[PATCH] drm/i915: restore backlight precision when converting from opregion

[Daniel Vetter]

On Mon, Apr 28, 2014 at 11:19:29AM +0800, Aaron Lu wrote:
> When we set backlight on behalf of ACPI opregion, we will convert the
> backlight value in the 0-255 range defined in opregion to the actual
> hardware level. Commit 22505b82a2 (drm/i915: avoid brightness overflow
> when doing scale) is meant to fix the overflow problem when doing the
> conversion, but it also caused a problem that the converted hardware
> level doesn't quite represent the intended value: say user wants maximum
> backlight level(255 in opregion's range), then we will calculate the
> actual hardware level to be: level = freq / max * level, where freq is
> the hardware's max backlight level(937 on an user's box), and max and
> level are all 255. The converted value should be 937 but the above
> calculation will yield 765.
> 
> To fix this issue, just use 64 bits to do the calculation to keep the
> precision and avoid overflow at the same time.
> 
> Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=72491
> Reported-and-tested-by: Nico Schottelius <nico-bugzilla.kernel.org at schottelius.org>
> Signed-off-by: Aaron Lu <aaron.lu at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index a953b081ee38..bdd2f24b7a6b 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -502,10 +502,7 @@ void intel_panel_set_backlight(struct intel_connector *connector, u32 level,
>  
>  	/* scale to hardware max, but be careful to not overflow */
>  	freq = panel->backlight.max;
> -	if (freq < max)
> -		level = level * freq / max;
> -	else
> -		level = freq / max * level;
> +	level = (u64)level * freq / max;

64bit divisions won't compile on 32bit. You need one of the DO_DIV macros,
or whatever they're called again. I pain, I know ;-)
-Daniel

>  
>  	panel->backlight.level = level;
>  	if (panel->backlight.device)
> -- 
> 1.9.0
> 

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