Laptop Panel brightness control

Matthew Garrett mjg59 at
Wed Jan 6 11:07:41 PST 2010

On Wed, Jan 06, 2010 at 02:04:48PM -0500, Adam Jackson wrote:

> In the glorious future, when the kernel grows DDC/CI support, we're
> going to end up with a DRM output property for backlight anyway.  And we
> definitely want to expose it as RANDR property, because backlight level
> is part of the session state for fast-user-switching.

Even with DDC/CI, it ought to be a backlight device so existing software 
works with it...

> But my major problem with exposing it as a backlight class device is the
> same problem I had with trying to use /proc/acpi/video/*/*/edid: The
> kernel doesn't give userspace enough information to know which backlight
> is bound to which output.

This is fixable. We just need to set the parent member to the 
appropriate output. The ACPI problem was that we don't have a good way 
of binding ACPI outputs to card outputs, which isn't an issue here.

> As with ACPI's EDID method, it's sort of academic, since there's not 
> many cases where it's ambiguous.  If you've only got one video card, 
> then the ACPI EDID methods are yours; if you've only got one 
> backlight, then the backlight is yours.  But I have no trouble 
> imagining (eg) a kiosk machine with two LVDS panels, and, then what.

We just need to populate things appropriately. When the DRM layer is 
handling things, there shouldn't be any ambiguity.

Matthew Garrett | mjg59 at

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