Re: [PATCH RFC net-next] net: phy: add Marvell PHY PTP support

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Tue, Jul 14, 2020 at 05:26:28PM +0100, Russell King wrote:
> Add PTP basic support for Marvell 88E151x PHYs.  These PHYs support
> timestamping the egress and ingress of packets, but does not support
> any packet modification, nor do we support any filtering beyond
> selecting packets that the hardware recognises as PTP/802.1AS.

A question has come up concerning the selection of PTP timestamping
sources within a network device.

I have the situation on a couple of devices where there are multiple
places that can do PTP timestamping:

- the PHY (slow to access, only event capture which may not be wired,
   doesn't seem to synchronise well - delay of 58000, frequency changes
   every second between +/-1500ppb.)
- the Ethernet MAC (fast to access, supports event capture and trigger
   generation which also may not be wired, synchronises well, delay of
   700, frequency changes every second +/- 40ppb.)

How do we deal with this situation - from what I can see from the
ethtool API, we have to make a choice about which to use.  How do we
make that choice?

It's not a case of "just implement one" since hardware may have both
available on a particular ethernet interface or just one available.

Do we need a property to indicate whether we wish to use the PHY
or MAC PTP stamping, or something more elaborate?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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