counter: Implement events_queue_size sysfs attribute

The events_queue_size sysfs attribute provides a way for users to
dynamically configure the Counter events queue size for the Counter
character device interface. The size is in number of struct
counter_event data structures. The number of elements will be rounded-up
to a power of 2 due to a requirement of the kfifo_alloc function called
during reallocation of the queue.

Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>

Documentation/ABI/testing/sysfs-bus-counter

@@ -233,6 +233,14 @@ Description:
 		shorter or equal to configured value are ignored. Value 0 means
 		filter is disabled.
 
+What:		/sys/bus/counter/devices/counterX/events_queue_size
+KernelVersion:	5.15
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Size of the Counter events queue in number of struct
+		counter_event data structures. The number of elements will be
+		rounded-up to a power of 2.
+
 What:		/sys/bus/counter/devices/counterX/name
 KernelVersion:	5.2
 Contact:	linux-iio@vger.kernel.org

drivers/counter/counter-chrdev.c

@@ -3,6 +3,7 @@
  * Generic Counter character device interface
  * Copyright (C) 2020 William Breathitt Gray
  */
+#include <linux/bitops.h>
 #include <linux/cdev.h>
 #include <linux/counter.h>
 #include <linux/err.h>
@@ -336,6 +337,9 @@ static int counter_chrdev_open(struct inode *inode, struct file *filp)
 							    typeof(*counter),
 							    chrdev);
 
+	if (test_and_set_bit_lock(0, counter->chrdev_lock))
+		return -EBUSY;
+
 	get_device(&counter->dev);
 	filp->private_data = counter;
 
@@ -352,6 +356,7 @@ static int counter_chrdev_release(struct inode *inode, struct file *filp)
 		return err;
 
 	put_device(&counter->dev);
+	clear_bit_unlock(0, counter->chrdev_lock);
 
 	return 0;
 }
@@ -376,6 +381,7 @@ int counter_chrdev_add(struct counter_device *const counter)
 	mutex_init(&counter->events_lock);
 
 	/* Initialize character device */
+	clear_bit(0, counter->chrdev_lock);
 	cdev_init(&counter->chrdev, &counter_fops);
 
 	/* Allocate Counter events queue */

drivers/counter/counter-sysfs.c

@@ -3,11 +3,13 @@
  * Generic Counter sysfs interface
  * Copyright (C) 2020 William Breathitt Gray
  */
+#include <linux/bitops.h>
 #include <linux/counter.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/gfp.h>
 #include <linux/kernel.h>
+#include <linux/kfifo.h>
 #include <linux/kstrtox.h>
 #include <linux/list.h>
 #include <linux/string.h>
@@ -785,12 +787,49 @@ static int counter_num_counts_read(struct counter_device *counter, u8 *val)
 	return 0;
 }
 
+static int counter_events_queue_size_read(struct counter_device *counter,
+					  u64 *val)
+{
+	*val = kfifo_size(&counter->events);
+	return 0;
+}
+
+static int counter_events_queue_size_write(struct counter_device *counter,
+					   u64 val)
+{
+	DECLARE_KFIFO_PTR(events, struct counter_event);
+	int err = 0;
+
+	/* Verify chrdev is not currently being used */
+	if (test_and_set_bit_lock(0, counter->chrdev_lock))
+		return -EBUSY;
+
+	/* Allocate new events queue */
+	err = kfifo_alloc(&events, val, GFP_KERNEL);
+	if (err)
+		goto exit_early;
+
+	/* Swap in new events queue */
+	kfifo_free(&counter->events);
+	counter->events.kfifo = events.kfifo;
+
+exit_early:
+	clear_bit_unlock(0, counter->chrdev_lock);
+
+	return err;
+}
+
 static struct counter_comp counter_num_signals_comp =
 	COUNTER_COMP_DEVICE_U8("num_signals", counter_num_signals_read, NULL);
 
 static struct counter_comp counter_num_counts_comp =
 	COUNTER_COMP_DEVICE_U8("num_counts", counter_num_counts_read, NULL);
 
+static struct counter_comp counter_events_queue_size_comp =
+	COUNTER_COMP_DEVICE_U64("events_queue_size",
+				counter_events_queue_size_read,
+				counter_events_queue_size_write);
+
 static int counter_sysfs_attr_add(struct counter_device *const counter,
 				  struct counter_attribute_group *cattr_group)
 {
@@ -829,6 +868,12 @@ static int counter_sysfs_attr_add(struct counter_device *const counter,
 	if (err < 0)
 		return err;
 
+	/* Create events_queue_size attribute */
+	err = counter_attr_create(dev, cattr_group,
+				  &counter_events_queue_size_comp, scope, NULL);
+	if (err < 0)
+		return err;
+
 	/* Create an attribute for each extension */
 	for (i = 0; i < counter->num_ext; i++) {
 		ext = &counter->ext[i];

include/linux/counter.h

@@ -297,6 +297,7 @@ struct counter_ops {
  * @events:		queue of detected Counter events
  * @events_wait:	wait queue to allow blocking reads of Counter events
  * @events_lock:	lock to protect Counter events queue read operations
+ * @chrdev_lock:	lock to limit chrdev to a single open at a time
  */
 struct counter_device {
 	const char *name;
@@ -323,6 +324,7 @@ struct counter_device {
 	DECLARE_KFIFO_PTR(events, struct counter_event);
 	wait_queue_head_t events_wait;
 	struct mutex events_lock;
+	DECLARE_BITMAP(chrdev_lock, 1);
 };
 
 int counter_register(struct counter_device *const counter);