On 26.02.2021 15:26:33, Torin Cooper-Bennun wrote:

On Fri, Feb 26, 2021 at 03:00:54PM +0100, Marc Kleine-Budde wrote:

quoted

https://github.com/hartkopp/M_CAN-User-Manual-History

Great, thanks. I can see this also sits in the linux-can repo which is
handy.

quoted

quoted

    Note: With CAN FD an external counter is required for timestamp
    generation (TSS = “10”)

I suggest to check on the tcan hardware, what's really in the register.

I've just verified that internal timestamping works on the TCAN4550 (in
both RX FIFO and TX event FIFO), with FD and BRS both enabled. The
16-bit timestamps are generated specifically from the nomimal bit times,
and you can prescale up to 16x. \o/

NICE! \o/

Is timestamp-wrapping a concern for rx-offloading?

No, as long as you provide a proper u32 timestamp, which means wrap
around at 0xffffffff to 0x0. As the tcan4x5x has only a 16 bit
timestamp, shift it to full 32 bit, like this:

https://elixir.bootlin.com/linux/v5.11/source/drivers/net/can/flexcan.c#L805

On the mcp with true 32 bit timestamp there's a wraparound every 107
seconds at 40 MHz external clock (0xffffffff / 40000000).

The tcan has the TSS.TCP as a prescaler and increments in bit time,
which is 5MHz max. This results in a wrap around every 13ms (0xffff /
5000000) with a prescaler of 1. With a prescaler of 16, you can increase
this to 209ms (0xffff / 5000000 * 16), which gives an accuracy of 2
bytes.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |