From: Vladimir Oltean <olteanv@gmail.com>
To: "Y.b. Lu" <yangbo.lu@nxp.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
Richard Cochran <richardcochran@gmail.com>,
Vladimir Oltean <vladimir.oltean@nxp.com>,
"David S . Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
Kurt Kanzenbach <kurt@linutronix.de>,
Andrew Lunn <andrew@lunn.ch>,
Vivien Didelot <vivien.didelot@gmail.com>,
Florian Fainelli <f.fainelli@gmail.com>,
Claudiu Manoil <claudiu.manoil@nxp.com>,
Alexandre Belloni <alexandre.belloni@bootlin.com>,
"UNGLinuxDriver@microchip.com" <UNGLinuxDriver@microchip.com>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [net-next 3/3] net: mscc: ocelot: support PTP Sync one-step timestamping
Date: Tue, 20 Apr 2021 11:20:41 +0300 [thread overview]
Message-ID: <20210420082041.tcthzr6ubtdk6ztf@skbuf> (raw)
In-Reply-To: <AM7PR04MB6885B46EB5C55BD1B92DD746F8489@AM7PR04MB6885.eurprd04.prod.outlook.com>
On Tue, Apr 20, 2021 at 07:33:39AM +0000, Y.b. Lu wrote:
> > > + /* For two-step timestamp, retrieve ptp_cmd in DSA_SKB_CB_PRIV
> > > + * and timestamp ID in clone->cb[0].
> > > + * For one-step timestamp, retrieve ptp_cmd in DSA_SKB_CB_PRIV.
> > > + */
> > > + u8 *ptp_cmd = DSA_SKB_CB_PRIV(skb);
> >
> > This is fine in the sense that it works, but please consider creating something
> > similar to sja1105:
> >
> > struct ocelot_skb_cb {
> > u8 ptp_cmd; /* For both one-step and two-step timestamping */
> > u8 ts_id; /* Only for two-step timestamping */ };
> >
> > #define OCELOT_SKB_CB(skb) \
> > ((struct ocelot_skb_cb *)DSA_SKB_CB_PRIV(skb))
> >
> > And then access as OCELOT_SKB_CB(skb)->ptp_cmd,
> > OCELOT_SKB_CB(clone)->ts_id.
> >
> > and put a comment to explain that this is done in order to have common code
> > between Felix DSA and Ocelot switchdev. Basically Ocelot will not use the first
> > 8 bytes of skb->cb, but there's enough space for this to not make any
> > difference. The original skb will hold only ptp_cmd, the clone will only hold
> > ts_id, but it helps to have the same structure in place.
> >
> > If you create this ocelot_skb_cb structure, I expect the comment above to be
> > fairly redundant, you can consider removing it.
> >
>
> You're right to define the structure.
> Considering patch #1, move skb cloning to drivers, and populate DSA_SKB_CB(skb)->clone if needs to do so (per suggestion).
> Can we totally drop dsa_skb_cb in dsa core? The only usage of it is holding a skb clone pointer, for only felix and sja1105.
> Actually we can move such pointer in <device>_skb_cb, instead of reserving the space of skb for any drivers.
>
> Do you think so?
The trouble with skb->cb is that it isn't zero-initialized. But somebody
needs to initialize the clone pointer to NULL, otherwise you don't know
if this is a valid pointer or not. Because dsa_skb_tx_timestamp() is
called before p->xmit(), the driver has no way to initialize the clone
pointer by itself. So this was done directly from dsa_slave_xmit(), and
not from any driver-specific hook. So this is why there is a
DSA_SKB_CB(skb)->clone and not SJA1105_SKB_CB(skb)->clone. The
alternative would be to memset(skb->cb, 0, 48) which is a bit
sub-optimal because it initializes more than it needs. Alternatively, it
might be possible to introduce a new property in struct dsa_device_ops
which holds sizeof(struct sja1105_skb_cb), and the generic code will
only zero-initialize this number of bytes.
I don't know, if you can get it to work in a way that does not incur a
noticeable performance penalty, I'm okay with whatever you come up with.
prev parent reply other threads:[~2021-04-20 8:20 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 12:36 [net-next 0/3] Support ocelot PTP Sync one-step timestamping Yangbo Lu
2021-04-16 12:36 ` [net-next 1/3] net: dsa: optimize tx timestamp request handling Yangbo Lu
2021-04-18 9:18 ` Vladimir Oltean
2021-04-18 15:11 ` Richard Cochran
2021-04-20 7:39 ` Y.b. Lu
2021-04-20 7:48 ` Y.b. Lu
2021-04-18 15:06 ` Richard Cochran
2021-04-20 7:40 ` Y.b. Lu
2021-04-18 15:08 ` Richard Cochran
2021-04-19 6:46 ` Kurt Kanzenbach
2021-04-16 12:36 ` [net-next 2/3] net: mscc: ocelot: convert to ocelot_port_txtstamp_request() Yangbo Lu
2021-04-18 9:27 ` Vladimir Oltean
2021-04-16 12:36 ` [net-next 3/3] net: mscc: ocelot: support PTP Sync one-step timestamping Yangbo Lu
2021-04-18 9:46 ` Vladimir Oltean
2021-04-20 7:33 ` Y.b. Lu
2021-04-20 8:20 ` Vladimir Oltean [this message]
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