linux-kernel - Re: [PATCH] acpi/video: lcd_get_level_current doen't return current level

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Date:	Sun, 31 Mar 2013 12:16:00 +0400
From:	Artem Savkov <artem.savkov@...il.com>
To:	Danny Baumann <dannybaumann@....de>
Cc:	aaron.lu@...el.com, rafael.j.wysocki@...el.com,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] acpi/video: lcd_get_level_current doen't return current
 level

On Sun, Mar 31, 2013 at 09:04:21AM +0200, Danny Baumann wrote:
> Hi,
> 
> Artem Savkov <artem.savkov@...il.com> schrieb:
> 
> >On Sun, Mar 31, 2013 at 08:15:14AM +0200, Danny Baumann wrote:
> >> Artem Savkov <artem.savkov@...il.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?

Yes, you are right. Both my patch and the version before your patches
result in initial acpi_video_device_lcd_get_level_current call ending
with invalid level being returned and bqc disabled, so they both just
use *level = brightness->curr after that.

-- 
Regards,
Artem
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