Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute

[William Breathitt Gray <vilhelm.gray@gmail.com>]

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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.

> > +	if (err)
> > +		return err;
> > +
> > +	raw_spin_lock_irqsave(&counter->events_list_lock, flags);
> > +
> > +	/* Swap in new events queue */
> > +	kfifo_free(&counter->events);
> > +	counter->events.kfifo = events.kfifo;
> > +
> > +	raw_spin_unlock_irqrestore(&counter->events_list_lock, flags);
> > +
> > +	return 0;
> > +}
> > +
> >  static int counter_get_data(struct counter_device *const counter,
> >  			    const struct counter_comp_node *const comp_node,
> >  			    u64 *const value)
> > diff --git a/drivers/counter/counter-chrdev.h b/drivers/counter/counter-chrdev.h
> > index cf5a318fe540..ff7fb0191852 100644
> > --- a/drivers/counter/counter-chrdev.h
> > +++ b/drivers/counter/counter-chrdev.h
> > @@ -12,5 +12,7 @@
> >  int counter_chrdev_add(struct counter_device *const counter,
> >  		       const dev_t counter_devt);
> >  void counter_chrdev_remove(struct counter_device *const counter);
> > +int counter_chrdev_realloc_queue(struct counter_device *const counter,
> > +				 size_t queue_size);
> >  
> >  #endif /* _COUNTER_CHRDEV_H_ */
> > diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
> > index 0cb3dba950bc..9abc821a3871 100644
> > --- a/drivers/counter/counter-sysfs.c
> > +++ b/drivers/counter/counter-sysfs.c
> > @@ -13,6 +13,7 @@
> >  #include <linux/sysfs.h>
> >  #include <linux/types.h>
> >  
> > +#include "counter-chrdev.h"
> >  #include "counter-sysfs.h"
> >  
> >  /**
> > @@ -737,12 +738,30 @@ static int counter_num_counts_read(struct counter_device *counter, u8 *val)
> >  	return 0;
> >  }
> >  
> > +static int counter_events_queue_size_read(struct counter_device *counter,
> > +					  u64 *val)
> > +{
> > +	*val = counter->events.kfifo.mask + 1;
> 
> kfifo_size() rather than open coding it?

Ack.

William Breathitt Gray

> > +	return 0;
> > +}
> > +
> > +static int counter_events_queue_size_write(struct counter_device *counter,
> > +					   u64 val)
> > +{
> > +	return counter_chrdev_realloc_queue(counter, val);
> > +}
> > +
> >  static struct counter_comp counter_num_signals_comp =
> >  	COUNTER_COMP_DEVICE_U8("num_signals", counter_num_signals_read, NULL);
> >  
> >  static struct counter_comp counter_num_counts_comp =
> >  	COUNTER_COMP_DEVICE_U8("num_counts", counter_num_counts_read, NULL);
> >  
> > +static struct counter_comp counter_events_queue_size_comp =
> > +	COUNTER_COMP_DEVICE_U64("events_queue_size",
> > +				counter_events_queue_size_read,
> > +				counter_events_queue_size_write);
> > +
> >  static int counter_sysfs_attr_add(struct counter_device *const counter,
> >  				  struct counter_attribute_group *group)
> >  {
> > @@ -781,6 +800,12 @@ static int counter_sysfs_attr_add(struct counter_device *const counter,
> >  	if (err < 0)
> >  		return err;
> >  
> > +	/* Create num_counts attribute */
> > +	err = counter_attr_create(dev, group, &counter_events_queue_size_comp,
> > +				  scope, NULL);
> > +	if (err < 0)
> > +		return err;
> > +
> >  	/* Create an attribute for each extension */
> >  	for (i = 0; i < counter->num_ext; i++) {
> >  		ext = counter->ext + i;
> 

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