netdev - Re: [PATCH v2 6/7] net: dsa: b53: Add logic for TX timestamping

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Date:   Wed, 10 Nov 2021 08:14:32 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <olteanv@...il.com>,
        Martin Kaistra <martin.kaistra@...utronix.de>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Richard Cochran <richardcochran@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 6/7] net: dsa: b53: Add logic for TX timestamping

Hi Vladimir,

On Tue Nov 09 2021, Vladimir Oltean wrote:
>> +void b53_port_txtstamp(struct dsa_switch *ds, int port, struct sk_buff *skb)
>> +{
>> +	struct b53_device *dev = ds->priv;
>> +	struct b53_port_hwtstamp *ps = &dev->ports[port].port_hwtstamp;
>> +	struct sk_buff *clone;
>> +	unsigned int type;
>> +
>> +	type = ptp_classify_raw(skb);
>> +
>> +	if (type != PTP_CLASS_V2_L2)
>> +		return;
>> +
>> +	if (!test_bit(B53_HWTSTAMP_ENABLED, &ps->state))
>> +		return;
>> +
>> +	clone = skb_clone_sk(skb);
>> +	if (!clone)
>> +		return;
>> +
>> +	if (test_and_set_bit_lock(B53_HWTSTAMP_TX_IN_PROGRESS, &ps->state)) {
>
> Is it ok if you simply don't timestamp a second skb which may be sent
> while the first one is in flight, I wonder? What PTP profiles have you
> tested with? At just one PTP packet at a time, the switch isn't giving
> you a lot.

PTP only generates a couple of messages per second which need to be
timestamped. Therefore, this behavior shouldn't be a problem.

hellcreek (and mv88e6xxx) do the same thing, simply because the device
can only hold only one Tx timestamp. If we'd allow more than one PTP
packet in flight, there will be correlation problems. I've tested with
default and gPTP profile without any problems. What PTP profiles do have
in mind?

> Is it a hardware limitation?

Not for the b53. It will generate status frames for each to be
timestamped packet. However, I don't see the need to allow more than one
Tx packet per port to be timestamped at the moment.

Thanks,
Kurt

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