| Message ID | 20191227004435.21692-3-olteanv@gmail.com |
|---|---|
| State | Superseded |
| Delegated to: | David Miller |
| Headers | show |
| Series | The DSA TX timestamping situation | expand |
On Fri, Dec 27, 2019 at 02:44:35AM +0200, Vladimir Oltean wrote: > Why? Well, in short, the PTP API in place today is a bit rudimentary and > relies on applications to retrieve the TX timestamps by polling the > error queue and looking at the cmsg structure. But there is no timestamp > identification of any sorts (except whether it's HW or SW), you don't > know how many more timestamps are there to come, which one is this one, > from whom it is, etc. IOW, you can only get one HW time stamp from the stack. This is a fundamental limitation of the so_timestamping API. If user space really wanted multiple HW and SW time stamps, then some new way would have to be invented, but it would be lots of work. IMHO, as a practical matter, multiple time stamps would be interesting from a profiling point of view, but less so for synchronization applications. > So it is a fact of life that PTP timestamping on the DSA master is > incompatible with timestamping on the switch MAC. And if the switch > supports PTP, taking the timestamps from the switch MAC is highly > preferable anyway, due to the fact that those don't contain the queuing > latencies of the switch. So just disallow PTP on the DSA master if there > is any PTP-capable switch attached. I agreed that MAC time stamps are not very useful when there is a PTP switch in front. Acked-by: Richard Cochran <richardcochran@gmail.com>
diff --git a/net/dsa/master.c b/net/dsa/master.c index 3255dfc97f86..bd44bde272f4 100644 --- a/net/dsa/master.c +++ b/net/dsa/master.c @@ -197,6 +197,35 @@ static int dsa_master_get_phys_port_name(struct net_device *dev, return 0; } +static int dsa_master_ioctl(struct net_device *dev, struct ifreq *ifr, int cmd) +{ + struct dsa_port *cpu_dp = dev->dsa_ptr; + struct dsa_switch *ds = cpu_dp->ds; + struct dsa_switch_tree *dst; + int err = -EOPNOTSUPP; + struct dsa_port *dp; + + dst = ds->dst; + + switch (cmd) { + case SIOCGHWTSTAMP: + case SIOCSHWTSTAMP: + /* Deny PTP operations on master if there is at least one + * switch in the tree that is PTP capable. + */ + list_for_each_entry(dp, &dst->ports, list) + if (dp->ds->ops->port_hwtstamp_get || + dp->ds->ops->port_hwtstamp_set) + return -EBUSY; + break; + } + + if (cpu_dp->orig_ndo_ops && cpu_dp->orig_ndo_ops->ndo_do_ioctl) + err = cpu_dp->orig_ndo_ops->ndo_do_ioctl(dev, ifr, cmd); + + return err; +} + static int dsa_master_ethtool_setup(struct net_device *dev) { struct dsa_port *cpu_dp = dev->dsa_ptr; @@ -249,6 +278,7 @@ static int dsa_master_ndo_setup(struct net_device *dev) memcpy(ops, cpu_dp->orig_ndo_ops, sizeof(*ops)); ops->ndo_get_phys_port_name = dsa_master_get_phys_port_name; + ops->ndo_do_ioctl = dsa_master_ioctl; dev->netdev_ops = ops;
It is possible to kill PTP on a DSA switch completely and absolutely, until a reboot, with a simple command: tcpdump -i eth2 -j adapter_unsynced where eth2 is the switch's DSA master. Why? Well, in short, the PTP API in place today is a bit rudimentary and relies on applications to retrieve the TX timestamps by polling the error queue and looking at the cmsg structure. But there is no timestamp identification of any sorts (except whether it's HW or SW), you don't know how many more timestamps are there to come, which one is this one, from whom it is, etc. And the "-j adapter_unsynced" flag of tcpdump enables hardware timestamping. So let's imagine what happens when the DSA master decides it wants to deliver TX timestamps to the skb's socket too: - The timestamp that the user space sees is taken by the DSA master. Whereas the RX timestamp will eventually be overwritten by the DSA switch. So the RX and TX timestamps will be in different time bases (aka garbage). - The user space applications have no way to deal with the second (real) TX timestamp finally delivered by the DSA switch, or even to know to wait for it. Take ptp4l from the linuxptp project, for example. This is its behavior after running tcpdump, before the patch: ptp4l[172]: [6469.594] Unexpected data on socket err queue: ptp4l[172]: [6469.693] rms 8 max 16 freq -21257 +/- 11 delay 748 +/- 0 ptp4l[172]: [6469.711] Unexpected data on socket err queue: ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 03 aa 05 00 fd ptp4l[172]: 0030 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: [6469.721] Unexpected data on socket err queue: ptp4l[172]: 0000 01 80 c2 00 00 0e 00 1f 7b 63 02 48 88 f7 10 02 ptp4l[172]: 0010 00 2c 00 00 02 00 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 01 c6 b1 00 fd ptp4l[172]: 0030 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: [6469.838] Unexpected data on socket err queue: ptp4l[172]: 0000 01 80 c2 00 00 0e 00 1f 7b 63 02 48 88 f7 10 02 ptp4l[172]: 0010 00 2c 00 00 02 00 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 03 aa 06 00 fd ptp4l[172]: 0030 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: [6469.848] Unexpected data on socket err queue: ptp4l[172]: 0000 01 80 c2 00 00 0e 00 1f 7b 63 02 48 88 f7 13 02 ptp4l[172]: 0010 00 36 00 00 02 00 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 04 1a 45 05 7f ptp4l[172]: 0030 00 00 5e 05 41 32 27 c2 1a 68 00 04 9f ff fe 05 ptp4l[172]: 0040 de 06 00 01 ptp4l[172]: [6469.855] Unexpected data on socket err queue: ptp4l[172]: 0000 01 80 c2 00 00 0e 00 1f 7b 63 02 48 88 f7 10 02 ptp4l[172]: 0010 00 2c 00 00 02 00 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 01 c6 b2 00 fd ptp4l[172]: 0030 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: [6469.974] Unexpected data on socket err queue: ptp4l[172]: 0000 01 80 c2 00 00 0e 00 1f 7b 63 02 48 88 f7 10 02 ptp4l[172]: 0010 00 2c 00 00 02 00 00 00 00 00 00 00 00 00 00 00 ptp4l[172]: 0020 00 00 00 1f 7b ff fe 63 02 48 00 03 aa 07 00 fd ptp4l[172]: 0030 00 00 00 00 00 00 00 00 00 00 The ptp4l program itself is heavily patched to show this (more details here [0]). Otherwise, by default it just hangs. On the other hand, with the DSA patch to disallow HW timestamping applied: tcpdump -i eth2 -j adapter_unsynced tcpdump: SIOCSHWTSTAMP failed: Device or resource busy So it is a fact of life that PTP timestamping on the DSA master is incompatible with timestamping on the switch MAC. And if the switch supports PTP, taking the timestamps from the switch MAC is highly preferable anyway, due to the fact that those don't contain the queuing latencies of the switch. So just disallow PTP on the DSA master if there is any PTP-capable switch attached. [0]: https://sourceforge.net/p/linuxptp/mailman/message/36880648/ Fixes: 0336369d3a4d ("net: dsa: forward hardware timestamping ioctls to switch driver") Signed-off-by: Vladimir Oltean <olteanv@gmail.com> --- net/dsa/master.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+)