Re: [PATCH v1] counter: interrupt-cnt: add counter_push_event()
From: David Lechner
Date: Mon Dec 27 2021 - 10:16:42 EST
On 12/24/21 10:07 PM, William Breathitt Gray wrote:
On Wed, Dec 15, 2021 at 10:08:53AM +0100, David Jander wrote:
Dear William,
On Wed, 15 Dec 2021 17:48:26 +0900
William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
On Wed, Dec 08, 2021 at 05:10:35PM +0100, David Jander wrote:
Dear Uwe,
On Wed, 8 Dec 2021 14:59:02 +0100
Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
Hello David,
On Tue, Dec 07, 2021 at 08:16:02AM +0100, David Jander wrote:
On Mon, 6 Dec 2021 13:24:18 -0600
David Lechner <david@xxxxxxxxxxxxxx> wrote:
On 11/24/21 7:58 PM, William Breathitt Gray wrote:
On Wed, Nov 24, 2021 at 08:27:20AM +0100, Oleksij Rempel wrote:
Hi William,
On Wed, Nov 24, 2021 at 03:09:05PM +0900, William Breathitt Gray wrote:
On Tue, Nov 23, 2021 at 02:45:40PM +0100, Oleksij Rempel wrote:
Add counter_push_event() to notify user space about new pulses
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
---
drivers/counter/interrupt-cnt.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/counter/interrupt-cnt.c b/drivers/counter/interrupt-cnt.c
index 8514a87fcbee..b237137b552b 100644
--- a/drivers/counter/interrupt-cnt.c
+++ b/drivers/counter/interrupt-cnt.c
@@ -31,6 +31,8 @@ static irqreturn_t interrupt_cnt_isr(int irq, void *dev_id)
atomic_inc(&priv->count);
+ counter_push_event(&priv->counter, COUNTER_EVENT_OVERFLOW, 0);
+
return IRQ_HANDLED;
}
--
2.30.2
Hi Oleksij,
It looks like this is pushing a COUNTER_EVENT_OVERFLOW event every time
an interrupt is handled, which I suspect is not what you want to happen.
The COUNTER_EVENT_OVERFLOW event indicates a count value overflow event,
so you'll need to check for a count value overflow before pushing the
event.
It would be good idea to implement a ceiling extension as well (you can
use the COUNTER_COMP_CEILING() macro) so that users can configure the
particular point where the value overflows.
Thank you!
What would be the best and resource effective strategy for periodically
getting frequency of interrupts/pulses? This is actual information which is
needed for my use case.
So far, I was pushing every event to the user space, which is working
but probably not the most resource effective method of doing it.
Regards,
Oleskij
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We could introduce a new Counter change-of-state event type which would
trigger whenever the count value changes, but I agree with you that this
is likely not the best way for us to derive the frequency of the
interrupts due to the indirection of handling and parsing the event
data.
Instead, perhaps introducing a "frequency" or "period" Count extension
would make more sense here. This extension could report the value delta
between counts, or alternatively the time delta from which you can
derive frequency. Regarding implementation, you can store the previous
value in a variable, updating it whenever an interrupt occurs, and
compute the particular delta every time a read is requested by the user.
David Lechner is implementing something similar for the TI eQEP driver
to expose speed, so I'm CCing them here in case this is of interest to
them.
Based on my experience, I would recommend that counter drivers be as
"thin" as possible. They shouldn't try to provide any information that
the hardware itself doesn't provide. In other words, the kernel should
provide userspace the information needed to calculate the speed/rate
but not try to do the actual calculation in the kernel. Inevitably
there are nuances for specific use cases that can't all possibly be
handled by such an implementation.
I completely agree with this. While interrupts aren't really meant for
measuring frequency, and this being somewhat of a mis-use of something, it is
still possible to do and very useful in many cases. That said, while the
counter framework is AFAIK the best fit for this, the main use-case for this
driver is measuring wheel speed (and similar "speeds"). For this, the minimum
amount of information the driver needs to provide user-space with to do
reliable calculations, is high-resolution time-stamps of GPIO events. A simple
counter is not suited, because there can be glitches that need to be detected.
