[PATCH net 0/2] The DSA TX timestamping situation

[PATCH net 0/2] The DSA TX timestamping situation

[Vladimir Oltean]

This series is the moral v2 of "[PATCH net] net: dsa: sja1105: Fix
double delivery of TX timestamps to socket error queue" [0] which did
not manage to convince public opinion (actually it didn't convince me
neither).

This fixes PTP timestamping on one particular board, where the DSA
switch is sja1105 and the master is gianfar. Unfortunately there is no
way to make the fix more general without committing logical
inaccuracies: the SKBTX_IN_PROGRESS flag does serve a purpose, even if
the sja1105 driver is not using it now: it prevents delivering a SW
timestamp to the app socket when the HW timestamp will be provided. So
not setting this flag (the approach from v1) might create avoidable
complications in the future (not to mention that there isn't any
satisfactory explanation on why that would be the correct solution).

So the goal of this change set is to create a more strict framework for
DSA master devices when attached to PTP switches, and to fix the first
master driver that is overstepping its duties and is delivering
unsolicited TX timestamps.

[0]: https://www.spinics.net/lists/netdev/msg619699.html

Vladimir Oltean (2):
  gianfar: Fix TX timestamping with stacked (DSA and PHY) drivers
  net: dsa: Deny PTP on master if switch supports it

 drivers/net/ethernet/freescale/gianfar.c | 10 +++++---
 net/dsa/master.c                         | 30 ++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

-- 
2.17.1

[PATCH net 1/2] gianfar: Fix TX timestamping with stacked (DSA and PHY) drivers

[Vladimir Oltean]

The driver wrongly assumes that it is the only entity that can set the
SKBTX_IN_PROGRESS bit of the current skb. Therefore, in the
gfar_clean_tx_ring function, where the TX timestamp is collected if
necessary, the aforementioned bit is used to discriminate whether or not
the TX timestamp should be delivered to the socket's error queue.

But a stacked driver such as a DSA switch or a PTP-capable PHY can
also set SKBTX_IN_PROGRESS, which is actually exactly what it should do
in order to denote that the hardware timestamping process is undergoing.

Therefore, gianfar would misinterpret the "in progress" bit as being its
own, and deliver a second skb clone in the socket's error queue,
completely throwing off a PTP process which is not expecting to receive
it.

There have been discussions [0] as to whether non-MAC drivers need or not to
set SKBTX_IN_PROGRESS at all (whose purpose is to avoid sending 2
timestamps, a sw and a hw one, to applications which only expect one).
But as of this patch, there are at least 2 PTP drivers that would break
in conjunction with gianfar: the sja1105 DSA switch and the TI PHYTER
(dp83640). So until we reach a conclusion, fix the gianfar driver to not
do stuff based on flags set by others and not intended for it.

[0]: https://www.spinics.net/lists/netdev/msg619699.html

Fixes: f0ee7acfcdd4 ("gianfar: Add hardware TX timestamping support")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
---
 drivers/net/ethernet/freescale/gianfar.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/freescale/gianfar.c b/drivers/net/ethernet/freescale/gianfar.c
index 72868a28b621..7d08bf6370ae 100644
--- a/drivers/net/ethernet/freescale/gianfar.c
+++ b/drivers/net/ethernet/freescale/gianfar.c
@@ -2205,13 +2205,17 @@ static void gfar_clean_tx_ring(struct gfar_priv_tx_q *tx_queue)
 	skb_dirtytx = tx_queue->skb_dirtytx;
 
 	while ((skb = tx_queue->tx_skbuff[skb_dirtytx])) {
+		bool do_tstamp;
+
+		do_tstamp = (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
+			    priv->hwts_tx_en;
 
 		frags = skb_shinfo(skb)->nr_frags;
 
 		/* When time stamping, one additional TxBD must be freed.
 		 * Also, we need to dma_unmap_single() the TxPAL.
 		 */
-		if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS))
+		if (unlikely(do_tstamp))
 			nr_txbds = frags + 2;
 		else
 			nr_txbds = frags + 1;
@@ -2225,7 +2229,7 @@ static void gfar_clean_tx_ring(struct gfar_priv_tx_q *tx_queue)
 		    (lstatus & BD_LENGTH_MASK))
 			break;
 
-		if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS)) {
+		if (unlikely(do_tstamp)) {
 			next = next_txbd(bdp, base, tx_ring_size);
 			buflen = be16_to_cpu(next->length) +
 				 GMAC_FCB_LEN + GMAC_TXPAL_LEN;
@@ -2235,7 +2239,7 @@ static void gfar_clean_tx_ring(struct gfar_priv_tx_q *tx_queue)
 		dma_unmap_single(priv->dev, be32_to_cpu(bdp->bufPtr),
 				 buflen, DMA_TO_DEVICE);
 
-		if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS)) {
+		if (unlikely(do_tstamp)) {
 			struct skb_shared_hwtstamps shhwtstamps;
 			u64 *ns = (u64 *)(((uintptr_t)skb->data + 0x10) &
 					  ~0x7UL);
-- 
2.17.1

[PATCH net 2/2] net: dsa: Deny PTP on master if switch supports it

[Vladimir Oltean]

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(+)

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;
 
-- 
2.17.1

Re: [PATCH net 2/2] net: dsa: Deny PTP on master if switch supports it

[Richard Cochran]

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>

Re: [PATCH net 1/2] gianfar: Fix TX timestamping with stacked (DSA and PHY) drivers

[Richard Cochran]

On Fri, Dec 27, 2019 at 02:44:34AM +0200, Vladimir Oltean wrote:

> But a stacked driver such as a DSA switch or a PTP-capable PHY can
> also set SKBTX_IN_PROGRESS, which is actually exactly what it should do
> in order to denote that the hardware timestamping process is undergoing.

Please remove the text about the PHY.  This driver does not call
skb_tx_timestamp(), and so it isn't possible for a PHY driver to set
the flag.
 
> There have been discussions [0] as to whether non-MAC drivers need or not to
> set SKBTX_IN_PROGRESS at all (whose purpose is to avoid sending 2
> timestamps, a sw and a hw one, to applications which only expect one).
> But as of this patch, there are at least 2 PTP drivers that would break
> in conjunction with gianfar: the sja1105 DSA switch and the TI PHYTER
> (dp83640).

Again, please drop the bit about the phyter.  It is a non-issue here.
The clash with the DSA layer is reason enough for this patch.

> Fixes: f0ee7acfcdd4 ("gianfar: Add hardware TX timestamping support")
> Signed-off-by: Vladimir Oltean <olteanv@gmail.com>

Acked-by: Richard Cochran <richardcochran@gmail.com>