Comment 11 for bug 397617

(In reply to comment #10)

> Agreed, we don't want to expose multiple methods (especially native register
> banging methods) in the KMS case.
Yes. It is reasonable that only one backlight method is exported in KMS mode. The ACPI backlight I/F can be the primiary control method. But which control method can be exposed when there is no ACPI backlight I/F?
If no control method is exposed to xrandr tool, the user will complain that backlight brightness can't be controlled.
> > (In reply to comment #8)
> > > How about exposing the ACPI backlight I/F to userland in the
> > > drmmode_dislay.c when the KMS mode is used? It means that the ACPI backlight
> > > I/F is hook up in 2D driver when it exists. Only when there is no ACPI I/F,
> > > other backlight control mode is exposed to xrandr. (For example:
> > > legacy/combo/native).
> > >
> > ...
> I don't think anyone wants to do direct ACPI access from drmmode_display.c;
> going through /sys/class/backlight is the best approach.
What I said is that the ACPI backlight I/F is hook up in drmmode_display.c. In such case when backlight brightness is changed through xrandr tool, the /sys/class/backlight is used.
In fact in UMS mode when the brightness is changed through xrandr tool, the /sys/class/backlight is hook up in the 2D driver.

> No this sounds very similar to what I'm proposing. In we should have either
> /sys/class/backlight/acpi_video0
> or
> /sys/class/backlight/platform_foo (e.g. toshiba or sony specific driver)
> or
> /sys/class/backlight/i915 (for legacy i2c case)
> As Matthew pointed out we'll need to make sure to prefer the ACPI or platform
> driver (probably in platform, ACPI, i915 order) to interact properly with the
> firmware.
Very good idea.
It seems that a new backlight interface should be registered in i915 driver. Should this also be registered in UMS mode?
Another issue is whether the /sys/class/backlight/ should be hook up by xrandr tool. If it should be hook up, which should be selected when there exists multiple interfaces? For example: acpi_video0, i915

   Thanks.