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 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 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 > > > > Signed-off-by: William Breathitt Gray > > > > --- > > > > 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; William Breathitt Gray