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

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Date:   Wed, 2 Feb 2022 09:17:57 -0600
From:   David Lechner <david@...hnology.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>,
        William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     David Jander <david@...tonic.nl>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        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()

On 2/2/22 6:32 AM, Oleksij Rempel wrote:
> 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
> 

Couldn't the flush be implicit in the `read()` implementation
instead of requiring a separate sysfs attribute to trigger it?