linux-kernel - Re: [PATCH] radeon: Expose backlight class device for legacy LVDS encoder

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Date:	Wed, 8 Sep 2010 18:03:20 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Alex Deucher <alexdeucher@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Michel Dänzer <michel@...nzer.net>,
	dri-devel@...ts.freedesktop.org,
	Michel Dänzer <daenzer@...are.com>
Subject: Re: [PATCH] radeon: Expose backlight class device for legacy LVDS
	encoder

On Wed, Sep 08, 2010 at 12:58:32PM -0400, Alex Deucher wrote:

> The only problem with this is that not all oems use the internal
> backlight controller; systems that don't need to use the acpi methods.

That's why we expose the backlight type. Userspace should use the acpi 
or platform mechanism when available, with this being a last-ditch 
fallback.

>  On atombios systems there is a bit in the
> ATOM_FIRMWARE_CAPABILITY_ACCESS struct in the FirmwareInfo data table
> to determine whether the backlight is controlled by the GPU or some
> external mechanism.  Combios may have something similar.  If the
> backlight is controlled via the GPU, it can be adjusting using the
> atom OutputControl and TransmitterControl control tables depending on
> the GPU family.  However, if the driver chooses to control the
> backlight itself, it needs to set the appropriate bit in the bios
> scratch regs to tell the firmware not to attempt to change the
> backlight itself.

If there's support for probing this more reliably then I'm all for that, 
but I'm not keen on taking over control if the BIOS has previous 
asserted it.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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