netdev - Re: [PATCH RFC net-next] net: phy: add Marvell PHY PTP support [multicast/DSA issues]

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Message-ID: <20230302180616.7bcfc1ef@kmaincent-XPS-13-7390>
Date:   Thu, 2 Mar 2023 18:06:16 +0100
From:   Köry Maincent <kory.maincent@...tlin.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Richard Cochran <richardcochran@...il.com>, andrew@...n.ch,
        davem@...emloft.net, f.fainelli@...il.com, hkallweit1@...il.com,
        netdev@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH RFC net-next] net: phy: add Marvell PHY PTP support
 [multicast/DSA issues]

On Thu, 2 Mar 2023 08:49:32 -0800
Jakub Kicinski <kuba@...nel.org> wrote:

> On Thu, 2 Mar 2023 11:49:26 +0000 Russell King (Oracle) wrote:
> > (In essence, because of all the noise when trying the Marvell PHY with
> > ptp4l, I came to the conlusion that NTP was a far better solution to
> > time synchronisation between machines than PTP would ever be due to
> > the nose induced by MDIO access. However, I should also state that I
> > basically gave up with PTP in the end because hardware support is
> > overall poor, and NTP just works - and I'd still have to run NTP for
> > the machines that have no PTP capabilities. PTP probably only makes
> > sense if one has a nice expensive grand master PTP clock on ones
> > network, and all the machines one wants to synchronise have decent
> > PTP implementations.)  
> 
> Don't wanna waste too much of your time with the questions since
> I haven't done much research but - wouldn't MAC timestamp be a better
> choice more often (as long as it's a real, to-spec PTP stamp)? 
> Are we picking PHY for historical reasons?
> 
> Not that flipping the default would address the problem of regressing
> some setups..

I have measured it with the Marvell PHY and MACB MAC but it is the contrary on
my side:
https://lkml.kernel.org/netdev/20230302113752.057a3213@kmaincent-XPS-13-7390/
Also PHY default seems more logical as it is nearer to the physical link, but
still I am interesting by the answer as I am not a PTP expert. Is really PTP
MAC often more precise than PTP PHY?

Regards,
Köry