+++ /dev/null
- 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. ;)
-
+++ /dev/null
-/*
- * include/linux/pwm.h
- *
- * Copyright (C) 2008 Bill Gatliff < bgat@billgatliff.com>
- *
- * This program is free software; you may redistribute and/or modify
- * it under the terms of the GNU General Public License version 2, as
- * published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- * General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
- * USA
- */
-#ifndef __LINUX_PWM_H
-#define __LINUX_PWM_H
-
-enum {
- PWM_CONFIG_DUTY_TICKS = BIT(0),
- PWM_CONFIG_PERIOD_TICKS = BIT(1),
- PWM_CONFIG_POLARITY = BIT(2),
- PWM_CONFIG_START = BIT(3),
- PWM_CONFIG_STOP = BIT(4),
-
- PWM_CONFIG_HANDLER = BIT(5),
-
- PWM_CONFIG_DUTY_NS = BIT(6),
- PWM_CONFIG_DUTY_PERCENT = BIT(7),
- PWM_CONFIG_PERIOD_NS = BIT(8),
-};
-
-struct pwm_channel;
-struct work_struct;
-
-typedef int (*pwm_handler_t)(struct pwm_channel *p, void *data);
-typedef void (*pwm_callback_t)(struct pwm_channel *p);
-
-struct pwm_channel_config {
- int config_mask;
- unsigned long duty_ticks;
- unsigned long period_ticks;
- int polarity;
-
- pwm_handler_t handler;
-
- unsigned long duty_ns;
- unsigned long period_ns;
- int duty_percent;
-};
-
-struct pwm_device {
- struct list_head list;
- spinlock_t list_lock;
- struct device *dev;
- struct module *owner;
- struct pwm_channel *channels;
-
- const char *bus_id;
- int nchan;
-
- int (*request) (struct pwm_channel *p);
- void (*free) (struct pwm_channel *p);
- int (*config) (struct pwm_channel *p,
- struct pwm_channel_config *c);
- int (*config_nosleep)(struct pwm_channel *p,
- struct pwm_channel_config *c);
- int (*synchronize) (struct pwm_channel *p,
- struct pwm_channel *to_p);
- int (*unsynchronize)(struct pwm_channel *p,
- struct pwm_channel *from_p);
- int (*set_callback) (struct pwm_channel *p,
- pwm_callback_t callback);
-};
-
-int pwm_register(struct pwm_device *pwm);
-int pwm_unregister(struct pwm_device *pwm);
-
-enum {
- FLAG_REQUESTED = 0,
- FLAG_STOP = 1,
-};
-
-struct pwm_channel {
- struct list_head list;
- struct pwm_device *pwm;
- const char *requester;
- pid_t pid;
- int chan;
- unsigned long flags;
- unsigned long tick_hz;
-
- spinlock_t lock;
- struct completion complete;
-
- pwm_callback_t callback;
-
- struct work_struct handler_work;
- pwm_handler_t handler;
- void *handler_data;
-
- int active_high;
- unsigned long period_ticks;
- unsigned long duty_ticks;
-};
-
-struct gpio_pwm_platform_data {
- int gpio;
-};
-
-struct pwm_channel *
-pwm_request(const char *bus_id, int chan,
- const char *requester);
-
-void pwm_free(struct pwm_channel *pwm);
-
-int pwm_config_nosleep(struct pwm_channel *pwm,
- struct pwm_channel_config *c);
-
-int pwm_config(struct pwm_channel *pwm,
- struct pwm_channel_config *c);
-
-unsigned long pwm_ns_to_ticks(struct pwm_channel *pwm,
- unsigned long nsecs);
-
-unsigned long pwm_ticks_to_ns(struct pwm_channel *pwm,
- unsigned long ticks);
-
-int pwm_set_period_ns(struct pwm_channel *pwm,
- unsigned long period_ns);
-
-unsigned long int pwm_get_period_ns(struct pwm_channel *pwm);
-
-int pwm_set_duty_ns(struct pwm_channel *pwm,
- unsigned long duty_ns);
-
-int pwm_set_duty_percent(struct pwm_channel *pwm,
- int percent);
-
-unsigned long pwm_get_duty_ns(struct pwm_channel *pwm);
-
-int pwm_set_polarity(struct pwm_channel *pwm,
- int active_high);
-
-int pwm_start(struct pwm_channel *pwm);
-
-int pwm_stop(struct pwm_channel *pwm);
-
-int pwm_set_handler(struct pwm_channel *pwm,
- pwm_handler_t handler,
- void *data);
-
-int pwm_synchronize(struct pwm_channel *p,
- struct pwm_channel *to_p);
-
-
-int pwm_unsynchronize(struct pwm_channel *p,
- struct pwm_channel *from_p);
-
-
-#endif /* __LINUX_PWM_H */
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