Re: [PATCH] drm/i915: intel_backlight scale() math WA v2

From: "Eoff, Ullysses A" <ullysses.a.eoff@intel.com>
To: Jani Nikula <jani.nikula@linux.intel.com>,
	Hans de Goede <hdegoede@redhat.com>,
	Joe Konno <joe.konno@linux.intel.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH] drm/i915: intel_backlight scale() math WA v2
Date: Wed, 24 Sep 2014 17:41:53 +0000	[thread overview]
Message-ID: <8B5BDC21C48FAF4CA5D7E3FE1A63A9087085B853@ORSMSX111.amr.corp.intel.com> (raw)
In-Reply-To: <87d2al6j6n.fsf@intel.com>

> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of Jani Nikula
> Sent: Wednesday, September 24, 2014 10:08 AM
> To: Hans de Goede; Joe Konno; intel-gfx@lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: intel_backlight scale() math WA v2
> 
> On Wed, 24 Sep 2014, Hans de Goede <hdegoede@redhat.com> wrote:
> > Hi,
> >
> > On 09/24/2014 05:54 PM, Joe Konno wrote:
> >> From: Joe Konno <joe.konno@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
> >
> > I wonder why do scaling at all, why not simply shift hw_min - hw_max range
> > to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace
> > to (hw_max - hw_min) ?
> 
> Mostly in preparation for a future where we expose an arbitrary range,
> say 0..100 or 0..255 to the userspace.
> 

The problem with this scaling method is that scaling from user level to hw level and
back to user level is ambiguous since there isn't a 1:1 mapping between the user
range and the hw range.

On the other hand, this patch does fix a bug in the currently used method (scaling).
That, at least, is an improvement nonetheless.

U. Artie

> BR,
> Jani.
> 
> 
> >
> > Regards,
> >
> > Hans
> >
> >
> >>
> >> Signed-off-by: Joe Konno <joe.konno@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);
> >>  	target_val += target_min;
> >>
> >>  	return target_val;
> >>
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> --
> Jani Nikula, Intel Open Source Technology Center
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