On Mon, Sep 23, 2013 at 10:27:06PM +0200, Sylwester Nawrocki wrote:
On 09/23/2013 06:37 PM, Oliver Schinagl wrote:
On 09/23/13 16:45, Sylwester Nawrocki wrote:
Hi,
I would like to have a short discussion on LED flash devices support in the kernel. Currently there are two APIs: the V4L2 and LED class API exposed by the kernel, which I believe is not good from user space POV. Generic applications will need to implement both APIs. I think we should decide whether to extend the led class API to add support for more advanced LED controllers there or continue to use the both APIs with overlapping functionality. There has been some discussion about this on the ML, but without any consensus reached [1].
What about the linux-pwm framework and its support for the backlight via dts?
Or am I talking way to uninformed here. Copying backlight to flashlight with some minor modification sounds sensible in a way...
I'd assume we don't need yet another user interface for the LEDs ;) AFAICS the PWM subsystem exposes pretty much raw interface in sysfs. The PWM LED controllers are already handled in the leds-class API, there is the leds_pwm driver (drivers/leds/leds-pwm.c).
I'm adding linux-pwm and linux-leds maintainers at Cc so someone may correct me if I got anything wrong.
The PWM subsystem is most definitely not a good fit for this. The only thing it provides is a way for other drivers to access a PWM device and use it for some specific purpose (pwm-backlight, leds-pwm).
The sysfs support is a convenience for people that needs to use a PWM in a way for which no driver framework exists, or for which it doesn't make sense to write a driver. Or for testing.
Presumably, what we need is a few enhancements to support in a standard way devices like MAX77693, LM3560 or MAX8997. There is already a led class driver for the MAX8997 LED controller (drivers/leds/leds-max8997.c), but it uses some device-specific sysfs attributes.
Thus similar devices are currently being handled by different subsystems. The split between the V4L2 Flash and the leds class API WRT to Flash LED controller drivers is included in RFC [1], it seems still up to date.
Perhaps it would make sense for V4L2 to be able to use a LED as exposed by the LED subsystem and wrap it so that it can be integrated with V4L2? If functionality is missing from the LED subsystem I suppose that could be added.
If I understand correctly, the V4L2 subsystem uses LEDs as flashes for camera devices. I can easily imagine that there are devices out there which provide functionality beyond what a regular LED will provide. So perhaps for things such as mobile phones, which typically use a plain LED to illuminate the surroundings, an LED wrapped into something that emulates the flash functionality could work. But I doubt that the LED subsystem is a good fit for anything beyond that.
Thierry