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174 lines
7.1 KiB
174 lines
7.1 KiB
====================================== |
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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 the getter, devm_pwm_get(), devm_of_pwm_get(), |
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devm_fwnode_pwm_get(), 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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There is also a usage_power setting: If set, the PWM driver is only required to |
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maintain the power output but has more freedom regarding signal form. |
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If supported by the driver, the signal can be optimized, for example to improve |
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EMI by phase shifting the individual channels of a chip. |
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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 that was passed to the |
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last invocation of pwm_apply_state() using pwm_get_state(). Note this is |
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different to what the driver has actually implemented if the request cannot be |
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satisfied exactly with the hardware in use. There is currently no way for |
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consumers to get the actually implemented settings. |
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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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