Re: [PATCH net] net: fall back to skb_iif for the timestamping pktinfo if_index

[JR Lanteigne <root@dnim.dev>]

Willem de Bruijn wrote:
> It may be possible to detect whether an skb_iif points to a physical
> or aggregate device (e.g., through lower_dev)? If so, then the value
> can still be used in most cases.

That only guesses less often (tunnels register no lower adjacency).
I agree with Miroslav: 0 when the physical interface is unknown.

The userspace check fixes chrony only; every other consumer of
SCM_TIMESTAMPING_PKTINFO has to rediscover that if_index can be 0
and grow its own IP_PKTINFO fallback. The kernel is the one place a
fix reaches all of them.

The gap on cpsw: one napi serves both ports, the driver delivers
with netif_receive_skb() so the id is never set, and it would name
port 1 for every frame if it were. The driver knows the source port
per packet (CPDMA_RX_SOURCE_PORT) and needs a way to hand it to
put_ts_pktinfo(), e.g. via skb_shared_hwtstamps.