[linux-2.6-block.git] / Documentation / pwm.txt

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.
Commit	Line	Data
0c2498f1 SH	1	Pulse Width Modulation (PWM) interface
	2
	3	This provides an overview about the Linux PWM interface
	4
	5	PWMs are commonly used for controlling LEDs, fans or vibrators in
	6	cell phones. PWMs with a fixed purpose have no need implementing
	7	the Linux PWM API (although they could). However, PWMs are often
	8	found as discrete devices on SoCs which have no fixed purpose. It's
	9	up to the board designer to connect them to LEDs or fans. To provide
	10	this kind of flexibility the generic PWM API exists.
	11
	12	Identifying PWMs
	13	----------------
	14
	15	Users of the legacy PWM API use unique IDs to refer to PWM devices. One
	16	goal of the new PWM framework is to get rid of this global namespace.
	17
	18	Using PWMs
	19	----------
	20
	21	A PWM can be requested using pwm_request() and freed after usage with
	22	pwm_free(). After being requested a PWM has to be configured using
	23
	24	int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
	25
	26	To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
	27
	28	Implementing a PWM driver
	29	-------------------------
	30
	31	Currently there are two ways to implement pwm drivers. Traditionally
	32	there only has been the barebone API meaning that each driver has
	33	to implement the pwm_*() functions itself. This means that it's impossible
	34	to have multiple PWM drivers in the system. For this reason it's mandatory
	35	for new drivers to use the generic PWM framework.
	36	A new PWM device can be added using pwmchip_add() and removed again with
	37	pwmchip_remove(). pwmchip_add() takes a filled in struct pwm_chip as
	38	argument which provides the ops and the pwm id to the framework.
	39
	40	Locking
	41	-------
	42
	43	The PWM core list manipulations are protected by a mutex, so pwm_request()
	44	and pwm_free() may not be called from an atomic context. Currently the
	45	PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
	46	pwm_config(), so the calling context is currently driver specific. This
	47	is an issue derived from the former barebone API and should be fixed soon.
	48
	49	Helpers
	50	-------
	51
	52	Currently a PWM can only be configured with period_ns and duty_ns. For several
	53	use cases freq_hz and duty_percent might be better. Instead of calculating
	54	this in your driver please consider adding appropriate helpers to the framework.