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.rem...@pengutronix.de>
Signed-off-by: William Breathitt Gray <vilhelm.g...@gmail.com>
---
 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 57b051ba53a9..a97d85e97184 100644
--- a/Documentation/ABI/testing/sysfs-bus-counter
+++ b/Documentation/ABI/testing/sysfs-bus-counter
@@ -224,6 +224,14 @@ Description:
                Read-only attribute that indicates the component ID of the
                respective extension or Synapse.
 
+What:          /sys/bus/counter/devices/counterX/events_queue_size
+KernelVersion: 5.13
+Contact:       linux-...@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-...@vger.kernel.org
diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
index d5f92b2d3bb4..d9f123ca0ed6 100644
--- a/drivers/counter/counter-chrdev.c
+++ b/drivers/counter/counter-chrdev.c
@@ -311,6 +311,9 @@ 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;
 
@@ -327,6 +330,7 @@ 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;
-- 
2.30.2

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