linux-kernel - Re: [PATCH] i915: Quirk out disconnected backlight

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Message-ID: <CACxGe6szLVwsdNOvpB9v8pGDrygu+Go5Vf3bvpE19v3+Hkv4ww@mail.gmail.com>
Date:	Fri, 14 Sep 2012 14:24:36 +0100
From:	Grant Likely <grant.likely@...retlab.ca>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	David Airlie <airlied@...ux.ie>,
	Matthew Garrett <mjg@...hat.com>
Subject: Re: [PATCH] i915: Quirk out disconnected backlight

On Fri, Sep 14, 2012 at 2:12 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> On Fri, 2012-09-14 at 14:09 +0100, Grant Likely wrote:
>> On Fri, Sep 14, 2012 at 2:01 PM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
>> > On Fri, 14 Sep 2012 13:57:06 +0100, Grant Likely <grant.likely@...retlab.ca> wrote:
>> >> Some platforms (for instance MacbookPros) have custom backlight drivers
>> >> and don't use the integrated i915 backlight control. This patch adds a
>> >> quirk to disable registering the intel backlight when unused on a
>> >> platform.
>> >>
>> >> Tested on MacbookPro8,3. Without this patch both the intel_backlight and
>> >> gmux_backlight devices get registered and userspace doesn't know which
>> >> it should use.
>> >
>> > Userspace is informed throught the backlight/type property.
>>
>> Perhaps, but userspace (Ubuntu) isn't doing anything with it, and it
>> still remains that it makes no sense whatsoever to register a
>> backlight device that doesn't exist.
>
> Indeed. Userspace (well, gnome-settings-daemon) will use the backlight
> provided by X, in preference to anything it finds
> in /sys/class/backlight. So if the Intel one is present (and thus
> exposed via X) then userspace will never bother with comparing types and
> choosing the sanest backlight to use.
>
> See https://bugzilla.redhat.com/show_bug.cgi?id=752595

In that bug you mention that the intel backlight sets a bogus max of
'1' when a backlight isn't present. I saw that too here. Here's the
offending code:

u32 intel_panel_get_max_backlight(struct drm_device *dev)
{
        struct drm_i915_private *dev_priv = dev->dev_private;
        u32 max;

        max = i915_read_blc_pwm_ctl(dev_priv);
        if (max == 0) {
                /* XXX add code here to query mode clock or hardware clock
                 * and program max PWM appropriately.
                 */
                pr_warn_once("fixme: max PWM is zero\n");
                return 1;
        }

I used a quirk in my patch, but I could instead change the driver to
bail here instead of trying to limp along.

g.
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