWM_CTL & LBB regs, but I'm worried about pushing it too quickly since it 
seems that every machine I fix the backlight control for breaks at least one 
other one...

Thanks,
Jesse
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