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

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Date:   Thu, 14 Jan 2021 13:22:17 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: ethtool: allow MAC drivers to override
 ethtool get_ts_info

On Thu, Jan 14, 2021 at 04:55:06AM -0800, Richard Cochran wrote:
> On Sun, Jan 10, 2021 at 11:13:44AM +0000, Russell King wrote:
> > This allows network drivers such as mvpp2 to use their more accurate
> > timestamping implementation than using a less accurate implementation
> > in the PHY. Network drivers can opt to defer to phylib by returning
> > -EOPNOTSUPP.
> 
> My expectation is that PHY time stamping is more accurate than MAC
> time stamping.

PHY time stamping may be a "more accurate" location to get timestamps,
in terms of the hardware, but when you consider the entire setup, that
is not necessarily the case.

> > This change will be needed if the Marvell PHY drivers add support for
> > PTP.
> 
> Huh? The mvpp2 appears to be a MAC.  If this device has integrated
> PHYs then I don't see the issue.  If your board has the nvpp2 device
> with the dp83640 PHYTER, then don't you want to actually use the
> PHYTER?

You seem to be adding more information way beyond what I'm saying.

No, there aren't integrated PHYs. There's an external PHY - a Marvell
88e151x which has what I would call rudimentary stamping abilities,
whereas the mvpp2 has advanced stamping abilities.

You implemented the Marvell timestamping in DSA, so you know what the
Marvell offering there looks like and what it is capable of. That same
hardware appears in some Marvell PHYs.

The mvpp2 hardware (which has support already merged after you acked
the TAI part, and failed to provide comments on the mvpp2 part - so
davem gave up waiting) is capable of:
- stamping every received packet irrespective of its type
- stamping any transmitted packet or only those we wish to stamp
- inserting a timestamp into the packet (aka one-step, although that
  isn't implemented that yet)
- correcting the hardware time counter tick rate

There is considerable noise in reading the hardware timestamp counter
from the PHYs - caused by latency over the MDIO bus, which makes the
achievable accuracy lower. That noise is very much reduced when reading
the hardware timestamp counter from mvpp2 - where we can implement
mvpp22_tai_gettimex64(). Therefore, the achieved accuracy from mvpp2 is
higher than from a PHY.

We had already discussed this patch last year, and you agreed with it
then. What has changed?

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