On Sat, Feb 27, 2021 at 03:18:47PM +0000, Jonathan Cameron wrote:

On Fri, 26 Feb 2021 09:03:48 +0900
William Breathitt Gray [off-list ref] wrote:

quoted

On Sun, Feb 21, 2021 at 03:51:40PM +0000, Jonathan Cameron wrote:

quoted

On Thu, 18 Feb 2021 19:32:16 +0900
William Breathitt Gray [off-list ref] wrote:
  

quoted

On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote:  

quoted

On Fri, 12 Feb 2021 21:13:44 +0900
William Breathitt Gray [off-list ref] wrote:
    

quoted

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 <redacted>
---
 Documentation/ABI/testing/sysfs-bus-counter |  8 +++++++
 drivers/counter/counter-chrdev.c            | 23 +++++++++++++++++++
 drivers/counter/counter-chrdev.h            |  2 ++
 drivers/counter/counter-sysfs.c             | 25 +++++++++++++++++++++
 4 files changed, 58 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
index 847e96f19d19..f6cb2a8b08a7 100644
--- a/Documentation/ABI/testing/sysfs-bus-counter
+++ b/Documentation/ABI/testing/sysfs-bus-counter

@@ -212,6 +212,14 @@ Description:
 		both edges:
 			Any state transition.
 
+What:		/sys/bus/counter/devices/counterX/events_queue_size
+KernelVersion:	5.13
+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

diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
index 16f02df7f73d..53eea894e13f 100644
--- a/drivers/counter/counter-chrdev.c
+++ b/drivers/counter/counter-chrdev.c

@@ -375,6 +375,29 @@ void counter_chrdev_remove(struct counter_device *const counter)
 	cdev_del(&counter->chrdev);
 }
 
+int counter_chrdev_realloc_queue(struct counter_device *const counter,
+				 size_t queue_size)
+{
+	int err;
+	DECLARE_KFIFO_PTR(events, struct counter_event);
+	unsigned long flags;
+
+	/* Allocate new events queue */
+	err = kfifo_alloc(&events, queue_size, GFP_ATOMIC);    

Is there any potential for losing events?    

We take the events_list_lock down below so we're safe against missing an
event, but past events currently unread in the queue will be lost.

Shortening the size of the queue is inherently a destructive process if
we have more events in the current queue than can fit in the new queue.
Because we a liable to lose some events in such a case, I think it's
best to keep the behavior of this reallocation consistent and have it
provide a fresh empty queue every time, as opposed to sometimes dropping
events and sometimes not.

I also suspect an actual user would be setting the size of their queue
to the required amount before they begin watching events, rather than
adjusting it sporadically during a live operation.
 

Absolutely agree.   As such I wonder if you are better off enforcing this
behaviour?  If the cdev is open for reading, don't allow the fifo to be
resized. 

Jonathan  

I can't really think of a good reason not to, so let's enforce it: if
the cdev is open, then we'll return an EINVAL if the user attempts to
resize the queue.

What is a good way to check for this condition? Should I just call
kref_read() and see if it's greater than 1? For example, in
counter_chrdev_realloc_queue():

	if (kref_read(&counter->dev.kobj.kref) > 1)
		return -EINVAL;

In theory at least you might want the kobj.kref to be incremented
for other reasons than just open.   So to keep different concepts
separate, perhaps it's worth a separate variable somewhere to
track whether the file is open currently.

However, it's reasonable (I think) to assume the kref will have a
minimum value if open, so perhaps what you suggest works fine.

Jonathan

I noticed an open() operation could occur right after this check, so
we'll need a mutex here to ensure the the queue size is not modified
during use. Because of that, I'll create a separate variable to track
this and use a mutex_trylock() instead of the kref to test whether to
return -EBUSY.

William Breathitt Gray