                       Generic PWM Device API

                          February 1, 2010
                            Bill Gatliff
                        <bgat@billgatliff.com>


The code in drivers/pwm and include/linux/pwm/ implements an API for
applications involving pulse-width-modulation signals.  This document
describes how the API implementation facilitates both PWM-generating
devices, and users of those devices.


Motivation

The primary goals for implementing the "generic PWM API" are to
consolidate the various PWM implementations within a consistent and
redundancy-reducing framework, and to facilitate the use of
hotpluggable PWM devices.

Previous PWM-related implementations within the Linux kernel achieved
their consistency via cut-and-paste, but did not need to (and didn't)
facilitate more than one PWM-generating device within the system---
hotplug or otherwise.  The Generic PWM Device API might be most
appropriately viewed as an update to those implementations, rather
than a complete rewrite.


Challenges

One of the difficulties in implementing a generic PWM framework is the
fact that pulse-width-modulation applications involve real-world
signals, which often must be carefully managed to prevent destruction
of hardware that is linked to those signals.  A DC motor that
experiences a brief interruption in the PWM signal controlling it
might destructively overheat; it could suddenly change speed, losing
synchronization with a sensor; it could even suddenly change direction
or torque, breaking the mechanical device connected to it.

(A generic PWM device framework is not directly responsible for
preventing the above scenarios: that responsibility lies with the
hardware designer, and the application and driver authors.  But it
must to the greatest extent possible make it easy to avoid such
problems).

A generic PWM device framework must accommodate the substantial
differences between available PWM-generating hardware devices, without
becoming sub-optimal for any of them.

Finally, a generic PWM device framework must be relatively
lightweight, computationally speaking.  Some PWM users demand
high-speed outputs, plus the ability to regulate those outputs
quickly.  A device framework must be able to "keep up" with such
hardware, while still leaving time to do real work.

The Generic PWM Device API is an attempt to meet all of the above
requirements.  At its initial publication, the API was already in use
managing small DC motors, sensors and solenoids through a
custom-designed, optically-isolated H-bridge driver.


Functional Overview

The Generic PWM Device API framework is implemented in
include/linux/pwm/pwm.h and drivers/pwm/pwm.c.  The functions therein
use information from pwm_device, pwm_channel and pwm_channel_config
structures to invoke services in PWM peripheral device drivers.
Consult drivers/pwm/atmel-pwm.c for an example driver.

There are two classes of adopters of the PWM framework:

  "Users" -- those wishing to employ the API merely to produce PWM
  signals; once they have identified the appropriate physical output
  on the platform in question, they don't care about the details of
  the underlying hardware

  "Driver authors" -- those wishing to bind devices that can generate
  PWM signals to the Generic PWM Device API, so that the services of
  those devices become available to users. Assuming the hardware can
  support the needs of a user, driver authors don't care about the
  details of the user's application

Generally speaking, users will first invoke pwm_request() to obtain a
handle to a PWM device.  They will then pass that handle to functions
like pwm_duty_ns() and pwm_period_ns() to set the duty cycle and
period of the PWM signal, respectively.  They will also invoke
pwm_start() and pwm_stop() to turn the signal on and off.

The Generic PWM API framework also provides a sysfs interface to PWM
devices, which is adequate for basic application needs and testing.

Driver authors fill out a pwm_device structure, which describes the
capabilities of the PWM hardware being constructed--- including the
number of distinct output "channels" the peripheral offers.  They then
invoke pwm_register() (usually from within their device's probe()
handler) to make the PWM API aware of their device.  The framework
will call back to the methods described in the pwm_device structure as
users begin to configure and utilize the hardware.

Note that PWM signals can be produced by a variety of peripherals,
beyond the true "PWM hardware" offered by many system-on-chip devices.
Other possibilities include timer/counters with compare-match
capabilities, carefully-programmed synchronous serial ports
(e.g. SPI), and GPIO pins driven by kernel interval timers.  With a
proper pwm_device structure, these devices and pseudo-devices can all
be accommodated by the Generic PWM Device API framework.


Using the API to Generate PWM Signals -- Basic Functions for Users


pwm_request() -- Returns a pwm_channel pointer, which is subsequently
passed to the other user-related PWM functions.  Once requested, a PWM
channel is marked as in-use and subsequent requests prior to
pwm_free() will fail.

