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SubjectRe: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute
On Thu, 18 Feb 2021 19:32:16 +0900
William Breathitt Gray <vilhelm.gray@gmail.com> 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@gmail.com> 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@pengutronix.de>
> > > Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
> > > ---
> > > 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


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