linux-kernel - Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time stamping layer be selectable

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Message-ID: <20231122165955.tujcadked5bgqjet@skbuf>
Date:   Wed, 22 Nov 2023 18:59:55 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Köry Maincent <kory.maincent@...tlin.com>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Radu Pirea <radu-nicolae.pirea@....nxp.com>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Claudiu Beznea <claudiu.beznea@...on.dev>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Simon Horman <horms@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time
 stamping layer be selectable

On Wed, Nov 22, 2023 at 08:54:59AM -0800, Jakub Kicinski wrote:
> On Wed, 22 Nov 2023 16:36:18 +0200 Vladimir Oltean wrote:
> > @Jakub, for your long-term "MAC timestamps for PTP, DMA for everything else".
> > How do you see this? I guess we need some sort of priority function in
> > the UAPI between hwtstamp providers.
> > 
> > And even with that, I think the enums that we currently have for filters
> > are not specific enough. The most we could expose is:
> > 
> >                       MAC provider                      DMA provider
> > 
> > hwtstamp_rx_filters   HWTSTAMP_FILTER_PTP_V2_EVENT      HWTSTAMP_FILTER_ALL
> > tx_type               HWTSTAMP_TX_ON                    HWTSTAMP_TX_ON
> > 
> > but it isn't clear: for PTP, does the DMA provider give you an RX
> > timestamp too?
> 
> If we phrase it as "precise / approximate" rather than "MAC / DMA" - it
> seems fairly intuitive to give the best timestamp available for a given
> packet, no?

I wouldn't be so sure. The alternative interpretation "for PTP, give me
timestamps from both sources" also sounds reasonable for the distant
future where that will be possible (with proper cmsg identification).
But I don't see how to distinguish the two - the filters, expressed in
these terms, would be the same.

> > What about a TX timestamp?
> 
> I was thinking - socket flag to make packets for a given socket request
> precise timestamps.

So the ptp4l source code would have to be modified to still work with
the same precision as before? I'm not seeing this through.