packet: doc: update timestamping part

Bring the timestamping section in sync with the implementation.
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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.