linux-kernel - Re: [PATCH 3/3] i915: Don't provide ACPI backlight interface if firmware expects Windows 8

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Date:	Fri, 14 Jun 2013 17:29:48 +0000
From:	Matthew Garrett <matthew.garrett@...ula.com>
To:	Aaron Lu <aaron.lu@...el.com>
CC:	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	"daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Zhang Rui <rui.zhang@...el.com>,
	Aaron Lu <aaron.lwe@...il.com>,
	Alex Deucher <alexdeucher@...il.com>
Subject: Re: [PATCH 3/3] i915: Don't provide ACPI backlight interface if
 firmware expects Windows 8

On Fri, 2013-06-14 at 14:47 +0800, Aaron Lu wrote:

> What about a priority based solution? We can introduce a new field named
> priority to backlight_device and instead of calling another module's
> function like the unregister one here(which cause unnecessary module
> dependency), we only need to boost priority for its own interface. This
> field will be exported to sysfs, so user can change it during runtime
> too. And we can also introduce a new kernel command line as
> backlight.force_interface=raw/firmware/platform, to overcome the limited
> functionality provided by acpi_backlight=video/vendor, which does not
> involve GPU's interface.

How would that work with existing userspace?

> And we can place the quirk code in backlight layer instead of individual
> backlight functionality provider module. Suppose we have a backlight
> manager there, for all win8 systems, we can boost the raw type's
> priority on its registration, so no need to add code in
> intel/amd/etc./'s GPU driver code.

But we'd need to add code to every piece of userspace that currently
uses the backlight, right?

> With priority based solution, all backlight control interfaces stay,
> the priority field is an indication given by kernel to user space.

We shouldn't export interfaces if we don't expect them to work.

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