261 lines
10 KiB
Plaintext
261 lines
10 KiB
Plaintext
Generic PWM Device API
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February 1, 2010
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Bill Gatliff
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<bgat@billgatliff.com>
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The code in drivers/pwm and include/linux/pwm/ implements an API for
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applications involving pulse-width-modulation signals. This document
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describes how the API implementation facilitates both PWM-generating
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devices, and users of those devices.
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Motivation
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The primary goals for implementing the "generic PWM API" are to
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consolidate the various PWM implementations within a consistent and
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redundancy-reducing framework, and to facilitate the use of
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hotpluggable PWM devices.
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Previous PWM-related implementations within the Linux kernel achieved
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their consistency via cut-and-paste, but did not need to (and didn't)
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facilitate more than one PWM-generating device within the system---
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hotplug or otherwise. The Generic PWM Device API might be most
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appropriately viewed as an update to those implementations, rather
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than a complete rewrite.
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Challenges
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One of the difficulties in implementing a generic PWM framework is the
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fact that pulse-width-modulation applications involve real-world
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signals, which often must be carefully managed to prevent destruction
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of hardware that is linked to those signals. A DC motor that
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experiences a brief interruption in the PWM signal controlling it
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might destructively overheat; it could suddenly change speed, losing
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synchronization with a sensor; it could even suddenly change direction
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or torque, breaking the mechanical device connected to it.
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(A generic PWM device framework is not directly responsible for
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preventing the above scenarios: that responsibility lies with the
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hardware designer, and the application and driver authors. But it
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must to the greatest extent possible make it easy to avoid such
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problems).
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A generic PWM device framework must accommodate the substantial
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differences between available PWM-generating hardware devices, without
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becoming sub-optimal for any of them.
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Finally, a generic PWM device framework must be relatively
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lightweight, computationally speaking. Some PWM users demand
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high-speed outputs, plus the ability to regulate those outputs
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quickly. A device framework must be able to "keep up" with such
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hardware, while still leaving time to do real work.
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The Generic PWM Device API is an attempt to meet all of the above
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requirements. At its initial publication, the API was already in use
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managing small DC motors, sensors and solenoids through a
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custom-designed, optically-isolated H-bridge driver.
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Functional Overview
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The Generic PWM Device API framework is implemented in
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include/linux/pwm/pwm.h and drivers/pwm/pwm.c. The functions therein
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use information from pwm_device, pwm_channel and pwm_channel_config
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structures to invoke services in PWM peripheral device drivers.
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Consult drivers/pwm/atmel-pwm.c for an example driver.
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There are two classes of adopters of the PWM framework:
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"Users" -- those wishing to employ the API merely to produce PWM
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signals; once they have identified the appropriate physical output
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on the platform in question, they don't care about the details of
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the underlying hardware
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"Driver authors" -- those wishing to bind devices that can generate
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PWM signals to the Generic PWM Device API, so that the services of
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those devices become available to users. Assuming the hardware can
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support the needs of a user, driver authors don't care about the
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details of the user's application
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Generally speaking, users will first invoke pwm_request() to obtain a
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handle to a PWM device. They will then pass that handle to functions
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like pwm_duty_ns() and pwm_period_ns() to set the duty cycle and
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period of the PWM signal, respectively. They will also invoke
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pwm_start() and pwm_stop() to turn the signal on and off.
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The Generic PWM API framework also provides a sysfs interface to PWM
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devices, which is adequate for basic application needs and testing.
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Driver authors fill out a pwm_device structure, which describes the
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capabilities of the PWM hardware being constructed--- including the
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number of distinct output "channels" the peripheral offers. They then
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invoke pwm_register() (usually from within their device's probe()
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handler) to make the PWM API aware of their device. The framework
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will call back to the methods described in the pwm_device structure as
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users begin to configure and utilize the hardware.
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Note that PWM signals can be produced by a variety of peripherals,
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beyond the true "PWM hardware" offered by many system-on-chip devices.
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Other possibilities include timer/counters with compare-match
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capabilities, carefully-programmed synchronous serial ports
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(e.g. SPI), and GPIO pins driven by kernel interval timers. With a
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proper pwm_device structure, these devices and pseudo-devices can all
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be accommodated by the Generic PWM Device API framework.
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Using the API to Generate PWM Signals -- Basic Functions for Users
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pwm_request() -- Returns a pwm_channel pointer, which is subsequently
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passed to the other user-related PWM functions. Once requested, a PWM
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channel is marked as in-use and subsequent requests prior to
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pwm_free() will fail.
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The names used to refer to PWM devices are defined by driver authors.
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Typically they are platform device bus identifiers, and this
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convention is encouraged for consistency.
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pwm_free() -- Marks a PWM channel as no longer in use. The PWM device
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is stopped before it is released by the API.
