linux-kernel - Re: [PATCH 2/3] tegra: pwm-backlight: add tegra pwm-bl driver

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Date:	Thu, 24 Jan 2013 15:10:08 +0900
From:	Alex Courbot <acourbot@...dia.com>
To:	Thierry Reding <thierry.reding@...onic-design.de>,
	Richard Purdie <rpurdie@...ys.net>
CC:	Stephen Warren <swarren@...dotorg.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Mark Zhang <markz@...dia.com>,
	"gnurou@...il.com" <gnurou@...il.com>
Subject: Re: [PATCH 2/3] tegra: pwm-backlight: add tegra pwm-bl driver

On Wednesday 23 January 2013 17:45:39 Alex Courbot wrote:
> > I'm confused. Why would you want to call into pwm_bl directly? If we're
> > going to split this up into separate platform devices, why not look up a
> > given backlight device and use the backlight API on that? The pieces of
> > the puzzle are all there: you can use of_find_backlight_by_node() to
> > obtain a backlight device from a device tree node, so I'd expect the DT
> > to look something like this:
> > 	backlight: backlight {
> > 	
> > 		compatible = "pwm-backlight";
> > 		...
> > 	
> > 	};
> 
> This would still prevent any power control from the backlight driver. I.e.
> if someone sets the brightness to 0 through sysfs, we cannot power the
> backlight off as pwm-backlight cannot control more than the PWM without
> platform callbacks. Backlight could only be powered off as a result of a fb
> blank event.

In order to solve this, how about adding a backlight notifier call chain to 
broadcast backlight events in a fashion similar to what is done in 
fbmem/fbcon? Then backlight_update_status() could send events like 
BACKLIGHT_EARLY_EVENT_UPDATE and BACKLIGHT_EVENT_UPDATE to which panel drivers 
could subscribe in order to power the backlight up and down as needed.

Then as the backlight is mentioned in the panel's DT node,

>       panel: panel {
>               compatible = "...";
>               ...
>               backlight = <&backlight>;
>               ...
>       };

the panel's driver could listen to backlight-related events and do its stuff 
transparently, without changing anything to the backlight drivers. This would 
be a good way to replace pwm-backlight's callbacks for platforms that use the 
DT, and would also be applicable to any backlight class device.

Generally speaking, having a mean to monitor backlights state in the kernel 
does not seem too crazy, especially since we already have a way to notify user 
space through backlight_generate_event().

Richard, does that sound ok to you?

Alex.

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