net-timestamp: COMPLETION timestamp on packet tx completion

Add SOF_TIMESTAMPING_TX_COMPLETION, for requesting a software timestamp
when hardware reports a packet completed.

Completion tstamp is useful for Bluetooth, as hardware timestamps do not
exist in the HCI specification except for ISO packets, and the hardware
has a queue where packets may wait. In this case the software SND
timestamp only reflects the kernel-side part of the total latency
(usually small) and queue length (usually 0 unless HW buffers
congested), whereas the completion report time is more informative of
the true latency.

It may also be useful in other cases where HW TX timestamps cannot be
obtained and user wants to estimate an upper bound to when the TX
probably happened.

Signed-off-by: Pauli Virtanen <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

Documentation/networking/timestamping.rst

···
   cumulative acknowledgment. The mechanism ignores SACK and FACK.
   This flag can be enabled via both socket options and control messages.
 
+SOF_TIMESTAMPING_TX_COMPLETION:
+  Request tx timestamps on packet tx completion.  The completion
+  timestamp is generated by the kernel when it receives packet a
+  completion report from the hardware. Hardware may report multiple
+  packets at once, and completion timestamps reflect the timing of the
+  report and not actual tx time. This flag can be enabled via both
+  socket options and control messages.
+
 
 1.3.2 Timestamp Reporting
 ^^^^^^^^^^^^^^^^^^^^^^^^^

include/linux/skbuff.h

···
 	/* device driver is going to provide hardware time stamp */
 	SKBTX_IN_PROGRESS = 1 << 2,
 
-	/* reserved */
-	SKBTX_RESERVED = 1 << 3,
+	/* generate software time stamp on packet tx completion */
+	SKBTX_COMPLETION_TSTAMP = 1 << 3,
 
 	/* generate wifi status information (where possible) */
 	SKBTX_WIFI_STATUS = 1 << 4,
···
 
 #define SKBTX_ANY_SW_TSTAMP	(SKBTX_SW_TSTAMP    | \
 				 SKBTX_SCHED_TSTAMP | \
-				 SKBTX_BPF)
+				 SKBTX_BPF          | \
+				 SKBTX_COMPLETION_TSTAMP)
 #define SKBTX_ANY_TSTAMP	(SKBTX_HW_TSTAMP | \
 				 SKBTX_ANY_SW_TSTAMP)
 

include/uapi/linux/errqueue.h

···
 	SCM_TSTAMP_SND,		/* driver passed skb to NIC, or HW */
 	SCM_TSTAMP_SCHED,	/* data entered the packet scheduler */
 	SCM_TSTAMP_ACK,		/* data acknowledged by peer */
+	SCM_TSTAMP_COMPLETION,	/* packet tx completion */
 };
 
 #endif /* _UAPI_LINUX_ERRQUEUE_H */

include/uapi/linux/net_tstamp.h

···
 	SOF_TIMESTAMPING_BIND_PHC = (1 << 15),
 	SOF_TIMESTAMPING_OPT_ID_TCP = (1 << 16),
 	SOF_TIMESTAMPING_OPT_RX_FILTER = (1 << 17),
+	SOF_TIMESTAMPING_TX_COMPLETION = (1 << 18),
 
-	SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_OPT_RX_FILTER,
+	SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_TX_COMPLETION,
 	SOF_TIMESTAMPING_MASK = (SOF_TIMESTAMPING_LAST - 1) |
 				 SOF_TIMESTAMPING_LAST
 };
···
 #define SOF_TIMESTAMPING_TX_RECORD_MASK	(SOF_TIMESTAMPING_TX_HARDWARE | \
 					 SOF_TIMESTAMPING_TX_SOFTWARE | \
 					 SOF_TIMESTAMPING_TX_SCHED | \
-					 SOF_TIMESTAMPING_TX_ACK)
+					 SOF_TIMESTAMPING_TX_ACK | \
+					 SOF_TIMESTAMPING_TX_COMPLETION)
 
 /**
  * struct so_timestamping - SO_TIMESTAMPING parameter

net/core/skbuff.c

···
 						    SKBTX_SW_TSTAMP);
 	case SCM_TSTAMP_ACK:
 		return TCP_SKB_CB(skb)->txstamp_ack & TSTAMP_ACK_SK;
+	case SCM_TSTAMP_COMPLETION:
+		return skb_shinfo(skb)->tx_flags & SKBTX_COMPLETION_TSTAMP;
 	}
 
 	return false;

net/ethtool/common.c

···
 	[const_ilog2(SOF_TIMESTAMPING_BIND_PHC)]     = "bind-phc",
 	[const_ilog2(SOF_TIMESTAMPING_OPT_ID_TCP)]   = "option-id-tcp",
 	[const_ilog2(SOF_TIMESTAMPING_OPT_RX_FILTER)] = "option-rx-filter",
+	[const_ilog2(SOF_TIMESTAMPING_TX_COMPLETION)] = "tx-completion",
 };
 static_assert(ARRAY_SIZE(sof_timestamping_names) == __SOF_TIMESTAMPING_CNT);
 

net/socket.c

···
 	if (tsflags & SOF_TIMESTAMPING_TX_SCHED)
 		flags |= SKBTX_SCHED_TSTAMP;
 
+	if (tsflags & SOF_TIMESTAMPING_TX_COMPLETION)
+		flags |= SKBTX_COMPLETION_TSTAMP;
+
 	*tx_flags = flags;
 }
 EXPORT_SYMBOL(__sock_tx_timestamp);