linux-kernel - Re: Strange effect with i915 backlight controller

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Date:	Sun, 13 Nov 2011 17:24:09 +0100
From:	Daniel Mack <zonque@...il.com>
To:	Takashi Iwai <tiwai@...e.de>
CC:	dri-devel@...ts.freedesktop.org, jbarnes@...tuousgeek.org,
	harald@...hat.com, Keith Packard <keithp@...thp.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Strange effect with i915 backlight controller

Hi Takashi,

On 11/10/2011 04:39 PM, Takashi Iwai wrote:
> At Thu, 10 Nov 2011 16:11:29 +0100,
> Daniel Mack wrote:
>>
>> On 11/08/2011 01:57 AM, Daniel Mack wrote:
>>> Didn't get any response yet, hence copying LKML for a broader audience.
>>
>> Nobody, really?
>>
>> This is a rather annoying regression, as touching the brightness keys
>> appearantly switches off the whole machine. I'm sure this is trivial to
>> fix, I just don't have the insight of this driver and the chipset.
> 
> I vaguely remember that the bit 0 is invalid on some old chips.
> Maybe 915GM is one of them, as it's gen3?  If so, the patch like below
> may work.

Thank you for looking into this.

> ---
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 499d4c0..be952d1 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -249,8 +249,11 @@ static void intel_panel_actually_set_backlight(struct drm_device *dev, u32 level
>  	if (IS_PINEVIEW(dev)) {
>  		tmp &= ~(BACKLIGHT_DUTY_CYCLE_MASK - 1);
>  		level <<= 1;
> -	} else
> +	} else {
>  		tmp &= ~BACKLIGHT_DUTY_CYCLE_MASK;
> +		if (INTEL_INFO(dev)->gen < 4)
> +			tmp &= ~1;
> +	}
>  	I915_WRITE(BLC_PWM_CTL, tmp | level);
>  }
>  

This seems to be the right intention, but the value you want to modify
under this condition is 'level', not 'tmp'. With this amendment of your
patch, things work perfectly fine here.

You can add my

  Reported-and-tested-by: Daniel Mack <zonque@...il.com>

and please Cc: stable so we get this to the users asap.


Thanks!

Daniel
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