55 lines
2.1 KiB
Plaintext
55 lines
2.1 KiB
Plaintext
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Pulse Width Modulation (PWM) interface
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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. One
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goal of the new PWM framework is to get rid of this global namespace.
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Using PWMs
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----------
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A PWM can be requested using pwm_request() and freed after usage with
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pwm_free(). After being requested a PWM has to be configured using
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int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
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To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
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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 device can be added using pwmchip_add() and removed again with
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pwmchip_remove(). pwmchip_add() takes a filled in struct pwm_chip as
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argument which provides the ops and the pwm id to the framework.
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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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