[Intel-gfx] eDP display control registers in Linux kernel

[Michael Leuchtenburg]

On Thu, Feb 26, 2015 at 3:54 AM, Jani Nikula <jani.nikula at intel.com> wrote:
>
> On Thu, 26 Feb 2015, Michael Leuchtenburg <michael at slashhome.org> wrote:
> > Okay, here's the results:
> > 0000: 12 0a 02 41 00 00 00 00 00 00 00 00 00 0b 00
> > 0070: 01 00
> > 0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > 0100: 0a 02 00 09 09 00 00 00 01 00 00
> > 0200: 01 00 77 00 01 01 55 00
> > 0600: 01
> > 0700: 02
> > 0701: 88 40 00 00
> > 0720: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > 0732: 00 00
> >
> > If I read that right, it looks like it is dynamic backlight capable -
0x702
> > | (1 << 6).
>
> That's correct, your panel is dynamic backlight *capable*, but the
> dynamic backlight *enable* (bit 4 in 0x721) is not set. The power-on
> default value is also disabled, and the i915 driver does not touch it
> for sure.
>
> It is of course (I wish it wasn't "of course") possible the BIOS enables
> that in some circumstances; please try the dump again when you've
> observed the brightness swings for sure, and check for the DBC enable
> bit.

No changes, even while the brightness is in the process of changing. Is it
possible there's some other bit used on this hardware? The Broadwell chips
are pretty new. I haven't had a chance to look for specs yet.

>
> Also, are you sure the swings are content adaptive, and not ambient
> light adaptive? Have you tried locating the ambient light sensor on the
> laptop (possibly somewhere in the bezel around the display or keyboard)
> and covering or ensuring constant lighting of it?

There's no ambient light adaptation advertised for this laptop and people
have complained of its lack in Windows. That said, there's two points on
the bezel which could potentially contain a sensor. I think one is a camera
and the other is a microphone, but I tried covering them with a finger and
observed no change in the behavior. If there is such a sensor hidden in the
keyboard, it is *very* well hidden. Covering the entire keyboard doesn't
cause a change in brightness so I suspect it's not an ambient sensor.

Michael
On Thu, 26 Feb 2015, Michael Leuchtenburg <michael at slashhome.org> wrote:
> Okay, here's the results:
> 0000: 12 0a 02 41 00 00 00 00 00 00 00 00 00 0b 00
> 0070: 01 00
> 0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 0100: 0a 02 00 09 09 00 00 00 01 00 00
> 0200: 01 00 77 00 01 01 55 00
> 0600: 01
> 0700: 02
> 0701: 88 40 00 00
> 0720: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 0732: 00 00
>
> If I read that right, it looks like it is dynamic backlight capable -
0x702
> | (1 << 6).

That's correct, your panel is dynamic backlight *capable*, but the
dynamic backlight *enable* (bit 4 in 0x721) is not set. The power-on
default value is also disabled, and the i915 driver does not touch it
for sure.

It is of course (I wish it wasn't "of course") possible the BIOS enables
that in some circumstances; please try the dump again when you've
observed the brightness swings for sure, and check for the DBC enable
bit.

Also, are you sure the swings are content adaptive, and not ambient
light adaptive? Have you tried locating the ambient light sensor on the
laptop (possibly somewhere in the bezel around the display or keyboard)
and covering or ensuring constant lighting of it?


BR,
Jani.


--
Jani Nikula, Intel Open Source Technology Center
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