linux-kernel - Re: [PATCH] ACPI / video: check _DOD list when creating backlight device

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Message-ID: <54786294.5030105@intel.com>
Date:	Fri, 28 Nov 2014 19:55:00 +0800
From:	Aaron Lu <aaron.lu@...el.com>
To:	Brian Norris <computersforpeace@...il.com>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>, ntrrgc@...il.com,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI / video: check _DOD list when creating backlight
 device

On 11/28/2014 05:59 PM, Brian Norris wrote:
> On Thu, Oct 09, 2014 at 04:27:55PM +0800, Aaron Lu wrote:
>> On 10/01/2014 04:18 AM, Rafael J. Wysocki wrote:
>>> On Tuesday, September 30, 2014 02:10:17 PM Aaron Lu wrote:
>>>> The _DOD method lists which video output device is currently attached so
>>>> we should only care about them and ignore others. An user recently
>>>> reported that there are two acpi_video interfaces appeared on his system
>>>> and one of them doesn't work. From the acpidump, it is found that there
>>>> are more than one video output devices that have _BCM control method but
>>>> the _DOD lists only one of them. So this patch checks if the video output
>>>> device is in the _DOD list and will not create backlight device if it is
>>>> not in the list. Also, we consider the broken _DOD case(reflected by the
>>>> video->attached_count is 0) and do not change behaviour for those broken
>>>> _DOD systems.
>>>>
>>>> Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=84111
>>>> Reported-and-tested-by: ntrrgc@...il.com
>>>> Signed-off-by: Aaron Lu <aaron.lu@...el.com>
>>>
>>> This looks reasonable to me, but I'm a little afraid that it may break
>>> systems which forget to list valid interfaces in the _DOD.
>>
>> Indeed, this is possible.
> 
> And indeed, there is a regression! My Dell Latituded E6410's backlight
> control no longer works after this commit, and I get messages like this
> instead:
> 
> [   57.214610] ACPI: Failed to switch the brightness
> 
> If I revert this commit, my backlight controls work again. Also, I
> regain a cooling device (?) that was being ignored:
> 
> [    1.332682] acpi device:02: registered as cooling_device0
> 
> Do you need any additional info to handle the regression, or should we
> just revert the patch?

Please attach acpidump, dmesg with video.dyndbg="module video +pft" in
kernel cmdline, list the /sys/class/backlight with and without this
commit, thanks.

-Aaron

> 
> Brian
> 

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