On Tue, Oct 06, 2020 at 03:56:31PM +0200, Kurt Kanzenbach wrote: > On Tue Oct 06 2020, Vladimir Oltean wrote: > > On Tue, Oct 06, 2020 at 03:30:36PM +0200, Kurt Kanzenbach wrote: > >> That's the point. The user (or anybody else) cannot disable hardware > >> stamping, because it is always performed. So, why should it be allowed > >> to disable it even when it cannot be disabled? > > > > Because your driver's user can attach a PTP PHY to your switch port, and > > the network stack doesn't support multiple TX timestamps attached to the > > same skb. They'll want the TX timestamp from the PHY and not from your > > switch. > > Yeah, sure. That use case makes sense. What's the problem exactly? The SO_TIMESTAMPING / SO_TIMESTAMPNS cmsg socket API simply doesn't have any sort of identification for a hardware TX timestamp (where it came from). So when you'll poll for TX timestamps, you'll receive a TX timestamp from the PHY and another one from the switch, and those will be in a race with one another, so you won't know which one is which.
