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

[Oliver Hartkopp]

On 16.04.23 17:35, Marc Kleine-Budde wrote:
> On 16.04.2023 14:33:11, Oliver Hartkopp wrote:
> > On 4/14/23 20:20, Marc Kleine-Budde wrote:
> > > On 13.04.2023 17:30:51, Judith Mendez wrote:
> > > > Add a hrtimer to MCAN struct. Each MCAN will have its own
> > > > hrtimer instantiated if there is no hardware interrupt found.
> > > >
> > > > The hrtimer will generate a software interrupt every 1 ms. In
> > >
> > > Are you sure about the 1ms?
>
> I had the 5ms that are actually used in the code in mind. But this is a
> good calculation.

@Judith: Can you acknowledge the value calculation?

> > 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.

> > if there is some FIFO involved, right?
>
> Yes, the mcan core has a FIFO. In the current driver the FIFO
> configuration is done via device tree and fixed after that. And I don't
> know the size of the available RAM in the mcan IP core on that TI SoC.
>
> Marc

Best regards,
Oliver