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

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Date:	Fri, 2 Aug 2013 01:44:51 -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 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
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.

-- 
Felipe Contreras
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