[PATCH v7] pwm: Add support for pwmchip devices for faster and easier userspace access

Uwe Kleine-König
2025-04-16T09:43:35Z

With this change each pwmchip defining the new-style waveform callbacks
can be accessed from userspace via a character device. Compared to the
sysfs-API this is faster and allows to pass the whole configuration in a
single ioctl allowing atomic application and thus reducing glitches.

On an STM32MP13 I see:

root@DistroKit:~ time pwmtestperf
real 0m 1.27s
user 0m 0.02s
sys 0m 1.21s
root@DistroKit:~ rm /dev/pwmchip0
root@DistroKit:~ time pwmtestperf
real 0m 3.61s
user 0m 0.27s
sys 0m 3.26s

pwmtestperf does essentially:

for i in 0 .. 50000:
pwm_set_waveform(duty_length_ns=i, period_length_ns=50000, duty_offset_ns=0)

and in the presence of /dev/pwmchip0 is uses the ioctls introduced here,
without that device it uses /sys/class/pwm/pwmchip0.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
---
Changes since v6
(https://lore.kernel.org/linux-pwm/cover.1744120697.git.ukleinek@kernel.org/):
- Trivially rebase to current pwm/for-next
- Improve error handling and reporting for too high pwmchip ids
- Plug a memory leak
- Drop unneeded breaks in ioctl function
- Documentation updates
- Update commit log with recent performance measurement
- After some thinking and internal discussion with David, let
PWM_IOCTL_SETEXACTWF return 1 instead of a negative errno when the
exact waveform cannot be implemented. The motivation is that this
situation should be reliably distinguishable from other errors.

Thanks to David for his feedback.

Best regards
Uwe

drivers/pwm/core.c | 324 +++++++++++++++++++++++++++++++++++++--
include/linux/pwm.h | 3 +
include/uapi/linux/pwm.h | 53 +++++++
3 files changed, 365 insertions(+), 15 deletions(-)
create mode 100644 include/uapi/linux/pwm.h

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index cec325bdffa5..780f401fe380 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -23,9 +23,13 @@

#include <dt-bindings/pwm/pwm.h>

+#include <uapi/linux/pwm.h>
+
#define CREATE_TRACE_POINTS
#include <trace/events/pwm.h>

+#define PWM_MINOR_COUNT 256
+
/* protects access to pwm_chips */
static DEFINE_MUTEX(pwm_lock);

@@ -1960,20 +1964,9 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
}
EXPORT_SYMBOL_GPL(pwm_get);

-/**
- * pwm_put() - release a PWM device
- * @pwm: PWM device
- */
-void pwm_put(struct pwm_device *pwm)
+static void __pwm_put(struct pwm_device *pwm)
{
- struct pwm_chip *chip;
-
- if (!pwm)
- return;
-
- chip = pwm->chip;
-
- guard(mutex)(&pwm_lock);
+ struct pwm_chip *chip = pwm->chip;

/*
* Trigger a warning if a consumer called pwm_put() twice.
@@ -1994,6 +1987,20 @@ void pwm_put(struct pwm_device *pwm)

module_put(chip->owner);
}
+
+/**
+ * pwm_put() - release a PWM device
+ * @pwm: PWM device
+ */
+void pwm_put(struct pwm_device *pwm)
+{
+ if (!pwm)
+ return;
+
+ guard(mutex)(&pwm_lock);
+
+ __pwm_put(pwm);
+}
EXPORT_SYMBOL_GPL(pwm_put);

static void devm_pwm_release(void *pwm)
@@ -2063,6 +2070,276 @@ struct pwm_device *devm_fwnode_pwm_get(struct device *dev,
}
EXPORT_SYMBOL_GPL(devm_fwnode_pwm_get);

