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-rw-r--r--Documentation/networking/timestamping.txt52
1 files changed, 32 insertions, 20 deletions
diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt
index 661d3c316a17..048c92b487f6 100644
--- a/Documentation/networking/timestamping.txt
+++ b/Documentation/networking/timestamping.txt
@@ -21,26 +21,38 @@ has such a feature).
SO_TIMESTAMPING:
-Instructs the socket layer which kind of information is wanted. The
-parameter is an integer with some of the following bits set. Setting
-other bits is an error and doesn't change the current state.
-
-SOF_TIMESTAMPING_TX_HARDWARE: try to obtain send time stamp in hardware
-SOF_TIMESTAMPING_TX_SOFTWARE: if SOF_TIMESTAMPING_TX_HARDWARE is off or
- fails, then do it in software
-SOF_TIMESTAMPING_RX_HARDWARE: return the original, unmodified time stamp
- as generated by the hardware
-SOF_TIMESTAMPING_RX_SOFTWARE: if SOF_TIMESTAMPING_RX_HARDWARE is off or
- fails, then do it in software
-SOF_TIMESTAMPING_RAW_HARDWARE: return original raw hardware time stamp
-SOF_TIMESTAMPING_SYS_HARDWARE: return hardware time stamp transformed to
- the system time base
-SOF_TIMESTAMPING_SOFTWARE: return system time stamp generated in
- software
-
-SOF_TIMESTAMPING_TX/RX determine how time stamps are generated.
-SOF_TIMESTAMPING_RAW/SYS determine how they are reported in the
-following control message:
+Instructs the socket layer which kind of information should be collected
+and/or reported. The parameter is an integer with some of the following
+bits set. Setting other bits is an error and doesn't change the current
+state.
+
+Four of the bits are requests to the stack to try to generate
+timestamps. Any combination of them is valid.
+
+SOF_TIMESTAMPING_TX_HARDWARE: try to obtain send time stamps in hardware
+SOF_TIMESTAMPING_TX_SOFTWARE: try to obtain send time stamps in software
+SOF_TIMESTAMPING_RX_HARDWARE: try to obtain receive time stamps in hardware
+SOF_TIMESTAMPING_RX_SOFTWARE: try to obtain receive time stamps in software
+
+The other three bits control which timestamps will be reported in a
+generated control message. If none of these bits are set or if none of
+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_RAW_HARDWARE: report hwtimeraw if available
+
+It is worth noting that timestamps may be collected for reasons other
+than being requested by a particular socket with
+SOF_TIMESTAMPING_[TR]X_(HARD|SOFT)WARE. For example, most drivers that
+can generate hardware receive timestamps ignore
+SOF_TIMESTAMPING_RX_HARDWARE. It is still a good idea to set that flag
+in case future drivers pay attention.
+
+If timestamps are reported, they will appear in a control message with
+cmsg_level==SOL_SOCKET, cmsg_type==SO_TIMESTAMPING, and a payload like
+this:
struct scm_timestamping {
struct timespec systime;