Messages in this thread | | | Date | Wed, 2 Feb 2022 13:32:41 +0100 | From | Oleksij Rempel <> | Subject | Re: [PATCH v1] counter: interrupt-cnt: add counter_push_event() |
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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 -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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