forked from luck/tmp_suning_uos_patched
8138d2ddbc
In order to get rid of the global namespace for PWM devices, this commit provides an alternative method, similar to that of the regulator or clock frameworks, for registering a static mapping for PWM devices. This works by providing a table with a provider/consumer map in the board setup code. With the new pwm_get() and pwm_put() functions available, usage of pwm_request() and pwm_free() becomes deprecated. Reviewed-by: Shawn Guo <shawn.guo@linaro.org> Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com> Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
77 lines
2.8 KiB
Plaintext
77 lines
2.8 KiB
Plaintext
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.
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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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};
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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.
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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 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 provider by the chip and the chip-specific
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implementation of the supported PWM operations 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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