If user-space gets a buffer full of consecutive time-stamps (don't need to be
all of them, just a sample of n consecutive timestamps), as well as total
count, all needed calculations, glitch filtering, low-pass filtering, etc...
can be done in user-space just fine.
I've tried using gpio interrupts to try to calculate speed/rate in
the kernel before and it simply doesn't work reliably. Interrupts
get missed and the calculation will be off.
Exactly. Been there, done that.
For reliable speed calculations of a mechanical system, the properties of the
mechanical system need to be known, like physical limits of accelerations,
maximum (or minimum) speed, etc. The minimum set of input data needed by a
user-space application to do these calculations is total pulse count in
addition to a buffer of timestamps of n consecutive input events (raising or
falling edges on GPIO). So IMHO this is what the driver should provide, and
in the most resource-efficient way possible. This particular driver will be
used 3 times on the same SoC, with each up to 10-15k pulses per second. That
is a lot of interrupts for an embedded system, so they better consume as
little resources as possible. Filling a ring buffer with timestamps should be
possible, as long as no locking is involved. Locks in IRQ context must be
avoided at all costs, specially in this case.
For really slow counts (i.e. 1 count/second), I can see a use for
generating an event on each count though. For high rates, I would
just read the count every 100ms in usespace and divide the change in
the number of counts by the time period to get the rate.
For slow counts, I agree, but for high rates, I don't (see above). There can
be glitches and false events that can (and must) be effectively filtered out.
For that user-space needs to know the time of each event during the
measurement period.
No sure I understood the problem here. If you keep the driver as is and
in userspace just read out the counter value twice and measure the time
between the reads[1], you can calculate the average frequency of the
event in userspace.
Isn't that good enough?
No, I'm afraid it isn't, for two reasons:
1. These counters are often used in environments, where glitches can and do
happen. So sometimes there are fake events. The only way to tell fake from
real pulses, is to filter them. Unfortunately you need the timestamps of each
event for that.
2. Another reason for having time-stamps is the case when the frequency is low
and one still requires fast accurate measurements. In that case timestamps
provide a way of accurately measuring period time.
Best regards,
--
David Jander
Protonic Holland.
Keeping drivers focused on just exposing the hardware data and
functionality is likely the best path to choose, so my earlier
suggestion of a "frequency" extension would better be left for userspace
to handle.
I agree.
So in order to enable userspace to derive frequency, you need reliable
timestamps for enough consecutive events to provide an adequate size
sample of data on which to perform filtering and other such operations.
Ack.
If we add a COUNTER_EVENT_CHANGE_OF_STATE or similar, every count change
will generate an event with a logged timestamp. Is the problem with this
solution primarily that the Counter event queue is currently utilizing
spinlocks for synchronization?
Yes. Basically, since one can expect a very high amount of IRQs, it seems
paramount to eliminate any source of latency (spinlocks, etc...) from
interrupt context as well as to keep CPU load as low as technically possible.
If a spinlock is used, and at 10kHz pulses, on a moderately fast embedded SoC,
it is IMHO quite possible to have user-space cause the spinlock to be held for
more than 100 microseconds, thus causing a pulse to be missed. Not to mention
slight jitter introduced to the timestamps that can cause user-space to falsely
filter out events (a software PLL that doesn't correctly lock).
The ideal ISR in this case would only take a timestamp from a hardware TSC (or
similarly latency-free direct source) and put it into a circular buffer
without using locks, and maybe increase an unsigned long counter value (atomic
operation if MB's are correctly used) and nothing else.
If, for example, such a solution would require user-space access CPU
load (complexity) to increase by a factor of 10 or even more (in order to
avoid locks), this is likely still preferable, since the ISR is executed maybe
1000+ times more often than user-space accessing the driver.
Best regards,
--
David Jander
Protonic Holland.
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.
Would it be possible to make it so that userspace can't modify the
events_list when IRQs are enabled? Then we wouldn't have to add asecond event buffer.
IIRC, the only operations that modify events_list are when another
list replaces events_list when events are enabled and when
events_list is cleared when events are disabled. So as long as the
ordering is right with respect to enabling and disabling IRQs, it
seems like the spin lock should not be needed.