Signed-off-by: Bill Gatliff <bgat@xxxxxxxxxxxxxxx> --- Documentation/pwm.txt | 258 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 258 insertions(+), 0 deletions(-) create mode 100644 Documentation/pwm.txt diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt new file mode 100644 index 0000000..b8932dd --- /dev/null +++ b/Documentation/pwm.txt @@ -0,0 +1,258 @@ + Generic PWM Device API + + October 8, 2008 + Bill Gatliff + <bgat@xxxxxxxxxxxxxxx> + + + +The code in drivers/pwm and include/linux/pwm.h 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 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 accomodate 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 through a custom-designed, optically-isolated +H-bridge driver. + + + +Functional Overview + +The Generic PWM Device API framework is implemented in +include/linux/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 framework also provides a sysfs interface to PWM devices, which is +adequate for basic needs and testing. + +Driver authors fill out a pwm_device structure, which describes the +capabilities of the PWM hardware being driven--- 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 to configure and +use 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 accomodated 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. + + +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 plaintext 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. ;) + -- 1.5.6.5 -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
