linux-kernel - Re: rc6+ regression - backlight reset to 0 on boot after 7c0ea45be4f114d85ee35caeead8e1660699c46f

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Date:	Thu, 03 Apr 2008 09:59:39 +0800
From:	Zhao Yakui <yakui.zhao@...el.com>
To:	Andrey Borzenkov <arvidjaar@...l.ru>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: rc6+ regression - backlight reset to 0 on boot after
	7c0ea45be4f114d85ee35caeead8e1660699c46f

On Wed, 2008-04-02 at 22:53 +0400, Andrey Borzenkov wrote:
> Commit 7c0ea45be4f114d85ee35caeead8e1660699c46f registers ACPI backlight
> even if _BQC method (query current backlight level) is missing. The effect is:
> 
> during initialization video.c:acpi_video_device_find_cap() calls backlight_update_status(). It tries to fetch actual level via acpi_video_get_brightness() - but as _BQC is missing it just returns current value
> as stored in memory - i.e. zero, because it was never set. So effectively
> backlight_update_status() reset brightness to minimal value == 0.
> 
> This happens on Toshiba Portege 4000. Verified by reverting commit on top of
> rc8.
It seems that _BQC object is missing on the Toshiba Portege 4000. And
acpi video driver will continue to update the status of backlight even
when _BQC object is missing, which is inappropriate. 
Maybe it is more appropriate that OS doesn't update the status of
backlight in boot phase when _BQC object is missing.

Of course please attach the output of acpidump in kernel bugzilla.
http://bugzilla.kernel.org/show_bug.cgi?id=10387
> On a side note, it would be nice to fit backlight device into actual ACPI
> device tree instead of /devices/virtual.

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