Message-ID: <66d9b40ecd086_18ac212943@willemb.c.googlers.com.notmuch>
Date: Thu, 05 Sep 2024 09:37:18 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 willemdebruijn.kernel@...il.com, 
 shuah@...nel.org, 
 willemb@...gle.com
Cc: linux-kselftest@...r.kernel.org, 
 netdev@...r.kernel.org, 
 Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v4 1/4] net-timestamp: filter out report when
 setting SOF_TIMESTAMPING_SOFTWARE

Jason Xing wrote:
> From: Jason Xing <kernelxing@...cent.com>
> 
> introduce a new flag SOF_TIMESTAMPING_OPT_RX_FILTER in the receive
> path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> out rx software 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.
> 
> Quoting Willem to explain why we need the flag:
> "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."
> 
> 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 software timestamp report.
> 2. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_OPT_RX_FILTER
> while the skb is timestamped, it will stop reporting the rx software
> timestamp.
> 
> 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.
> 
> Suggested-by: Willem de Bruijn <willemb@...gle.com>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>

nit: Reviewed-by tags are only sticky if no changes are made.

> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> index 5e93cd71f99f..37ead02be3b1 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_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.

This raises the question: why would a process request
report flag SOF_TIMESTAMPING_SOFTWARE without generate flag
SOF_TIMESTAMPING_RX_SOFTWARE? The only sensible use case I see is when
it sets SOF_TIMSETAMPING_TX_SOFTWARE. Probably good to mention that.

May also be good to mention that existing applications sometimes set
SOF_TIMESTAMPING_SOFTWARE only, because they implicitly came to depend
on another (usually daemon) process to enable rx timestamps systemwide.

> +
> +  SOF_TIMESTAMPING_OPT_RX_FILTER prevents the application from being
> +  influenced by others and let the application choose whether to report
> +  the timestamp in the receive path or not.
> +

I'd drop this paragraph. It's more of a value statement.

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