diff options
| -rw-r--r-- | Documentation/networking/packet_mmap.txt | 41 |
1 files changed, 35 insertions, 6 deletions
diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt index 65efb85e49de..23dd80e82b8e 100644 --- a/Documentation/networking/packet_mmap.txt +++ b/Documentation/networking/packet_mmap.txt @@ -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. |
