On Thu, 25 Nov 2021 10:58:23 +0900 William Breathitt Gray <vilhelm.gray@xxxxxxxxx> 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 > > -- > > 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 | > > 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. Also something I am worried about, is the overhead it creates to generate such an event on each and every IRQ. Looking at counter_push_event(), I can see it using a spin-lock, besides quite a bit of code potentially being executed, depending on user-space. The lock can probably be held by non-IRQ code also, which can potentially introduce more latency and cause high-frequency interrupts to be delayed far too long. This particular driver uses atomic_inc() to increment a counter, which AFAIK on most machines should be a single instruction. The main application for this driver is to count pulses _fast_ with minimal CPU load. IMHO we should do better than potentially blocking on a spin-lock in IRQ context. I know this is akin to trying to do hard-real-time stuff in the kernel, but since its main application is for embedded systems that have a known and controllable interrupt environment most of the time, this can be done if one is careful to not do certain things in IRQ context, such as using locks. > 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. The original version of this driver used a circular buffer that stored the timestamps of the last 'n' interrupts. A user-space read action would copy this buffer repeatedly (max tries --> fail) until two copies are identical to ensure integrity avoiding the use of locks. This is of course dead ugly and I was hoping for a better solution. But to be better IMHO it must avoid locks in IRQ context at all costs. Having a sample 'n' consecutive of time-stamps in user-space, made frequency calculation, filtering and glitch detection quite simple. > 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. Best regards, -- David Jander Protonic Holland.
