linux-kernel - Re: [PATCH] acpi: video: improve quirk check

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Date:	Sat, 03 Aug 2013 01:47:52 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Felipe Contreras <felipe.contreras@...il.com>,
	Aaron Lu <aaron.lwe@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	Len Brown <lenb@...nel.org>, Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH] acpi: video: improve quirk check

On Friday, August 02, 2013 02:37:09 PM Felipe Contreras wrote:
> 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.
> 
> This causes all _BQC calls to return bogus values causing weird behavior
> from the user's perspective. For example: xbacklight -set 10; xbacklight
> -set 20; would flash to 90% and then slowly down to the desired level
> (20).
> 
> The solution is simple; test anything other than the first level (e.g.
> 1).
> 
> Signed-off-by: Felipe Contreras <felipe.contreras@...il.com>

Looks reasonable.

Aaron, what do you think?

Rafael


> ---
> 
> On top of this we might want to test yet another value, because br->levels[3]
> might be the current value (although very unlikely).
> 
>  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)
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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