166 lines
6.5 KiB
ReStructuredText
166 lines
6.5 KiB
ReStructuredText
======================================
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Pulse Width Modulation (PWM) interface
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======================================
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This provides an overview about the Linux PWM interface
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PWMs are commonly used for controlling LEDs, fans or vibrators in
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cell phones. PWMs with a fixed purpose have no need implementing
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the Linux PWM API (although they could). However, PWMs are often
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found as discrete devices on SoCs which have no fixed purpose. It's
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up to the board designer to connect them to LEDs or fans. To provide
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this kind of flexibility the generic PWM API exists.
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Identifying PWMs
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----------------
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Users of the legacy PWM API use unique IDs to refer to PWM devices.
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Instead of referring to a PWM device via its unique ID, board setup code
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should instead register a static mapping that can be used to match PWM
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consumers to providers, as given in the following example::
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static struct pwm_lookup board_pwm_lookup[] = {
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PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
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50000, PWM_POLARITY_NORMAL),
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};
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static void __init board_init(void)
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{
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...
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pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
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...
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}
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Using PWMs
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----------
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Legacy users can request a PWM device using pwm_request() and free it
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after usage with pwm_free().
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New users should use the pwm_get() function and pass to it the consumer
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device or a consumer name. pwm_put() is used to free the PWM device. Managed
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variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
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After being requested, a PWM has to be configured using::
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int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
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This API controls both the PWM period/duty_cycle config and the
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enable/disable state.
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The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
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around pwm_apply_state() and should not be used if the user wants to change
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several parameter at once. For example, if you see pwm_config() and
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pwm_{enable,disable}() calls in the same function, this probably means you
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should switch to pwm_apply_state().
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The PWM user API also allows one to query the PWM state with pwm_get_state().
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In addition to the PWM state, the PWM API also exposes PWM arguments, which
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are the reference PWM config one should use on this PWM.
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PWM arguments are usually platform-specific and allows the PWM user to only
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care about dutycycle relatively to the full period (like, duty = 50% of the
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period). struct pwm_args contains 2 fields (period and polarity) and should
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be used to set the initial PWM config (usually done in the probe function
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of the PWM user). PWM arguments are retrieved with pwm_get_args().
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All consumers should really be reconfiguring the PWM upon resume as
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appropriate. This is the only way to ensure that everything is resumed in
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the proper order.
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Using PWMs with the sysfs interface
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-----------------------------------
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If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
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interface is provided to use the PWMs from userspace. It is exposed at
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/sys/class/pwm/. Each probed PWM controller/chip will be exported as
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pwmchipN, where N is the base of the PWM chip. Inside the directory you
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will find:
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npwm
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The number of PWM channels this chip supports (read-only).
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export
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Exports a PWM channel for use with sysfs (write-only).
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unexport
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Unexports a PWM channel from sysfs (write-only).
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The PWM channels are numbered using a per-chip index from 0 to npwm-1.
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When a PWM channel is exported a pwmX directory will be created in the
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pwmchipN directory it is associated with, where X is the number of the
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channel that was exported. The following properties will then be available:
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period
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The total period of the PWM signal (read/write).
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Value is in nanoseconds and is the sum of the active and inactive
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time of the PWM.
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duty_cycle
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The active time of the PWM signal (read/write).
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Value is in nanoseconds and must be less than the period.
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polarity
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Changes the polarity of the PWM signal (read/write).
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Writes to this property only work if the PWM chip supports changing
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the polarity. The polarity can only be changed if the PWM is not
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enabled. Value is the string "normal" or "inversed".
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enable
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Enable/disable the PWM signal (read/write).
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- 0 - disabled
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- 1 - enabled
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Implementing a PWM driver
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-------------------------
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Currently there are two ways to implement pwm drivers. Traditionally
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there only has been the barebone API meaning that each driver has
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to implement the pwm_*() functions itself. This means that it's impossible
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to have multiple PWM drivers in the system. For this reason it's mandatory
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for new drivers to use the generic PWM framework.
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A new PWM controller/chip can be added using pwmchip_add() and removed
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again with pwmchip_remove(). pwmchip_add() takes a filled in struct
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pwm_chip as argument which provides a description of the PWM chip, the
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number of PWM devices provided by the chip and the chip-specific
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implementation of the supported PWM operations to the framework.
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When implementing polarity support in a PWM driver, make sure to respect the
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signal conventions in the PWM framework. By definition, normal polarity
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characterizes a signal starts high for the duration of the duty cycle and
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goes low for the remainder of the period. Conversely, a signal with inversed
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polarity starts low for the duration of the duty cycle and goes high for the
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remainder of the period.
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Drivers are encouraged to implement ->apply() instead of the legacy
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->enable(), ->disable() and ->config() methods. Doing that should provide
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atomicity in the PWM config workflow, which is required when the PWM controls
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a critical device (like a regulator).
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The implementation of ->get_state() (a method used to retrieve initial PWM
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state) is also encouraged for the same reason: letting the PWM user know
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about the current PWM state would allow him to avoid glitches.
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Drivers should not implement any power management. In other words,
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consumers should implement it as described in the "Using PWMs" section.
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Locking
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-------
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The PWM core list manipulations are protected by a mutex, so pwm_request()
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and pwm_free() may not be called from an atomic context. Currently the
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PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
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pwm_config(), so the calling context is currently driver specific. This
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is an issue derived from the former barebone API and should be fixed soon.
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Helpers
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-------
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Currently a PWM can only be configured with period_ns and duty_ns. For several
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use cases freq_hz and duty_percent might be better. Instead of calculating
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this in your driver please consider adding appropriate helpers to the framework.
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