Re: [RFC PATCH 5/5] can: m_can: Add hrtimer to generate software interrupt

[Mendez, Judith]

Hello Marc,

On 4/19/2023 1:13 AM, Marc Kleine-Budde wrote:
> 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.

Yes, it was a mistake, will send out a respin with the correct value, 
thanks.

regards,
Judith