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

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Message-ID: <51FB5826.80305@intel.com>
Date:	Fri, 02 Aug 2013 14:56:38 +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 02:44 PM, Felipe Contreras wrote:
> On Thu, Aug 1, 2013 at 11:59 PM, Aaron Lu <aaron.lu@...el.com> wrote:
>> On 08/02/2013 12:50 PM, Felipe Contreras wrote:
>>> On Thu, Aug 1, 2013 at 11:30 PM, Aaron Lu <aaron.lu@...el.com> wrote:
> 
>>>> Please see acpi_video_bqc_quirk, we set _BQC_use_index by revert the
>>>> level on a reversed _BCL, so we will need to revert level here too.
>>>
>>> I cannot parse that sentence, but nothing needs to change there; to
>>> find out if _BQC is using an index, we need to see if the returned
>>> value is the index of the level we are looking for, and to find that
>>> out we need the original list of levels, which can be found by
>>> reverting the already reverted list. If this wasn't the case there
>>
>> Yes, but instead of reverting the already reverted list, we revert the
>> returned value to get the correct index in the reverted list. But your
>> patch removes the revert, is it correct?
> 
> It removes the revert in acpi_video_bqc_value_to_level(), not
> acpi_video_bqc_quirk(). If I remove both reverts it doesn't work on

Of course I mean you remove the revert in acpi_video_bqc_value_to_level
as is shown in your patch.

> this machine, however, I think it works by accident.
> 
> 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 have a patch to enhance the quirk some time ago:
https://github.com/aaronlu/linux/commit/0a3d2c5b59caf80ae5bb1ca1fda0f7bf448b38c9
But at that time we have decided to remove acpi video interface if this
is a win8 system(all of these affected systems are win8), so I didn't
submit it. Can you please give it a test? Perhaps I can send it now.

-Aaron
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