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net: skb: free up one bit in tx_flags
The linked series wants to add skb tx completion timestamps. That needs a bit in skb_shared_info.tx_flags, but all are in use. A per-skb bit is only needed for features that are configured on a per packet basis. Per socket features can be read from sk->sk_tsflags. Per packet tsflags can be set in sendmsg using cmsg, but only those in SOF_TIMESTAMPING_TX_RECORD_MASK. Per packet tsflags can also be set without cmsg by sandwiching a send inbetween two setsockopts: val |= SOF_TIMESTAMPING_$FEATURE; setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); write(fd, buf, sz); val &= ~SOF_TIMESTAMPING_$FEATURE; setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val)); Changing a datapath test from skb_shinfo(skb)->tx_flags to skb->sk->sk_tsflags can change behavior in that case, as the tx_flags is written before the second setsockopt updates sk_tsflags. Therefore, only bits can be reclaimed that cannot be set by cmsg and are also highly unlikely to be used to target individual packets otherwise. Free up the bit currently used for SKBTX_HW_TSTAMP_USE_CYCLES. This selects between clock and free running counter source for HW TX timestamps. It is probable that all packets of the same socket will always use the same source. Link: https://lore.kernel.org/netdev/cover.1739988644.git.pav@iki.fi/ Signed-off-by: Willem de Bruijn <willemb@google.com> Reviewed-by: Jason Xing <kerneljasonxing@gmail.com> Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com> Link: https://patch.msgid.link/20250225023416.2088705-1-willemdebruijn.kernel@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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