Re: [PATCH net-next] net: ethtool: allow MAC drivers to override ethtool get_ts_info

On Thu, Jan 14, 2021 at 10:38:00PM +0000, Russell King - ARM Linux admin wrote:

So, I think the only way to prevent a regression with the code as
it is today is that we _never_ support PTP on Marvell PHYs - because
doing so _will_ break the existing MVPP2 driver's implementation and
cause a regression.

The situation isn't as bad as it seems.

For one thing, mvpp2 incorrectly selects NETWORK_PHY_TIMESTAMPING.
It really shouldn't.  I'm submitting a fix soon.

As long as the new PHY driver (or at least the PTP bit) depends on
NETWORK_PHY_TIMESTAMPING, then that allows users who _really_ want
that to enable the option at compile time.  This option adds extra
checks into the networking hot path, and almost everyone should avoid
enabling it.

Right now, there is no option: if a PHY supports PTP, then the only
option is to use the PHYs PTP. Which is utterly rediculous.

Unless you can see a way around it. Because I can't.

I still think the original and best method is to hide the two (and
with your new driver, three) esoteric PHY time stamping drivers behind
a Kconfig option, and structure the code to favor PHY devices.

The idea to favor the MACs, from back in July, doesn't really change
the fundamental limitations that

- MAC and PHY time stamping cannot work simultaneously, and

- Users of PHY devices (representing a tiny minority) must enable the
  otherwise uninteresting NETWORK_PHY_TIMESTAMPING option at compile
  time.

Thanks,
Richard