The free running time of physical clocks called cycles shall be used for
hardware timestamps to enable synchronisation.

Introduce new flag SKBTX_HW_TSTAMP_USE_CYCLES, which signals driver to
provide a TX timestamp based on cycles if cycles are supported.

Signed-off-by: Gerhard Engleder <redacted>
---
 include/linux/skbuff.h |  3 +++
 net/core/skbuff.c      |  2 ++
 net/socket.c           | 10 +++++++++-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 26538ceb4b01..f494ddbfc826 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -578,6 +578,9 @@ enum {
 	/* device driver is going to provide hardware time stamp */
 	SKBTX_IN_PROGRESS = 1 << 2,
 
+	/* generate hardware time stamp based on cycles if supported */
+	SKBTX_HW_TSTAMP_USE_CYCLES = 1 << 3,

 	/* generate wifi status information (where possible) */
 	SKBTX_WIFI_STATUS = 1 << 4,
 

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 10bde7c6db44..c0f8f1341c3f 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -4847,6 +4847,8 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
 		skb_shinfo(skb)->tx_flags |= skb_shinfo(orig_skb)->tx_flags &
 					     SKBTX_ANY_TSTAMP;
 		skb_shinfo(skb)->tskey = skb_shinfo(orig_skb)->tskey;
+	} else {
+		skb_shinfo(skb)->tx_flags &= ~SKBTX_HW_TSTAMP_USE_CYCLES;
 	}
 
 	if (hwtstamps)

diff --git a/net/socket.c b/net/socket.c
index 982eecad464c..1acebcb19e8f 100644
--- a/net/socket.c
+++ b/net/socket.c

@@ -683,9 +683,17 @@ void __sock_tx_timestamp(__u16 tsflags, __u8 *tx_flags)
 {
 	u8 flags = *tx_flags;
 
-	if (tsflags & SOF_TIMESTAMPING_TX_HARDWARE)
+	if (tsflags & SOF_TIMESTAMPING_TX_HARDWARE) {
 		flags |= SKBTX_HW_TSTAMP;
 
+		/* PTP hardware clocks can provide a free running time called
+		 * cycles as base for virtual clocks.