[Intel-gfx] [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness

[Eoff, Ullysses A]

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> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf Of Eoff, Ullysses A
> Sent: Thursday, November 20, 2014 9:34 AM
> To: Nikula, Jani; intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness
> 
> > -----Original Message-----
> > From: Nikula, Jani
> > Sent: Thursday, November 20, 2014 3:59 AM
> > To: intel-gfx at lists.freedesktop.org
> > Cc: Nikula, Jani; Eoff, Ullysses A
> > Subject: [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness
> >
> > Due to scaling, the userspace and hardware brightness ranges might not
> > have a 1:1 mapping, causing the backlight class sysfs actual_brightness
> > not match the brightness attribute just written. While this is not a
> > strict requirement per Documentation/ABI/stable/sysfs-class-backlight,
> > try the userspace->hardware scaling for a match first and return the
> > cached value to not confuse userspace.
> >
> > The problem was already mitigated by
> >
> > commit 673e7bbdb3920b62cfc6c710bea626b0a9b0f43a
> > Author: U. Artie Eoff <ullysses.a.eoff at intel.com>
> > Date:   Mon Sep 29 15:49:32 2014 -0700
> >
> >     drm/i915: intel_backlight scale() math WA
> >
> > but this should be more robust for cases where the userspace expects
> > actual_brightness to match the just written brightness.
> >
> > Reference: http://mid.gmane.org/1415737838-9640-1-git-send-email-ullysses.a.eoff@intel.com
> > Reference: https://bugzilla.kernel.org/show_bug.cgi?id=85861
> > Cc: U. Artie Eoff <ullysses.a.eoff at intel.com>
> > Signed-off-by: Jani Nikula <jani.nikula at intel.com>
> 
> Tested-by: U. Artie Eoff <ullysses.a.eoff at intel.com>
> 
> > ---
> >  drivers/gpu/drm/i915/intel_panel.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> > index 4d63839bd9b4..675b56e105e7 100644
> > --- a/drivers/gpu/drm/i915/intel_panel.c
> > +++ b/drivers/gpu/drm/i915/intel_panel.c
> > @@ -1024,7 +1024,17 @@ static int intel_backlight_device_get_brightness(struct backlight_device *bd)
> >  	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> >
> >  	hw_level = intel_panel_get_backlight(connector);
> > -	ret = scale_hw_to_user(connector, hw_level, bd->props.max_brightness);
> > +
> > +	/*
> > +	 * Check the userspace->hardware scaling for a match first to avoid
> > +	 * scaling errors in the userspace->hardware->userspace round-trip.
> > +	 */
> > +	if (hw_level == scale_user_to_hw(connector, bd->props.brightness,
> > +					 bd->props.max_brightness))
> > +		ret = bd->props.brightness;
> > +	else
> > +		ret = scale_hw_to_user(connector, hw_level,
> > +				       bd->props.max_brightness);
> >
> >  	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> >  	intel_runtime_pm_put(dev_priv);
> > --
> > 2.1.3
> 
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