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net-timestamp: document deprecated syststamp

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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>
This commit is contained in:
Willem de Bruijn
2014-07-14 17:55:30 -04:00
committed by David S. Miller
parent 11878b40ed
commit 26c4fdb052
2 changed files with 12 additions and 4 deletions
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10
Documentation/networking/timestamping.txt
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@@ -40,7 +40,7 @@ the set bits correspond to data that is available, then the control
message will not be generated:
SOF_TIMESTAMPING_SOFTWARE: report systime if available
SOF_TIMESTAMPING_SYS_HARDWARE: report hwtimetrans if available
SOF_TIMESTAMPING_SYS_HARDWARE: report hwtimetrans if available (deprecated)
SOF_TIMESTAMPING_RAW_HARDWARE: report hwtimeraw if available
It is worth noting that timestamps may be collected for reasons other
@@ -94,7 +94,13 @@ not perfect; as a consequence, sorting packets received via different
NICs by their hwtimetrans may differ from the order in which they were
received. hwtimetrans may be non-monotonic even for the same NIC.
Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
by the network device and will be empty without that support.
by the network device and will be empty without that support. This
field is DEPRECATED. Only one driver computes this value. New device
drivers must leave this zero. Instead, they can expose the hardware
clock device on the NIC directly as a HW PTP clock source, to allow
time conversion in userspace and optionally synchronize system time
with a userspace PTP stack such as linuxptp. For the PTP clock API,
see Documentation/ptp/ptp.txt.
SIOCSHWTSTAMP, SIOCGHWTSTAMP: