Hello,
I know it's possible to have Linux timestamp incoming datagrams as soon
as they are received, then for one to retrieve this timestamp later with
an ioctl command or a recvmsg call.
As far as I understand, one can either do
const int on = 1;
setsockopt(sock, SOL_SOCKET, SO_TIMESTAMP, &on, sizeof on);
then use recvmsg()
or not set the SO_TIMESTAMP socket option and just call
ioctl(sock, SIOCGSTAMP, &tv);
after each datagram has been received.
SIOCGSTAMP
Return a struct timeval with the receive timestamp of the last
packet passed to the user. This is useful for accurate round trip time
measurements. See setitimer(2) for a description of struct timeval.
As far as I understand, this timestamp is given by the CLOCK_REALTIME
clock. However, I would like to obtain a timestamp given by the
CLOCK_MONOTONIC clock.
Relevant parts of the code (I think):
net/core/dev.c
void net_enable_timestamp(void)
{
atomic_inc(&netstamp_needed);
}
void __net_timestamp(struct sk_buff *skb)
{
struct timeval tv;
do_gettimeofday(&tv);
skb_set_timestamp(skb, &tv);
}
static inline void net_timestamp(struct sk_buff *skb)
{
if (atomic_read(&netstamp_needed))
__net_timestamp(skb);
else {
skb->tstamp.off_sec = 0;
skb->tstamp.off_usec = 0;
}
}
do_gettimeofday() just calls __get_realtime_clock_ts()
Would it be possible to replace do_gettimeofday() by ktime_get_ts() with
the appropriate division by 1000 to convert the struct timespec back
into a struct timeval?
void __net_timestamp(struct sk_buff *skb)
{
struct timespec now;
struct timeval tv;
ktime_get_ts(&ts);
tv.tv_sec = now.tv_sec;
tv->tv_usec = now.tv_nsec/1000;
skb_set_timestamp(skb, &tv);
}
How many apps / drivers would this break?
Is there perhaps a different way to achieve this?
Regards.
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