+struct pwm_cdev_data {
+ struct pwm_chip *chip;
+ struct pwm_device *pwm[];
+};
+
+static int pwm_cdev_open(struct inode *inode, struct file *file)
+{
+ struct pwm_chip *chip = container_of(inode->i_cdev, struct pwm_chip, cdev);
+ struct pwm_cdev_data *cdata;
+
+ guard(mutex)(&pwm_lock);
+
+ if (!chip->operational)
+ return -ENXIO;
+
+ cdata = kzalloc(struct_size(cdata, pwm, chip->npwm), GFP_KERNEL);
+ if (!cdata)
+ return -ENOMEM;
+
+ cdata->chip = chip;
+
+ file->private_data = cdata;
+
+ return nonseekable_open(inode, file);
+}
+
+static int pwm_cdev_release(struct inode *inode, struct file *file)
+{
+ struct pwm_cdev_data *cdata = file->private_data;
+ unsigned int i;
+
+ for (i = 0; i < cdata->chip->npwm; ++i) {
+ struct pwm_device *pwm = cdata->pwm[i];
+
+ if (pwm) {
+ const char *label = pwm->label;
+
+ pwm_put(cdata->pwm[i]);
+ kfree(label);
+ }
+ }
+ kfree(cdata);
+
+ return 0;
+}
+
+static int pwm_cdev_request(struct pwm_cdev_data *cdata, unsigned int hwpwm)
+{
+ struct pwm_chip *chip = cdata->chip;
+
+ if (hwpwm >= chip->npwm)
+ return -EINVAL;
+
+ if (!cdata->pwm[hwpwm]) {
+ struct pwm_device *pwm = &chip->pwms[hwpwm];
+ const char *label;
+ int ret;
+
+ label = kasprintf(GFP_KERNEL, "pwm-cdev (pid=%d)", current->pid);
+ if (!label)
+ return -ENOMEM;
+
+ ret = pwm_device_request(pwm, label);
+ if (ret < 0) {
+ kfree(label);
+ return ret;
+ }
+
+ cdata->pwm[hwpwm] = pwm;
+ }
+
+ return 0;
+}
+
+static int pwm_cdev_free(struct pwm_cdev_data *cdata, unsigned int hwpwm)
+{
+ struct pwm_chip *chip = cdata->chip;
+
+ if (hwpwm >= chip->npwm)
+ return -EINVAL;
+
+ if (cdata->pwm[hwpwm]) {
+ struct pwm_device *pwm = cdata->pwm[hwpwm];
+ const char *label = pwm->label;
+
+ __pwm_put(pwm);
+
+ kfree(label);
+
+ cdata->pwm[hwpwm] = NULL;
+ }
+
+ return 0;
+}
+
+static struct pwm_device *pwm_cdev_get_requested_pwm(struct pwm_cdev_data *cdata,
+ u32 hwpwm)
+{
+ struct pwm_chip *chip = cdata->chip;
+
+ if (hwpwm >= chip->npwm)
+ return ERR_PTR(-EINVAL);
+
+ if (cdata->pwm[hwpwm])
+ return cdata->pwm[hwpwm];
+
+ return ERR_PTR(-EINVAL);
+}
+
+static long pwm_cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+ int ret = 0;
+ struct pwm_cdev_data *cdata = file->private_data;
+ struct pwm_chip *chip = cdata->chip;
+
+ guard(mutex)(&pwm_lock);
+
+ if (!chip->operational)
+ return -ENODEV;
+
+ switch (cmd) {
+ case PWM_IOCTL_REQUEST:
+ {
+ unsigned int hwpwm = arg;
+
+ return pwm_cdev_request(cdata, hwpwm);
+ }
+
+ case PWM_IOCTL_FREE:
+ {
+ unsigned int hwpwm = arg;
+
+ return pwm_cdev_free(cdata, hwpwm);
+ }
+
+ case PWM_IOCTL_ROUNDWF:
+ {
+ struct pwmchip_waveform cwf;
+ struct pwm_waveform wf;
+ struct pwm_device *pwm;
+
+ ret = copy_from_user(&cwf,
+ (struct pwmchip_waveform __user *)arg,
+ sizeof(cwf));
+ if (ret)
+ return -EFAULT;
+
+ if (cwf.__pad != 0)
+ return -EINVAL;
+
+ pwm = pwm_cdev_get_requested_pwm(cdata, cwf.hwpwm);
+ if (IS_ERR(pwm))
+ return PTR_ERR(pwm);
+
+ wf = (struct pwm_waveform) {
+ .period_length_ns = cwf.period_length_ns,
+ .duty_length_ns = cwf.duty_length_ns,
+ .duty_offset_ns = cwf.duty_offset_ns,
+ };
+
+ ret = pwm_round_waveform_might_sleep(pwm, &wf);
+ if (ret < 0)
+ return ret;
+
+ cwf = (struct pwmchip_waveform) {
+ .hwpwm = cwf.hwpwm,
+ .period_length_ns = wf.period_length_ns,
+ .duty_length_ns = wf.duty_length_ns,
+ .duty_offset_ns = wf.duty_offset_ns,
+ };
+
+ return copy_to_user((struct pwmchip_waveform __user *)arg,
+ &cwf, sizeof(cwf));
+ }
+
+ case PWM_IOCTL_GETWF:
+ {
+ struct pwmchip_waveform cwf;
+ struct pwm_waveform wf;
+ struct pwm_device *pwm;
+
+ ret = copy_from_user(&cwf,
+ (struct pwmchip_waveform __user *)arg,
+ sizeof(cwf));
+ if (ret)
+ return -EFAULT;
+
+ if (cwf.__pad != 0)
+ return -EINVAL;
+
+ pwm = pwm_cdev_get_requested_pwm(cdata, cwf.hwpwm);
+ if (IS_ERR(pwm))
+ return PTR_ERR(pwm);
+
+ ret = pwm_get_waveform_might_sleep(pwm, &wf);
+ if (ret)
+ return ret;
+
+ cwf = (struct pwmchip_waveform) {
+ .hwpwm = cwf.hwpwm,
+ .period_length_ns = wf.period_length_ns,
+ .duty_length_ns = wf.duty_length_ns,
+ .duty_offset_ns = wf.duty_offset_ns,
+ };
+
+ return copy_to_user((struct pwmchip_waveform __user *)arg,
+ &cwf, sizeof(cwf));
+ }
+
+ case PWM_IOCTL_SETROUNDEDWF:
+ case PWM_IOCTL_SETEXACTWF:
+ {
+ struct pwmchip_waveform cwf;
+ struct pwm_waveform wf;
+ struct pwm_device *pwm;
+
+ ret = copy_from_user(&cwf,
+ (struct pwmchip_waveform __user *)arg,
+ sizeof(cwf));
+ if (ret)
+ return -EFAULT;
+
+ if (cwf.__pad != 0)
+ return -EINVAL;
+
+ wf = (struct pwm_waveform){
+ .period_length_ns = cwf.period_length_ns,
+ .duty_length_ns = cwf.duty_length_ns,
+ .duty_offset_ns = cwf.duty_offset_ns,
+ };
+
+ if (!pwm_wf_valid(&wf))
+ return -EINVAL;
+
+ pwm = pwm_cdev_get_requested_pwm(cdata, cwf.hwpwm);
+ if (IS_ERR(pwm))
+ return PTR_ERR(pwm);
+
+ ret = pwm_set_waveform_might_sleep(pwm, &wf,
+ cmd == PWM_IOCTL_SETEXACTWF);
+
+ /*
+ * If userspace cares about rounding deviations it has
+ * to check the values anyhow, so simplify handling for
+ * them and don't signal uprounding. This matches the
+ * behaviour of PWM_IOCTL_ROUNDWF which also returns 0
+ * in that case.
+ * For PWM_IOCTL_SETEXACTWF the return value might be 1
+ * to signal failure in a way that is distinguishable
+ * from other errors.
+ */
+ if (cmd == PWM_IOCTL_SETROUNDEDWF && ret == 1)
+ return 0;
+ return ret;
+ }
+
+ default:
+ return -ENOTTY;
+ }
+}
+
+static const struct file_operations pwm_cdev_fileops = {
+ .open = pwm_cdev_open,
+ .release = pwm_cdev_release,
+ .owner = THIS_MODULE,
+ .unlocked_ioctl = pwm_cdev_ioctl,
+};
+
+static dev_t pwm_devt;
+
/**
* __pwmchip_add() - register a new PWM chip
* @chip: the PWM chip to add
@@ -2115,7 +2392,17 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
scoped_guard(pwmchip, chip)
chip->operational = true;