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pwm_period_ns() -- Specifies the PWM signal's period, in nanoseconds.
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pwm_duty_ns() -- Specifies the PWM signal's active duration, in nanoseconds.
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pwm_duty_percent() -- Specifies the PWM signal's active duration, as a
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percentage of the current period of the signal. NOTE: this value is
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not recalculated if the period of the signal is subsequently changed.
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pwm_start(), pwm_stop() -- Turns the PWM signal on and off. Except
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where stated otherwise by a driver author, signals are stopped at the
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end of the current period, at which time the output is set to its
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inactive state.
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pwm_polarity() -- Defines whether the PWM signal output's active
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region is "1" or "0". A 10% duty-cycle, polarity=1 signal will
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conventionally be at 5V (or 3.3V, or 1000V, or whatever the platform
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hardware does) for 10% of the period. The same configuration of a
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polarity=0 signal will be at 5V (or 3.3V, or ...) for 90% of the
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period.
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Using the API to Generate PWM Signals -- Advanced Functions
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pwm_config() -- Passes a pwm_channel_config structure to the
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associated device driver. This function is invoked by pwm_start(),
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pwm_duty_ns(), etc. and is one of two main entry points to the PWM
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driver for the hardware being used. The configuration change is
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guaranteed atomic if multiple configuration changes are specified.
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This function might sleep, depending on what the device driver has to
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do to satisfy the request. All PWM device drivers must support this
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entry point.
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pwm_config_nosleep() -- Passes a pwm_channel_config structure to the
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associated device driver. If the driver must sleep in order to
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implement the requested configuration change, -EWOULDBLOCK is
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returned. Users may call this function from interrupt handlers, for
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example. This is the other main entry point into the PWM hardware
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driver, but not all device drivers support this entry point.
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pwm_synchronize(), pwm_unsynchronize() -- "Synchronizes" two or more
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PWM channels, if the underlying hardware permits. (If it doesn't, the
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framework facilitates emulating this capability but it is not yet
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implemented). Synchronized channels will start and stop
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simultaneously when any single channel in the group is started or
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stopped. Use pwm_unsynchronize(..., NULL) to completely detach a
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channel from any other synchronized channels. By default, all PWM
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channels are unsynchronized.
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pwm_set_handler() -- Defines an end-of-period callback. The indicated
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function will be invoked in a worker thread at the end of each PWM
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period, and can subsequently invoke pwm_config(), etc. Must be used
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with extreme care for high-speed PWM outputs. Set the handler
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function to NULL to un-set the handler.
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Implementing a PWM Device API Driver -- Functions for Driver Authors
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Fill out the appropriate fields in a pwm_device structure, and submit
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to pwm_register():
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bus_id -- the plain-text name of the device. Users will bind to a
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channel on the device using this name plus the channel number. For
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example, the Atmel PWMC's bus_id is "atmel_pwmc", the same as used by
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the platform device driver (recommended). The first device registered
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thereby receives bus_id "atmel_pwmc.0", which is what you put in
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pwm_device.bus_id. Channels are then named "atmel_pwmc.0:[0-3]".
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(Hint: just use pdev->dev.bus_id in your probe() method).
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nchan -- the number of distinct output channels provided by the device.
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request -- (optional) Invoked each time a user requests a channel.
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Use to turn on clocks, clean up register states, etc. The framework
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takes care of device locking/unlocking; you will see only successful
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requests.
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free -- (optional) Callback for each time a user relinquishes a
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channel. The framework will have already stopped, unsynchronized and
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un-handled the channel. Use to turn off clocks, etc. as necessary.
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synchronize, unsynchronize -- (optional) Callbacks to
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synchronize/unsynchronize channels. Some devices provide this
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capability in hardware; for others, it can be emulated (see
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atmel_pwmc.c's sync_mask for an example).
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set_callback -- (optional) Invoked when a user requests a handler. If
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the hardware supports an end-of-period interrupt, invoke the function
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indicated during your interrupt handler. The callback function itself
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is always internal to the API, and does not map directly to the user's
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callback function.
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config -- Invoked to change the device configuration, always from a
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sleep-capable context. All the changes indicated must be performed
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atomically, ideally synchronized to an end-of-period event (so that
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you avoid short or long output pulses). You may sleep, etc. as
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necessary within this function.
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config_nosleep -- (optional) Invoked to change device configuration
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from within a context that is not allowed to sleep. If you cannot
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perform the requested configuration changes without sleeping, return
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-EWOULDBLOCK.
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Acknowledgements
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The author expresses his gratitude to the countless developers who
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have reviewed and submitted feedback on the various versions of the
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Generic PWM Device API code, and those who have submitted drivers and
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applications that use the framework. You know who you are. ;)
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