Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute
From: David Lechner
Date: Fri Feb 26 2021 - 19:15:38 EST
On 2/25/21 6:03 PM, William Breathitt Gray wrote:
On Sun, Feb 21, 2021 at 03:51:40PM +0000, Jonathan Cameron wrote:
On Thu, 18 Feb 2021 19:32:16 +0900
William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote:
On Fri, 12 Feb 2021 21:13:44 +0900
William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
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 | 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@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 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;
William Breathitt Gray
Wouldn't EBUSY make more sense?