Re: [PATCH RFC] pwm: add sysfs interface

From: Rob Landley
Date: Sat Feb 23 2013 - 23:22:47 EST

On 02/19/2013 08:27:41 AM, Lars Poeschel wrote:
From: Lars Poeschel <poeschel@xxxxxxxxxxx>

This adds a simple sysfs interface to the pwm subsystem. It is
heavily inspired by the gpio sysfs interface.


diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt
index 7d2b4c9..b349d16 100644
--- a/Documentation/pwm.txt
+++ b/Documentation/pwm.txt
@@ -45,6 +45,52 @@ 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().

Hm, read read read existing file...

So a PWM is a GPIO that blinks on its own, in hardware, without needing a kernel thread and a timer to tell it to? (I think? Correction, an output-only gpio...)

+Using PWMs with the sysfs interface
+You have to enable CONFIG_PWM_SYSFS in your kernel configuration to use
+the sysfs interface. It is exposed at /sys/class/pwm/. If there are pwm
+drivers loaded and these drivers successfully probed a chip, this chip
+is exported as pwmchipX . Note that a single driver can probe multiple chips.
+Inside the directory you can read these properties:
+base - This is the linux global start where the chips pwm channels get
+npwm - This is the number of pwm channels this chip supports.
+If you want to start using a pwm channel with sysfs first you have to
+export it. If you are finished with it and want to free the pwm for other
+uses, you can unexport it:
+export - Write the global pwm channel number to this file to start using
+the channel with sysfs.
+unexport - Write the global pwm channel number of the channel you are finshed
+with to this file to make the channel available for other uses.
+Once a pwm is exported a pwmX (X ranging from 0 to MAX_PWMS) directory appears
+with the following read/write properties inside to control the pwm:
+duty_ns - Write the number of nanoseconds the active portion of the pwm period
+should last to this file. This can not be longer than the period_ns.
+period_ns - Write the length of the pwm period in nanoseconds to this file.
+This includes the active and inactive portion of the pwm period and can not
+be smaller than duty_ns.
+polarity - The normal behaviour is to put out a high for the active portion of
+the pwm period. Write a 1 to this file to inverse the signal and output a low
+on the active portion. Write a 0 to switch back to the normal behaviour. The
+polarity can only be changed if the pwm is not running. This file is only
+visible if the underlying driver/device supports changing the polarity.
+run - Write a 1 to this file to start the pwm signal generation, write a 0 to
+stop it. Set your desired period_ns, duty_ns and polarity before starting the
+It is recommend to set the period_ns at first and duty_ns after that.
Implementing a PWM driver

Doc part looks good to me:

Acked-by Rob Landley <rob@xxxxxxxxxxx>

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index e513cd9..1c3432e 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -28,6 +28,21 @@ menuconfig PWM

if PWM

+config PWM_SYSFS
+ bool "/sys/class/pwm/... (sysfs interface)"
+ depends on SYSFS
+ help
+ Say Y here to add a sysfs interface for PWMs.
+ This exports directories and files to userspace using sysfs.

Given that that's what a sysfs interface _is_, does that last line actually add anything?

+ This way the PWM outputs of a device can easyly be used,


+ controlled and tested.

And again, this sentence isn't hugely helpful if you already know what sysfs is. Why not start here:

+ For every instance of an PWM capable
+ device there is a pwmchipX directory exported to
+ /sys/class/pwm. If you want to use a PWM, you have to export
+ it to sysfs, which is done by writing the number into
+ /sys/class/pwm/export. After that /sys/class/pwm/pwmX is
+ reaady to be used.


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