linux-kernel - Re: i915 backlight

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Date:	Mon, 5 Aug 2013 09:34:10 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Aaron Lu <aaron.lu@...el.com>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	Jani Nikula <jani.nikula@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	lkml <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>
Subject: Re: i915 backlight

On Fri, Aug 02, 2013 at 09:16:03AM +0800, Aaron Lu wrote:
> Hi Jani & Daniel,
> 
> It turned out there is an integer overflow problem, and the below patch
> fixed this problem on Acer Aspire 4732Z and thinkpad R61i.
> 
> From: Aaron Lu <aaron.lu@...el.com>
> Subject: [PATCH] drm/i915: avoid brightness overflow when doing scale
> 
> Some card's max brightness level is pretty large, e.g. on Acer Aspire
> 4732Z, the max level is 989910. If user space set a large enough level
> then the current scale done in intel_panel_set_backlight will cause an
> integer overflow and the scaled level will be mistakenly small, leaving
> user with an almost black screen. This patch fixes this problem.
> 
> Signed-off-by: Aaron Lu <aaron.lu@...el.com>
> ---
> Since the only external user of intel_panel_set_backlight is operation
> region code where the max will be a constant of 255, this patch fixes
> the problem by comparing freq and max and then do things accordingly
> instead of converting to 64 bits.

Yeah, makes sense. Picked up for -fixes, thanks for the patch.
-Daniel
> 
>  drivers/gpu/drm/i915/intel_panel.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 67e2c1f..7c674f0 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -498,7 +498,10 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level, u32 max)
>  	}
>  
>  	/* scale to hardware */
> -	level = level * freq / max;
> +	if (freq < max)
> +		level = level * freq / max;
> +	else
> +		level = freq / max * level;
>  
>  	dev_priv->backlight.level = level;
>  	if (dev_priv->backlight.device)
> -- 
> 1.8.3.1
> 
> 
> Hi Boris,
> 
> Since the sysfs interface works on your system, I think your problem
> should be different. Can you please file a bug for this? I can provide
> you with a debug patch and then see what happened. Please attach
> acpidump when filing the bug.
> 
> https://bugzilla.kernel.org, ACPI/Power-Video.
> 
> Thanks,
> Aaron
> 
> > 
> >>
> >> The ranges are different, though:
> >>
> >> intel_backlight/actual_brightness:1000
> >> intel_backlight/bl_power:0
> >> intel_backlight/brightness:1000
> >> intel_backlight/max_brightness:4437
> >> intel_backlight/type:raw
> >>
> >> acpi_video0/actual_brightness:41
> >> acpi_video0/bl_power:0
> >> acpi_video0/brightness:41
> >> acpi_video0/max_brightness:100
> >> acpi_video0/type:firmware
> > 
> > Yes, different interface has different brightness ranges and a value in
> > one range may turn out to be the same actual brightness level of another
> > value in another range.
> > 
> >>
> >> I guess I need to write me a dirty script for now ... :-)
> > 
> > :-)
> > 
> >>
> >> Thanks guys.
> >>
> > Thanks,
> > Aaron
> > --
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> > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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