[PATCH v12 15/17] counter: Implement events_queue_size sysfs attribute

[William Breathitt Gray]

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@xxxxxxxxxxxxxx>
Signed-off-by: William Breathitt Gray <vilhelm.gray@xxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-bus-counter |  8 ++++
 drivers/counter/counter-chrdev.c            |  4 ++
 drivers/counter/counter-sysfs.c             | 44 +++++++++++++++++++++
 include/linux/counter.h                     |  2 +
 4 files changed, 58 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
index e0e99adb0ecc..84ebb1ed28ed 100644
--- a/Documentation/ABI/testing/sysfs-bus-counter
+++ b/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@xxxxxxxxxxxxxxx
+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@xxxxxxxxxxxxxxx
diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
index 92805b1f65b8..13644c87d02a 100644
--- a/drivers/counter/counter-chrdev.c
+++ b/drivers/counter/counter-chrdev.c
@@ -323,6 +323,9 @@ static int counter_chrdev_open(struct inode *inode, struct file *filp)
 							    typeof(*counter),
 							    chrdev);
 
+	if (!mutex_trylock(&counter->chrdev_lock))
+		return -EBUSY;
+
 	get_device(&counter->dev);
 	filp->private_data = counter;
 
@@ -339,6 +342,7 @@ static int counter_chrdev_release(struct inode *inode, struct file *filp)
 		return err;
 
 	put_device(&counter->dev);
+	mutex_unlock(&counter->chrdev_lock);
 
 	return 0;
 }
diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
index eb1505bfbd89..bf038eff4587 100644
--- a/drivers/counter/counter-sysfs.c
+++ b/drivers/counter/counter-sysfs.c
@@ -8,7 +8,9 @@
 #include <linux/err.h>
 #include <linux/gfp.h>
 #include <linux/kernel.h>
+#include <linux/kfifo.h>
 #include <linux/list.h>
+#include <linux/mutex.h>
 #include <linux/string.h>
 #include <linux/sysfs.h>
 #include <linux/types.h>
@@ -782,12 +784,48 @@ 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)
+{
+	int err;
+	DECLARE_KFIFO_PTR(events, struct counter_event);
+
+	/* Verify chrdev is not currently being used */
+	if (!mutex_trylock(&counter->chrdev_lock))
+		return -EBUSY;
+
+	/* Allocate new events queue */
+	err = kfifo_alloc(&events, val, GFP_ATOMIC);
+	if (err)
+		return err;
+
+	/* Swap in new events queue */
+	kfifo_free(&counter->events);
+	counter->events.kfifo = events.kfifo;
+
+	mutex_unlock(&counter->chrdev_lock);
+
+	return 0;
+}
+
 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 *group)
 {
@@ -826,6 +864,12 @@ static int counter_sysfs_attr_add(struct counter_device *const counter,
 	if (err < 0)
 		return err;
 
+	/* Create num_counts attribute */
+	err = counter_attr_create(dev, 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;
diff --git a/include/linux/counter.h b/include/linux/counter.h
index 3f0bbe4ff702..854fcaf49c32 100644
--- a/include/linux/counter.h
+++ b/include/linux/counter.h
@@ -289,6 +289,7 @@ struct counter_ops {
  * @priv:		optional private data supplied by driver
  * @dev:		internal device structure
  * @chrdev:		internal character device structure
+ * @chrdev_lock:	lock to limit chrdev to a single open at a time
  * @events_list:	list of current watching Counter events
  * @events_list_lock:	lock to protect Counter events list operations
  * @next_events_list:	list of next watching Counter events
@@ -314,6 +315,7 @@ struct counter_device {
 
 	struct device dev;
 	struct cdev chrdev;
+	struct mutex chrdev_lock;
 	struct list_head events_list;
 	spinlock_t events_list_lock;
 	struct list_head next_events_list;
-- 
2.32.0