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net: corrected documentation for hardware time stamping 

The current documentation for hardware time stamping does not
correctly specify the available kernel functions since the
implementation was changed later on.

Signed-off-by: Patrick Loschmidt <Patrick.Loschmidt@oeaw.ac.at>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Patrick Loschmidt 2010-04-07 21:52:07 -07:00 committed by David S. Miller
parent b62226826b
commit 69298698c2
1 changed files with 46 additions and 30 deletions
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76
Documentation/networking/timestamping.txt
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@ -41,11 +41,12 @@ SOF_TIMESTAMPING_SOFTWARE: return system time stamp generated in
SOF_TIMESTAMPING_TX/RX determine how time stamps are generated.
SOF_TIMESTAMPING_RAW/SYS determine how they are reported in the
following control message:
struct scm_timestamping {
struct timespec systime;
struct timespec hwtimetrans;
struct timespec hwtimeraw;
};
struct scm_timestamping {
struct timespec systime;
struct timespec hwtimetrans;
struct timespec hwtimeraw;
};
recvmsg() can be used to get this control message for regular incoming
packets. For send time stamps the outgoing packet is looped back to
@ -87,12 +88,13 @@ by the network device and will be empty without that support.
SIOCSHWTSTAMP:
Hardware time stamping must also be initialized for each device driver
that is expected to do hardware time stamping. The parameter is:
that is expected to do hardware time stamping. The parameter is defined in
/include/linux/net_tstamp.h as:
struct hwtstamp_config {
int flags; /* no flags defined right now, must be zero */
int tx_type; /* HWTSTAMP_TX_* */
int rx_filter; /* HWTSTAMP_FILTER_* */
int flags; /* no flags defined right now, must be zero */
int tx_type; /* HWTSTAMP_TX_* */
int rx_filter; /* HWTSTAMP_FILTER_* */
};
Desired behavior is passed into the kernel and to a specific device by
@ -139,42 +141,56 @@ enum {
/* time stamp any incoming packet */
HWTSTAMP_FILTER_ALL,
/* return value: time stamp all packets requested plus some others */
HWTSTAMP_FILTER_SOME,
/* return value: time stamp all packets requested plus some others */
HWTSTAMP_FILTER_SOME,
/* PTP v1, UDP, any kind of event packet */
HWTSTAMP_FILTER_PTP_V1_L4_EVENT,
...
/* for the complete list of values, please check
* the include file /include/linux/net_tstamp.h
*/
};
DEVICE IMPLEMENTATION
A driver which supports hardware time stamping must support the
SIOCSHWTSTAMP ioctl. Time stamps for received packets must be stored
in the skb with skb_hwtstamp_set().
SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
the actual values as described in the section on SIOCSHWTSTAMP.
Time stamps for received packets must be stored in the skb. To get a pointer
to the shared time stamp structure of the skb call skb_hwtstamps(). Then
set the time stamps in the structure:
struct skb_shared_hwtstamps {
/* hardware time stamp transformed into duration
* since arbitrary point in time
*/
ktime_t hwtstamp;
ktime_t syststamp; /* hwtstamp transformed to system time base */
};
Time stamps for outgoing packets are to be generated as follows:
- In hard_start_xmit(), check if skb_hwtstamp_check_tx_hardware()
returns non-zero. If yes, then the driver is expected
to do hardware time stamping.
- In hard_start_xmit(), check if skb_tx(skb)->hardware is set no-zero.
If yes, then the driver is expected to do hardware time stamping.
- If this is possible for the skb and requested, then declare
that the driver is doing the time stamping by calling
skb_hwtstamp_tx_in_progress(). A driver not supporting
hardware time stamping doesn't do that. A driver must never
touch sk_buff::tstamp! It is used to store how time stamping
for an outgoing packets is to be done.
that the driver is doing the time stamping by setting the field
skb_tx(skb)->in_progress non-zero. You might want to keep a pointer
to the associated skb for the next step and not free the skb. A driver
not supporting hardware time stamping doesn't do that. A driver must
never touch sk_buff::tstamp! It is used to store software generated
time stamps by the network subsystem.
- As soon as the driver has sent the packet and/or obtained a
hardware time stamp for it, it passes the time stamp back by
calling skb_hwtstamp_tx() with the original skb, the raw
hardware time stamp and a handle to the device (necessary
to convert the hardware time stamp to system time). If obtaining
the hardware time stamp somehow fails, then the driver should
not fall back to software time stamping. The rationale is that
this would occur at a later time in the processing pipeline
than other software time stamping and therefore could lead
to unexpected deltas between time stamps.
- If the driver did not call skb_hwtstamp_tx_in_progress(), then
hardware time stamp. skb_hwtstamp_tx() clones the original skb and
adds the timestamps, therefore the original skb has to be freed now.
If obtaining the hardware time stamp somehow fails, then the driver
should not fall back to software time stamping. The rationale is that
this would occur at a later time in the processing pipeline than other
software time stamping and therefore could lead to unexpected deltas
between time stamps.
- If the driver did not call set skb_tx(skb)->in_progress, then
dev_hard_start_xmit() checks whether software time stamping
is wanted as fallback and potentially generates the time stamp.