Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute
From: William Breathitt Gray <hidden>
Date: 2021-02-26 00:05:08
Also in:
linux-iio, lkml
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 [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.orgdiff --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