Re: [PATCH net-next v3 1/2] net-timestamp: filter out report when setting SOF_TIMESTAMPING_SOFTWARE

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> introduce a new flag SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER in the
> receive path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> out rx timestamp report, especially after a process turns on
> netstamp_needed_key which can time stamp every incoming skb.
> 
> Previously, We found out if an application starts first which turns on
> netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
> could also get rx timestamp. Now we handle this case by introducing this
> new flag without breaking users.
> 
> In this way, we have two kinds of combination:
> 1. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_RX_SOFTWARE, it
> will surely allow users to get the rx timestamp report.
> 2. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER
> while the skb is timestamped, it will stop reporting the rx timestamp.

The one point this commit does not explain is why a process would want
to request software timestamp reporting, but not receive software
timestamp generation. The only use I see is when the application does
request SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE.
 
> Another thing about errqueue in this patch I have a few words to say:
> In this case, we need to handle the egress path carefully, or else
> reporting the tx timestamp will fail. Egress path and ingress path will
> finally call sock_recv_timestamp(). We have to distinguish them.
> Errqueue is a good indicator to reflect the flow direction.

Good find on skb_is_err_queue rather than open-coding PACKET_OUTGOING.

> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

> ---
> 1. Willem suggested this alternative way to solve the issue, so I
> added his Suggested-by tag here. Thanks!
> ---
>  Documentation/networking/timestamping.rst | 12 ++++++++++++
>  include/uapi/linux/net_tstamp.h           |  3 ++-
>  net/core/sock.c                           |  4 ++++
>  net/ethtool/common.c                      |  1 +
>  net/ipv4/tcp.c                            |  7 +++++--
>  net/socket.c                              |  5 ++++-
>  6 files changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> index 5e93cd71f99f..ef2a334d373e 100644
> --- a/Documentation/networking/timestamping.rst
> +++ b/Documentation/networking/timestamping.rst
> @@ -266,6 +266,18 @@ SOF_TIMESTAMPING_OPT_TX_SWHW:
>    two separate messages will be looped to the socket's error queue,
>    each containing just one timestamp.
>  
> +SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER:
> +  Used in the receive software timestamp. Enabling the flag along with
> +  SOF_TIMESTAMPING_SOFTWARE will not report the rx timestamp to the
> +  userspace so that it can filter out the case where one process starts
> +  first which turns on netstamp_needed_key through setting generation
> +  flags like SOF_TIMESTAMPING_RX_SOFTWARE, then another one only passing
> +  SOF_TIMESTAMPING_SOFTWARE report flag could also get the rx timestamp.
> +
> +  SOF_TIMESTAMPING_OPT_RX_SOFTWARE_FILTER prevents the application from
> +  being influenced by others and let the application finally choose
> +  whether to report the timestamp in the receive path or not.
> +

nit, so no need to revise, but for next time:

Documentation should describe the current state, not a history of
changes (log), so wording like "finally" does not belong.