net-timestamp: document deprecated syststamp

The SO_TIMESTAMPING API defines option SOF_TIMESTAMPING_SYS_HW.
This feature is deprecated. It should not be implemented by new
device drivers. Existing drivers do not implement it, either --
with one exception.

Driver developers are encouraged to expose the NIC hw clock as a
PTP HW clock source, instead, and synchronize system time to the
HW source.

The control flag cannot be removed due to being part of the ABI, nor
can the structure scm_timestamping that is returned. Due to the one
legacy driver, the internal datapath and structure are not removed.

This patch only clearly marks the interface as deprecated. Device
drivers should always return a syststamp value of zero.

Signed-off-by: Willem de Bruijn <willemb@google.com>

----

We can consider adding a WARN_ON_ONCE in__sock_recv_timestamp
if non-zero syststamp is encountered
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 890fb3307dd6..369430340ed9 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -211,7 +211,7 @@ static inline void skb_frag_size_sub(skb_frag_t *frag, int delta)
  * struct skb_shared_hwtstamps - hardware time stamps
  * @hwtstamp:   hardware time stamp transformed into duration
  *              since arbitrary point in time
- * @syststamp:  hwtstamp transformed to system time base
+ * @syststamp:  hwtstamp transformed to system time base (deprecated)
  *
  * Software time stamps generated by ktime_get_real() are stored in
  * skb->tstamp. The relation between the different kinds of time
@@ -222,7 +222,9 @@ static inline void skb_frag_size_sub(skb_frag_t *frag, int delta)
  * syststamp/tstamp/"syststamp from other device" comparison is
  * limited by the accuracy of the transformation into system time
  * base. This depends on the device driver and its underlying
- * hardware.
+ * hardware. The syststamp implementation is deprecated in favor
+ * of hwtstamps and hw PTP clock sources exposed directly to
+ * userspace.
  *
  * hwtstamps can only be compared against other hwtstamps from
  * the same device.