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packet: doc: update timestamping part 

Bring the timestamping section in sync with the implementation.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Daniel Borkmann 2013-04-23 00:39:32 +00:00 committed by David S. Miller
parent b9c32fb271
commit 2940b26bec
1 changed files with 35 additions and 6 deletions
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41
Documentation/networking/packet_mmap.txt
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@ -1016,10 +1016,11 @@ retry_block:
-------------------------------------------------------------------------------
The PACKET_TIMESTAMP setting determines the source of the timestamp in
the packet meta information. If your NIC is capable of timestamping
packets in hardware, you can request those hardware timestamps to used.
Note: you may need to enable the generation of hardware timestamps with
SIOCSHWTSTAMP.
the packet meta information for mmap(2)ed RX_RING and TX_RINGs. If your
NIC is capable of timestamping packets in hardware, you can request those
hardware timestamps to be used. Note: you may need to enable the generation
of hardware timestamps with SIOCSHWTSTAMP (see related information from
Documentation/networking/timestamping.txt).
PACKET_TIMESTAMP accepts the same integer bit field as
SO_TIMESTAMPING. However, only the SOF_TIMESTAMPING_SYS_HARDWARE
@ -1031,8 +1032,36 @@ SOF_TIMESTAMPING_RAW_HARDWARE if both bits are set.
req |= SOF_TIMESTAMPING_SYS_HARDWARE;
setsockopt(fd, SOL_PACKET, PACKET_TIMESTAMP, (void *) &req, sizeof(req))
If PACKET_TIMESTAMP is not set, a software timestamp generated inside
the networking stack is used (the behavior before this setting was added).
For the mmap(2)ed ring buffers, such timestamps are stored in the
tpacket{,2,3}_hdr structure's tp_sec and tp_{n,u}sec members. To determine
what kind of timestamp has been reported, the tp_status field is binary |'ed
with the following possible bits ...
TP_STATUS_TS_SYS_HARDWARE
TP_STATUS_TS_RAW_HARDWARE
TP_STATUS_TS_SOFTWARE
... that are equivalent to its SOF_TIMESTAMPING_* counterparts. For the
RX_RING, if none of those 3 are set (i.e. PACKET_TIMESTAMP is not set),
then this means that a software fallback was invoked *within* PF_PACKET's
processing code (less precise).
Getting timestamps for the TX_RING works as follows: i) fill the ring frames,
ii) call sendto() e.g. in blocking mode, iii) wait for status of relevant
frames to be updated resp. the frame handed over to the application, iv) walk
through the frames to pick up the individual hw/sw timestamps.
Only (!) if transmit timestamping is enabled, then these bits are combined
with binary | with TP_STATUS_AVAILABLE, so you must check for that in your
application (e.g. !(tp_status & (TP_STATUS_SEND_REQUEST | TP_STATUS_SENDING))
in a first step to see if the frame belongs to the application, and then
one can extract the type of timestamp in a second step from tp_status)!
If you don't care about them, thus having it disabled, checking for
TP_STATUS_AVAILABLE resp. TP_STATUS_WRONG_FORMAT is sufficient. If in the
TX_RING part only TP_STATUS_AVAILABLE is set, then the tp_sec and tp_{n,u}sec
members do not contain a valid value. For TX_RINGs, by default no timestamp
is generated!
See include/linux/net_tstamp.h and Documentation/networking/timestamping
for more information on hardware timestamps.