linux-kernel - Re: [PATCH v1] counter: interrupt-cnt: add counter_push

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Date:   Wed, 2 Feb 2022 13:32:41 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     David Jander <david@...tonic.nl>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        David Lechner <david@...hnology.com>,
        linux-iio@...r.kernel.org,
        Robin van der Gracht <robin@...tonic.nl>,
        linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH v1] counter: interrupt-cnt: add counter_push_event()

Hi William,

On Sat, Dec 25, 2021 at 01:07:44PM +0900, William Breathitt Gray wrote:
... 
> So the counter_push_event() function interacts with two spinlocks:
> events_list_lock and events_in_lock. The events_list_lock spinlock is
> necessary because userspace can modify the events_list list via the
> counter_enable_events() and counter_disable_events() functions. The
> events_in_lock spinlock is necessary because userspace can modify the
> events kfifo via the counter_events_queue_size_write() function.
> 
> A lockless solution for this might be possible if the driver maintains
> its own circular buffer as you suggest. The driver's IRQ handler can
> write to this circular buffer without calling the counter_push_event()
> function, and then flush the buffer to the Counter character device via
> a userspace write to a "flush_events" sysfs attribute or similar; this
> eliminates the need for the events_in_lock spinlock. The state of the
> events_list list can be captured in the driver's events_configure()
> callback and stored locally in the driver for reference, thus
> eliminating the need for the events_list_lock; interrupts can be
> disabled before the driver's local copy of events_list is modified.
> 
> With only one reader and one writer operating on the driver's buffer,
> you can use the normal kfifo_in and kfifo_out calls for lockless
> operations. Perhaps that is a way forward for this problem.

As proof of concept, I implemented the double buffered version with the
sysfs flush_events interface. Currently it feels kind of wired, I use
poll and wait until it timeouts to run the sysfs_flush_counter() to
trigger new data.

Here is example:
int main(void)
{
	ret = sysfs_enable_counter();
	...

	fd = open("/dev/counter0", O_RDWR);
	...

	ret = ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
	...

	ret = ioctl(fd, COUNTER_ENABLE_EVENTS_IOCTL);
	...

	for (;;) {
		struct pollfd fds[] = {
			{
				.fd = fd,
				.events = POLLIN,
			},
		};
		ssize_t i;

		/* wait for 10 sec */
		ret = poll(fds, ARRAY_SIZE(fds), DEFAULT_TIMEOUT_MS);
		if (ret == -EINTR)
			continue;
		else if (ret < 0)
			return -errno;
		else if (ret == 0) {
			sysfs_flush_counter(); <---- request to flush queued events from the driver
			continue;
		}

		ret = read(fd, event_data, sizeof(event_data));
		...

		for (i = 0; i < ret / (ssize_t)sizeof(event_data[0]); i++)
			/* process event */
			....
		}
	}

	return ret;
}

If it is still the only way to go, I'll send kernel patches.

Regards,
Oleksij
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