Commit 0c2498f1 authored by Sascha Hauer's avatar Sascha Hauer Committed by Thierry Reding
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pwm: Add PWM framework support 



This patch adds framework support for PWM (pulse width modulation) devices.

The is a barebone PWM API already in the kernel under include/linux/pwm.h,
but it does not allow for multiple drivers as each of them implements the
pwm_*() functions.

There are other PWM framework patches around from Bill Gatliff. Unlike
his framework this one does not change the existing API for PWMs so that
this framework can act as a drop in replacement for the existing API.

Why another framework?

Several people argue that there should not be another framework for PWMs
but they should be integrated into one of the existing frameworks like led
or hwmon. Unlike these frameworks the PWM framework is agnostic to the
purpose of the PWM. In fact, a PWM can drive a LED, but this makes the
LED framework a user of a PWM, like already done in leds-pwm.c. The gpio
framework also is not suitable for PWMs. Every gpio could be turned into
a PWM using timer based toggling, but on the other hand not every PWM hardware
device can be turned into a gpio due to the lack of hardware capabilities.

This patch does not try to improve the PWM API yet, this could be done in
subsequent patches.

Signed-off-by: default avatarSascha Hauer <s.hauer@pengutronix.de>
Acked-by: default avatarKurt Van Dijck <kurt.van.dijck@eia.be>
Reviewed-by: default avatarArnd Bergmann <arnd@arndb.de>
Reviewed-by: default avatarMatthias Kaehlcke <matthias@kaehlcke.net>
Reviewed-by: default avatarMark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: default avatarShawn Guo <shawn.guo@linaro.org>
[thierry.reding@avionic-design.de: fixup typos, kerneldoc comments]
Signed-off-by: default avatarThierry Reding <thierry.reding@avionic-design.de>
parent cfaf0251

Documentation/pwm.txt

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Pulse Width Modulation (PWM) interface



This provides an overview about the Linux PWM interface



PWMs are commonly used for controlling LEDs, fans or vibrators in

cell phones. PWMs with a fixed purpose have no need implementing

the Linux PWM API (although they could). However, PWMs are often

found as discrete devices on SoCs which have no fixed purpose. It's

up to the board designer to connect them to LEDs or fans. To provide

this kind of flexibility the generic PWM API exists.



Identifying PWMs

----------------



Users of the legacy PWM API use unique IDs to refer to PWM devices. One

goal of the new PWM framework is to get rid of this global namespace.



Using PWMs

----------



A PWM can be requested using pwm_request() and freed after usage with

pwm_free(). After being requested a PWM has to be configured using



int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);



To start/stop toggling the PWM output use pwm_enable()/pwm_disable().



Implementing a PWM driver

-------------------------



Currently there are two ways to implement pwm drivers. Traditionally

there only has been the barebone API meaning that each driver has

to implement the pwm_*() functions itself. This means that it's impossible

to have multiple PWM drivers in the system. For this reason it's mandatory

for new drivers to use the generic PWM framework.

A new PWM device can be added using pwmchip_add() and removed again with

pwmchip_remove(). pwmchip_add() takes a filled in struct pwm_chip as

argument which provides the ops and the pwm id to the framework.



Locking

-------



The PWM core list manipulations are protected by a mutex, so pwm_request()

and pwm_free() may not be called from an atomic context. Currently the

PWM core does not enforce any locking to pwm_enable(), pwm_disable() and

pwm_config(), so the calling context is currently driver specific. This

is an issue derived from the former barebone API and should be fixed soon.



Helpers

-------



Currently a PWM can only be configured with period_ns and duty_ns. For several

use cases freq_hz and duty_percent might be better. Instead of calculating

this in your driver please consider adding appropriate helpers to the framework.

MAINTAINERS

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@@ -5487,6 +5487,12 @@ S: Maintained 
F:	Documentation/video4linux/README.pvrusb2

F:	drivers/media/video/pvrusb2/



PWM core

M:	Sascha Hauer <s.hauer@pengutronix.de>

L:	linux-kernel@vger.kernel.org

S:	Maintained

F:	drivers/pwm/



PXA2xx/PXA3xx SUPPORT

M:	Eric Miao <eric.y.miao@gmail.com>

M:	Russell King <linux@arm.linux.org.uk>

drivers/Kconfig

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@@ -148,4 +148,6 @@ source "drivers/iio/Kconfig" 


source "drivers/vme/Kconfig"



source "drivers/pwm/Kconfig"



endmenu

drivers/Makefile

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@@ -8,6 +8,7 @@ 
# GPIO must come after pinctrl as gpios may need to mux pins etc

obj-y				+= pinctrl/

obj-y				+= gpio/

obj-y				+= pwm/

obj-$(CONFIG_PCI)		+= pci/

obj-$(CONFIG_PARISC)		+= parisc/

obj-$(CONFIG_RAPIDIO)		+= rapidio/

drivers/pwm/Kconfig

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menuconfig PWM

	bool "PWM Support"

	help

	  This enables PWM support through the generic PWM framework.

	  You only need to enable this, if you also want to enable

	  one or more of the PWM drivers below.



	  If unsure, say N.



if PWM



endif

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