On Mon, Dec 27, 2021 at 09:16:31AM -0600, David Lechner wrote: > 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 > >>>>>>>>> -- > >>>>>>>>> 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. > >>>>>>>> > >>>>>>>> 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. I think that could work. If IRQs are always disabled before events_list is modified, then there is never a risk of interacting with an invalid events_list and thus counter_push_events() won't need spinlocks. When IRQs are disabled we miss any possible events, but we are missing those already anyway when the events_list is modified. William Breathitt Gray
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