[Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA v2

[Eoff, Ullysses A]

> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf Of Jani Nikula
> Sent: Monday, September 29, 2014 6:07 AM
> To: Joe Konno; intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA v2
> 
> On Wed, 24 Sep 2014, Joe Konno <joe.konno at linux.intel.com> wrote:
> > From: Joe Konno <joe.konno at intel.com>
> >
> > Improper truncated integer division in the scale() function causes
> > actual_brightness != brightness. This (partial) work-around should be
> > sufficient for a majority of use-cases, but it is by no means a complete
> > solution.
> >
> > TODO: Determine how best to scale "user" values to "hw" values, and
> > vice-versa, when the ranges are of different sizes. That would be a
> > buggy scenario even with this work-around.
> >
> > The issue was introduced in the following (v3.17-rc1) commit:
> >
> >     6dda730 drm/i915: respect the VBT minimum backlight brightness
> >
> > v2: (thanks to Chris Wilson) clarify commit message, use rounded division
> > macro
> >
> > Signed-off-by: Joe Konno <joe.konno at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_panel.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> > index f17ada3..dcdfbb3 100644
> > --- a/drivers/gpu/drm/i915/intel_panel.c
> > +++ b/drivers/gpu/drm/i915/intel_panel.c
> > @@ -421,7 +421,7 @@ static uint32_t scale(uint32_t source_val,
> >  	/* avoid overflows */
> >  	target_val = (uint64_t)(source_val - source_min) *
> >  		(target_max - target_min);
> > -	do_div(target_val, source_max - source_min);
> > +	target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min);
> 
> This fails to build with CONFIG_X86_32=y:
> 
> drivers/built-in.o: In function `scale':
> intel_panel.c:(.text+0x12ab38): undefined reference to `__udivdi3'
> make: *** [vmlinux] Error 1
> 
> 

Do you have a recommended workaround?  Should we just
use the v1 technique instead?

U. Artie

> BR,
> Jani.
> 
> 
> >  	target_val += target_min;
> >
> >  	return target_val;
> > --
> > 2.1.0
> >
> > _______________________________________________
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> > Intel-gfx at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> --
> Jani Nikula, Intel Open Source Technology Center
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