netdev - Re: [PATCH net-next 0/5] PTP support for the SJA1105 DSA driver

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Message-ID: <CA+h21hr3+vUjS9_m=CtEbFeN9Bgxkg8b-4zuXSMnZXGtfUEOsQ@mail.gmail.com>
Date:   Sat, 1 Jun 2019 13:31:34 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 0/5] PTP support for the SJA1105 DSA driver

On Sat, 1 Jun 2019 at 08:07, Richard Cochran <richardcochran@...il.com> wrote:
>
> On Fri, May 31, 2019 at 09:12:03PM +0300, Vladimir Oltean wrote:
> > It won't work unless I make changes to dsa_switch_rcv.
>
> Or to the tagging code.
>
> > Right now taggers can only return a pointer to the skb, or NULL, case
> > in which DSA will free it.
>
> The tagger can re-write the skb.  Why not reform it into a PTP frame?
> This clever trick is what the phyter does in hardware.  See dp83640.c.
>

I think you're missing the point here.
If I dress the meta frame into a PTP frame (btw is there any
preferable event message for this purpose?) then sure, I'll make
dsa_skb_defer_rx_timestamp call my .port_rxtstamp and I can e.g. move
my state machine there.
The problem is that in the current DSA structure, I'll still have less
timestampable frames waiting for a meta frame than meta frames
themselves. This is because not all frames that the switch takes an RX
timestamp for will make it to my .port_rxtstamp (in fact that is what
my .can_timestamp patch changes). I can put the timestampable frame in
a 1-entry wait queue which I'll deplete upon arrival of the first meta
frame, but when I get meta frames and the wait queue is empty, it can
mean multiple things: either DSA didn't care about this timestamp
(ok), or the timestampable frame got reordered or dropped by the MAC,
or what have you (not ok). So I can't exclude the possibility that the
meta frame was holding a relevant timestamp.
Sure, I can dress the meta frame into whatever the previous
MAC-trapped frame was (PTP or not) and then I'll have .port_rxtstamp
function see a 1-to-1 correspondence with meta frames in case
everything works fine. But then I'll have non-PTP meta frames leaking
up the stack...

Regards,
-Vladimir

> Thanks,
> Richard