Re: [PATCH] acpi: video: fix reversed indexed BQC

[Aaron Lu <aaron.lu@intel.com>]

On 08/02/2013 03:59 PM, Felipe Contreras wrote:
> On Fri, Aug 2, 2013 at 1:56 AM, Aaron Lu <aaron.lu@intel.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.

-Aaron

> 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@gmail.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@gmail.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)
>