The names used to refer to PWM devices are defined by driver authors.
Typically they are platform device bus identifiers, and this
convention is encouraged for consistency.


pwm_free() -- Marks a PWM channel as no longer in use.  The PWM device
is stopped before it is released by the API.


pwm_period_ns() -- Specifies the PWM signal's period, in nanoseconds.


pwm_duty_ns() -- Specifies the PWM signal's active duration, in nanoseconds.


pwm_duty_percent() -- Specifies the PWM signal's active duration, as a
percentage of the current period of the signal.  NOTE: this value is
not recalculated if the period of the signal is subsequently changed.


pwm_start(), pwm_stop() -- Turns the PWM signal on and off.  Except
where stated otherwise by a driver author, signals are stopped at the
end of the current period, at which time the output is set to its
inactive state.


pwm_polarity() -- Defines whether the PWM signal output's active
region is "1" or "0".  A 10% duty-cycle, polarity=1 signal will
conventionally be at 5V (or 3.3V, or 1000V, or whatever the platform
hardware does) for 10% of the period.  The same configuration of a
polarity=0 signal will be at 5V (or 3.3V, or ...) for 90% of the
period.


Using the API to Generate PWM Signals -- Advanced Functions


pwm_config() -- Passes a pwm_channel_config structure to the
associated device driver.  This function is invoked by pwm_start(),
pwm_duty_ns(), etc. and is one of two main entry points to the PWM
driver for the hardware being used.  The configuration change is
guaranteed atomic if multiple configuration changes are specified.
This function might sleep, depending on what the device driver has to
do to satisfy the request.  All PWM device drivers must support this
entry point.


pwm_config_nosleep() -- Passes a pwm_channel_config structure to the
associated device driver.  If the driver must sleep in order to
implement the requested configuration change, -EWOULDBLOCK is
returned.  Users may call this function from interrupt handlers, for
example.  This is the other main entry point into the PWM hardware
driver, but not all device drivers support this entry point.


pwm_synchronize(), pwm_unsynchronize() -- "Synchronizes" two or more
PWM channels, if the underlying hardware permits.  (If it doesn't, the
framework facilitates emulating this capability but it is not yet
implemented).  Synchronized channels will start and stop
simultaneously when any single channel in the group is started or
stopped.  Use pwm_unsynchronize(..., NULL) to completely detach a
channel from any other synchronized channels.  By default, all PWM
channels are unsynchronized.


pwm_set_handler() -- Defines an end-of-period callback.  The indicated
function will be invoked in a worker thread at the end of each PWM
period, and can subsequently invoke pwm_config(), etc.  Must be used
with extreme care for high-speed PWM outputs.  Set the handler
function to NULL to un-set the handler.


Implementing a PWM Device API Driver -- Functions for Driver Authors


Fill out the appropriate fields in a pwm_device structure, and submit
to pwm_register():


bus_id -- the plain-text name of the device.  Users will bind to a
channel on the device using this name plus the channel number.  For
example, the Atmel PWMC's bus_id is "atmel_pwmc", the same as used by
the platform device driver (recommended).  The first device registered
thereby receives bus_id "atmel_pwmc.0", which is what you put in
pwm_device.bus_id.  Channels are then named "atmel_pwmc.0:[0-3]".
(Hint: just use pdev->dev.bus_id in your probe() method).


nchan -- the number of distinct output channels provided by the device.


request -- (optional) Invoked each time a user requests a channel.
Use to turn on clocks, clean up register states, etc.  The framework
takes care of device locking/unlocking; you will see only successful
requests.


free -- (optional) Callback for each time a user relinquishes a
channel.  The framework will have already stopped, unsynchronized and
un-handled the channel.  Use to turn off clocks, etc. as necessary.


synchronize, unsynchronize -- (optional) Callbacks to
synchronize/unsynchronize channels.  Some devices provide this
capability in hardware; for others, it can be emulated (see
atmel_pwmc.c's sync_mask for an example).


set_callback -- (optional) Invoked when a user requests a handler.  If
the hardware supports an end-of-period interrupt, invoke the function
indicated during your interrupt handler.  The callback function itself
is always internal to the API, and does not map directly to the user's
callback function.


config -- Invoked to change the device configuration, always from a
sleep-capable context.  All the changes indicated must be performed
atomically, ideally synchronized to an end-of-period event (so that
you avoid short or long output pulses).  You may sleep, etc. as
necessary within this function.


config_nosleep -- (optional) Invoked to change device configuration
from within a context that is not allowed to sleep.  If you cannot
perform the requested configuration changes without sleeping, return
-EWOULDBLOCK.


Acknowledgements


The author expresses his gratitude to the countless developers who
have reviewed and submitted feedback on the various versions of the
Generic PWM Device API code, and those who have submitted drivers and
applications that use the framework.  You know who you are.  ;)