linux-kernel - Re: [PATCH] acpi: video: fix reversed indexed BQC

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Message-ID: <51FB6D0D.4060407@intel.com>
Date:	Fri, 02 Aug 2013 16:25:49 +0800
From:	Aaron Lu <aaron.lu@...el.com>
To:	Felipe Contreras <felipe.contreras@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>, Len Brown <lenb@...nel.org>,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH] acpi: video: fix reversed indexed BQC

On 08/02/2013 04:14 PM, Felipe Contreras wrote:
> On Fri, Aug 2, 2013 at 3:06 AM, Aaron Lu <aaron.lu@...el.com> wrote:
>> On 08/02/2013 03:59 PM, Felipe Contreras wrote:
>>> On Fri, Aug 2, 2013 at 1:56 AM, Aaron Lu <aaron.lu@...el.com> wrote:
>>>> On 08/02/2013 02:44 PM, Felipe Contreras wrote:
>>>
>>>>> The initial _BCM commands don't work, so the level remains at 100%.
>>>>> Since the level is max_level, acpi_video_bqc_quirk() tries with the
>>>>> first level, which is 0, and 0 happens to be the index of 100.
>>>>>
>>>>> So _BQC is returning 100, which is not the index of 0 (what we tested
>>>>> for), but actually 100.
>>>>>
>>>>> I think the current code is correct, but acpi_video_bqc_quirk() should
>>>>> be testing br->levels[3], or anything other than 0/100 which can be
>>>>> easily confused.
>>>>>
>>>>> If so, the code would find that _BQC doesn't work on this machine (in
>>>>> win8 mode)... at least initially. My guess is that it only starts to
>>>>> work after acpi_video_bus_start_devices() is called.
>>>>>
>>>>> Forcing br->flags._BQC_use_index = 0 seems to work.
>>>>
>>>> Seems ASUS machines tend to have this issue:
>>>> https://bugzilla.kernel.org/show_bug.cgi?id=52951
>>>> https://bugzilla.kernel.org/show_bug.cgi?id=56711
>>>
>>> I don't see any real solution for the ACPI driver.
>>>
>>>> I have a patch to enhance the quirk some time ago:
>>>> https://github.com/aaronlu/linux/commit/0a3d2c5b59caf80ae5bb1ca1fda0f7bf448b38c9
>>>
>>> I think this is unnecessarily complicated; the comment makes it clear
>>
>> For your system, yes it is unnecessarily complicated. But since this is
>> a quirk, it better solves as many potential problems as possible, or we
>> would simply use a DMI entry to do the quirk.
> 
> The only difference between my patch and yours is that your patch
> checks that br->level[i] is not the current level, but that check is
> not necessary. If _BQC always returns the max level, all we need to do

_BQC does not always returns the max level.

> is pick another value, any other value, and br->level[3] works just
> fine.

For a _BCL only having 4 elements { 100, 40, 40, 100 }, the br->levels[3]
will be the max level. The example here may be too crazy to be true, but
since we are dealing with firmware, I tend to believe anything could
happen.

-Aaron
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