Re: [RFC CABC v3 PATCH 2/2] drm/i915: CABC support for backlight control

From: Daniel Vetter <daniel@ffwll.ch>
To: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Deepak M <m.deepak@intel.com>, intel-gfx@lists.freedesktop.org
Subject: Re: [RFC CABC v3 PATCH 2/2] drm/i915: CABC support for backlight control
Date: Mon, 14 Sep 2015 11:39:50 +0200	[thread overview]
Message-ID: <20150914093950.GZ3383@phenom.ffwll.local> (raw)
In-Reply-To: <87zj0tjiv1.fsf@intel.com>

On Fri, Sep 11, 2015 at 02:28:02PM +0300, Jani Nikula wrote:
> On Fri, 11 Sep 2015, Deepak M <m.deepak@intel.com> wrote:
> > In CABC (Content Adaptive Brightness Control) content grey level
> > scale can be increased while simultaneously decreasing
> > brightness of the backlight to achieve same perceived brightness.
> >
> > The CABC is not standardized and panel vendors are free to follow
> > their implementation. The CABC implementaion here assumes that the
> > panels use standard SW register for control.
> >
> > In this design there will be no PWM signal from the SoC and DCS
> > commands are sent to enable and control the backlight brightness.
> >
> > v2:
> > - Created a new backlight driver for cabc, which will be registered
> >   only when it cabc is supported by panel. (Daniel Vetter)
> 
> I don't know what Daniel's been telling you, but I think this does need
> to get bolted into the regular backlight control mechanism. We'll also
> need eDP specific backlight control, and there's the VLV/CVH pwm driver
> absed backlight control, so we need to cover this more gracefully than
> an if-else ladder anyway.
> 
> My idea all along has been that the backlight hooks in dev_priv.display
> need to be moved to connectors (probably intel_panel), and connector
> backlight setup can do what they want with these. All the boilerplate
> code for sysfs and scaling and so on would be there already.
> 
> I do not approve of the approach here.

The current design of backlights in linux is that userspace picks the
right backlight device. We've enabled/disabled the backlight pwm
ourselves too, but that was always just for the native backlight power
thing, not any of the others.

Imo this is the right approach since it fits into the current design.
Fixing the current design so that i915 can get at the right backlight
driver should imo be a separate series. I.e. yes this is what I
recommended roughly (but I didn't check details nor can I test with
xf86-video-intel to make sure it actually works correctly).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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