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

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Date:	Fri, 2 Aug 2013 03:14:02 -0500
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	Aaron Lu <aaron.lu@...el.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 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
is pick another value, any other value, and br->level[3] works just
fine.

-- 
Felipe Contreras
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