Re: [PATCH spi for-5.4 0/5] Deterministic SPI latency with NXP DSPI driver

On Thu, Aug 22, 2019 at 05:58:49PM +0300, Vladimir Oltean wrote:

I don't think I understand the problem here.

On the contrary, I do.

You'd have something like this:

Master (DSA master port)         Slave (switch CPU port)

    |                            |         Tstamps known
    |                            |         to slave
    |       Local_sync_req       |
 t1 |------\                     |         t1
    |       \-----\              |
    |              \-----\       |
    |                     \----->| t2      t1, t2
    |                            |
    |     Local_sync_resp /------| t3      t1, t2, t3
    |              /-----/       |
    |       /-----/              |
 t4 |<-----/                     |         t1, t2, t3, t4
    |                            |
    |                            |
    v           time             v

And who generates Local_sync_resp?

Also, what sort of frame is it?  PTP has no Sync request or response.

But you don't mean a TX timestamp at the egress of swp4 here, do you?

Yes, I do.
 

Why would that matter?

Because in order to synchronize to an external GM, you need to measure
two things:

1. the (unchanging) delay from MAC to MAC
2. the (per-packet) switch residence time

Thanks,
Richard