- ret = device_add(&chip->dev);
+ if (chip->ops->write_waveform) {
+ if (chip->id < PWM_MINOR_COUNT)
+ chip->dev.devt = MKDEV(MAJOR(pwm_devt), chip->id);
+ else
+ dev_warn(&chip->dev, "chip id too high to create a chardev\n");
+ }
+
+ cdev_init(&chip->cdev, &pwm_cdev_fileops);
+ chip->cdev.owner = owner;
+
+ ret = cdev_device_add(&chip->cdev, &chip->dev);
if (ret)
goto err_device_add;

@@ -2166,7 +2453,7 @@ void pwmchip_remove(struct pwm_chip *chip)
idr_remove(&pwm_chips, chip->id);
}

- device_del(&chip->dev);
+ cdev_device_del(&chip->cdev, &chip->dev);
}
EXPORT_SYMBOL_GPL(pwmchip_remove);

@@ -2310,9 +2597,16 @@ static int __init pwm_init(void)
{
int ret;

+ ret = alloc_chrdev_region(&pwm_devt, 0, PWM_MINOR_COUNT, "pwm");
+ if (ret) {
+ pr_err("Failed to initialize chrdev region for PWM usage\n");
+ return ret;
+ }
+
ret = class_register(&pwm_class);
if (ret) {
pr_err("Failed to initialize PWM class (%pe)\n", ERR_PTR(ret));
+ unregister_chrdev_region(pwm_devt, 256);
return ret;
}

diff --git a/include/linux/pwm.h b/include/linux/pwm.h
index bf0469b2201d..d8817afe95dc 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -2,6 +2,7 @@
#ifndef __LINUX_PWM_H
#define __LINUX_PWM_H

+#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/err.h>
#include <linux/module.h>
@@ -309,6 +310,7 @@ struct pwm_ops {
/**
* struct pwm_chip - abstract a PWM controller
* @dev: device providing the PWMs
+ * @cdev: &struct cdev for this device
* @ops: callbacks for this PWM controller
* @owner: module providing this chip
* @id: unique number of this PWM chip
@@ -323,6 +325,7 @@ struct pwm_ops {
*/
struct pwm_chip {
struct device dev;
+ struct cdev cdev;
const struct pwm_ops *ops;
struct module *owner;
unsigned int id;
diff --git a/include/uapi/linux/pwm.h b/include/uapi/linux/pwm.h
new file mode 100644
index 000000000000..1c3dc7a32e2e
--- /dev/null
+++ b/include/uapi/linux/pwm.h
@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */
+
+#ifndef _UAPI_PWM_H_
+#define _UAPI_PWM_H_
+
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+/**
+ * struct pwmchip_waveform - Describe a PWM waveform for a pwm_chip's PWM channel
+ * @hwpwm: per-chip relative index of the PWM device
+ * @__pad: padding, must be zero
+ * @period_length_ns: duration of the repeating period.
+ * A value of 0 represents a disabled PWM.
+ * @duty_length_ns: duration of the active part in each period
+ * @duty_offset_ns: offset of the rising edge from a period's start
+ */
+struct pwmchip_waveform {
+ __u32 hwpwm;
+ __u32 __pad;
+ __u64 period_length_ns;
+ __u64 duty_length_ns;
+ __u64 duty_offset_ns;
+};
+
+/* Reserves the passed hwpwm for exclusive control. */
+#define PWM_IOCTL_REQUEST _IO(0x75, 1)
+
+/* counter part to PWM_IOCTL_REQUEST */
+#define PWM_IOCTL_FREE _IO(0x75, 2)
+
+/*
+ * Modifies the passed wf according to hardware constraints. All parameters are
+ * rounded down to the next possible value, unless there is no such value, then
+ * values are rounded up. Note that zero isn't considered for rounding down
+ * period_length_ns.
+ */
+#define PWM_IOCTL_ROUNDWF _IOWR(0x75, 3, struct pwmchip_waveform)
+
+/* Get the currently implemented waveform */
+#define PWM_IOCTL_GETWF _IOWR(0x75, 4, struct pwmchip_waveform)
+
+/* Like PWM_IOCTL_ROUNDWF + PWM_IOCTL_SETEXACTWF in one go. */
+#define PWM_IOCTL_SETROUNDEDWF _IOW(0x75, 5, struct pwmchip_waveform)
+
+/*
+ * Program the PWM to emit exactly the passed waveform, subject only to rounding
+ * down each value less than 1 ns. Returns 0 on success, 1 if the waveform
+ * cannot be implemented exactly, or other negative error codes.
+ */
+#define PWM_IOCTL_SETEXACTWF _IOW(0x75, 6, struct pwmchip_waveform)
+
+#endif /* _UAPI_PWM_H_ */

base-commit: bde5547f2e87e6c71db79dc41e56aff3061e39a9
--
2.47.2