diff options
Diffstat (limited to 'Documentation/pwm.txt')
| -rw-r--r-- | Documentation/pwm.txt | 51 |
1 files changed, 48 insertions, 3 deletions
diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt index 7d2b4c9b544..ca895fd211e 100644 --- a/Documentation/pwm.txt +++ b/Documentation/pwm.txt @@ -19,7 +19,8 @@ should instead register a static mapping that can be used to match PWM consumers to providers, as given in the following example: static struct pwm_lookup board_pwm_lookup[] = { - PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL), + PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL, + 50000, PWM_POLARITY_NORMAL), }; static void __init board_init(void) @@ -39,12 +40,49 @@ New users should use the pwm_get() function and pass to it the consumer device or a consumer name. pwm_put() is used to free the PWM device. Managed variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist. -After being requested a PWM has to be configured using: +After being requested, a PWM has to be configured using: int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns); To start/stop toggling the PWM output use pwm_enable()/pwm_disable(). +Using PWMs with the sysfs interface +----------------------------------- + +If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs +interface is provided to use the PWMs from userspace. It is exposed at +/sys/class/pwm/. Each probed PWM controller/chip will be exported as +pwmchipN, where N is the base of the PWM chip. Inside the directory you +will find: + +npwm - The number of PWM channels this chip supports (read-only). + +export - Exports a PWM channel for use with sysfs (write-only). + +unexport - Unexports a PWM channel from sysfs (write-only). + +The PWM channels are numbered using a per-chip index from 0 to npwm-1. + +When a PWM channel is exported a pwmX directory will be created in the +pwmchipN directory it is associated with, where X is the number of the +channel that was exported. The following properties will then be available: + +period - The total period of the PWM signal (read/write). + Value is in nanoseconds and is the sum of the active and inactive + time of the PWM. + +duty_cycle - The active time of the PWM signal (read/write). + Value is in nanoseconds and must be less than the period. + +polarity - Changes the polarity of the PWM signal (read/write). + Writes to this property only work if the PWM chip supports changing + the polarity. The polarity can only be changed if the PWM is not + enabled. Value is the string "normal" or "inversed". + +enable - Enable/disable the PWM signal (read/write). + 0 - disabled + 1 - enabled + Implementing a PWM driver ------------------------- @@ -57,9 +95,16 @@ for new drivers to use the generic PWM framework. A new PWM controller/chip can be added using pwmchip_add() and removed again with pwmchip_remove(). pwmchip_add() takes a filled in struct pwm_chip as argument which provides a description of the PWM chip, the -number of PWM devices provider by the chip and the chip-specific +number of PWM devices provided by the chip and the chip-specific implementation of the supported PWM operations to the framework. +When implementing polarity support in a PWM driver, make sure to respect the +signal conventions in the PWM framework. By definition, normal polarity +characterizes a signal starts high for the duration of the duty cycle and +goes low for the remainder of the period. Conversely, a signal with inversed +polarity starts low for the duration of the duty cycle and goes high for the +remainder of the period. + Locking ------- |