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

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Date:	Fri, 2 Aug 2013 02:59:22 -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 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
that the purpose is to find out if _BQC is working, and this does the
trick:

>From 2bfa401b0a50fcde292ac0eb60cb6f857caf2fc6 Mon Sep 17 00:00:00 2001
From: Felipe Contreras <felipe.contreras@...il.com>
Date: Fri, 2 Aug 2013 02:27:44 -0500
Subject: [PATCH] acpi: video: improve quirk check

If the _BCL package is descending, the first level (br->levels[2]) will
be 0, and if the number of levels matches the number of steps, we might
confuse a returned level to mean the index.

For example:

  current_level = max_level = 100
  test_level = 0
  returned level = 100

In this case 100 means the level, not the index, and _BCM failed. But if
the _BCL package is descending, the index of level 0 is also 100, so we
assume _BQC is indexed, when it's not.

The solution is simple; test anything other than the first level (e.g.
1).

Signed-off-by: Felipe Contreras <felipe.contreras@...il.com>
---
 drivers/acpi/video.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index 0ec434d..e1284b8 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -689,7 +689,7 @@ static int acpi_video_bqc_quirk(struct
acpi_video_device *device,
 	 * Some systems always report current brightness level as maximum
 	 * through _BQC, we need to test another value for them.
 	 */
-	test_level = current_level == max_level ? br->levels[2] : max_level;
+	test_level = current_level == max_level ? br->levels[3] : max_level;

 	result = acpi_video_device_lcd_set_level(device, test_level);
 	if (result)
-- 
1.8.3.4

-- 
Felipe Contreras
--
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