Re: [RFC PATCH] pwm: TLC591xx PWM driver

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Aug 18, 2015 at 05:09:16PM +0200, Andrew Lunn wrote:
> > On the other hand, there's pwm_bl.c which give us backlight device
> > with PWM,
> 
> Lets look at this. A backlight device seems to do most of its work in
> the update_status callback. It is given a brightness in
> bl->props.brightness, which takes a value between 0 and
> props.max_brightness.
> 
> What pwm_bl.c does it then turn this brightness value into an
> artificial PWM configuration. Your proposed PWM driver then turns this
> back into a brightness, since you don't actually implement the period
> part of the PWM interface.
> 
> From an architecture point of view, doesn't an LED class device, which
> takes a brightness value, seem much more naturally?
> 
> It seems like implementing a generic led_bl.c driver would make sense.
> It would also allow some of the code in drivers/video/backlight/ to be
> eliminated. There seems to be both an LED class driver for lp55xx and
> a blacklight driver lp855x_bl.c. There are duplicate lp8788, 88pm860x,
> adp5520, da903x, da9052, hp6xx, and lm3533 drivers which might all be
> removed if a led_bl.c generic driver existed.
> 
> > and a GPIO over PWM sounds more sane to me than GPIO over LED.
> 
> Currently two LED class drivers are calling gpiochip_add:
> 
> ~/linux/drivers/leds$ grep gpiochip_add *.c
> leds-pca9532.c:	      	   err = gpiochip_add(&data->gpio);
> leds-tca6507.c:		   err = gpiochip_add(&tca->gpio);
> 
> The pca9532 has full GPIO capabilities, in as well as out. But it
> seems like tca6507 is output only. The TLC59108/TLC59116 is also
> output only. So a generic GPO driver on top of LED would make sense
> for these two, and save some code/bugs.
> 
> From a stand back, lets take a look at the architecture point of view,
> generic led_bl and gpio-led drivers seem to make sense.

I agree it's sensible for there to be a driver that can drive a
backlight from an LED. PWM is somewhat of a lower level API than the LED
API, so there are cases where LED fits better than PWM. This particular
case seems to be one of them.

Thierry

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