pwm: Add support for pwmchip devices for faster and easier userspace access

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>
Link: https://lore.kernel.org/r/ad4a4e49ae3f8ea81e23cac1ac12b338c3bf5c5b.1746010245.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

2 files changed, 56 insertions, 0 deletions

diff --git a/include/linux/pwm.h b/include/linux/pwm.h
index 63a17d2b4ec8..2492c91452f9 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>
@@ -311,6 +312,7 @@ 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
@@ -325,6 +327,7 @@ 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..182d59cc07ee
--- /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, -EDOM 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_ */