On 18.04.2023 15:59:57, Mendez, Judith wrote: > > > > > > > The "shortest" 11 bit CAN ID CAN frame is a Classical CAN frame with DLC = 0 > > > > > > > and 1 Mbit/s (arbitration) bitrate. This should be 48 bits @1Mbit => ~50 > > > > > > > usecs > > > > > > > > > > > > > > So it should be something about > > > > > > > > > > > > > > 50 usecs * (FIFO queue len - 2) > > > > > > > > > > > > Where does the "2" come from? > > > > > > > > > > I thought about handling the FIFO earlier than it gets completely "full". > > > > > > > > > > The fetching routine would need some time too and the hrtimer could also > > > > > jitter to some extend. > > > > > > > > I was assuming something like this. > > > > > > > > I would argue that the polling time should be: > > > > > > > > 50 µs * FIFO length - IRQ overhead. > > > > > > > > The max IRQ overhead depends on your SoC and kernel configuration. > > > > > > I just tried an educated guess to prevent the FIFO to be filled up > > > completely. How can you estimate the "IRQ overhead"? And how do you catch > > > the CAN frames that are received while the IRQ is handled? > > > > We're talking about polling, better call it "overhead" or "latency from > > timer expiration until FIFO has at least one frame room". This value > > depends on your system. > > > > It depends on many, many factors, SoC, Kernel configuration (preempt RT, > > powersaving, frequency scaling, system load. In your example it's 100 > > µs. I wanted to say there's an overhead (or latency) and we need enough > > space in the FIFO, to cover it. > > > > I am not sure how to estimate IRQ overhead, but FIFO length should be 64 > elements. Ok > 50 us * 62 is about 3.1 ms and we are using 1 ms timer polling interval. Sounds good. > Running a few benchmarks showed that using 0.5 ms timer polling interval > starts to take a toll on CPU load, that is why I chose 1 ms polling > interval. However in the code you use 5 ms. Marc -- Pengutronix e.K. | Marc Kleine-Budde | Embedded Linux | https://www.pengutronix.de | Vertretung Nürnberg | Phone: +49-5121-206917-129 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-9 |
Attachment:
signature.asc
Description: PGP signature
