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Subject: Re: [PATCH] acpi/video: lcd_get_level_current doen't return current level
From: Danny Baumann <dannybaumann@web.de>
Date: Sun, 31 Mar 2013 09:04:21 +0200
To: Artem Savkov <artem.savkov@gmail.com>
CC: aaron.lu@intel.com, rafael.j.wysocki@intel.com, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Hi,

Artem Savkov <artem.savkov@gmail.com> schrieb:

>On Sun, Mar 31, 2013 at 08:15:14AM +0200, Danny Baumann wrote:
>> Artem Savkov <artem.savkov@gmail.com> schrieb:
>> >acpi_video_device_lcd_get_level_current() called
>> >acpi_video_bqc_value_to_level()
>> >with "*level" as a second argument, resulting in level being
>returned
>> >based on
>> >initial input, not current brightness, breaking backlight controls.
>> I don't think this change is correct. As level was passed as
>parameter into the evaluation of _BQC, *level contains the AML returned
>brightness level afterwards, so it's correct to use it as an input to
>acpi_video_bqc_value_to_level(). Actually, the whole point of
>acpi_video_device_lcd_get_level_current() is to update
>device->brightness->curr, so it doesn't make sense to me to use it in
>that function.
>> 
>> What's the exact problem this patch tries to solve?
>
>I'm running a -next kernel on my laptop and couple of days ago keyboard
>backlight controls stopped working: only 2 lower brightness levels.
>I've
>debugged it a bit and found out that acpi_video_switch_brightness()
>calls
>acpi_video_device_lcd_get_level_current() with level uninitialized and
>always gets lowest posible value.

The point is: after the acpi_evaluate_object call, *level must contain the current brightness level, otherwise your BIOS is broken (this happens: e.g. my laptop always returns 100 from _BQC in Windows 8 mode). You can verify this easily by initializing level_current to some invalid value and checking it again after the call to _get_level_current. I'd be pretty surprised if the value didn't change. Also, if you look at [1], you'll see that the code operated on *level before as well.
This problem may have been obscured by the fact that the brightness device wasn't initialized properly before my patches went in. Does acpi_video_switch_brightness actually do anything for you when reverting the 3 newest commits of video.c?

Regards,

Danny

[1] https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/commit/drivers/acpi/video.c?h=linux-next&id=994fa63c5b126df6e9f31ef4e09000e2e243234b

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