[Intel-gfx] [PATCH 1/2 v3] drm/i915: intel_backlight scale() math WA

[Eoff, Ullysses A]

On Tue, 2014-09-30 at 10:04 +0200, Daniel Vetter wrote:
> On Mon, Sep 29, 2014 at 03:49:32PM -0700, U. Artie Eoff wrote:
> > 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
> > 
> > v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
> >     -Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
> >     -v1 and v2 originally authored by Joe Konno.
> > 
> > Signed-off-by: U. Artie Eoff <ullysses.a.eoff at intel.com>
> 
> Is there some bug report, internal jira, mailing list reference or similar
> about this?
> 
> Note that at least for OTC jira tasks we now want them to be added to
> commit message with e.g.
> 
> OTC-Jria: VIZ-4932
> 

Yes, the OTC-Jira task is: VIZ-4395.  I'll resubmit with amended commit
message.

> And I guess I should merge patch 2 before patch 1, right?

No, patch 1 before patch 2.

Thanks,
--U. Artie

> 
> Cheers, Daniel
> 
> > ---
> >  drivers/gpu/drm/i915/intel_panel.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> > index f17ada3..f7da913 100644
> > --- a/drivers/gpu/drm/i915/intel_panel.c
> > +++ b/drivers/gpu/drm/i915/intel_panel.c
> > @@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *dev)
> >  	}
> >  }
> >  
> > +#define DIV_ROUND_CLOSEST_ULL(ll, d)	\
> > +({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
> > +
> >  /**
> >   * scale - scale values from one range to another
> >   *
> > @@ -419,9 +422,8 @@ static uint32_t scale(uint32_t source_val,
> >  	source_val = clamp(source_val, source_min, source_max);
> >  
> >  	/* 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_ULL((uint64_t)(source_val - source_min) *
> > +			(target_max - target_min), source_max - source_min);
> >  	target_val += target_min;
> >  
> >  	return target_val;
> > -- 
> > 1.9.3
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>