netdev - Re: [PATCH v9 net-next 13/15] tcp: accecn: add tcpi_ecn_mode and tcpi_option2 in tcp

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Message-ID: <CANn89iKhAs3rMWFdmM-rma1wS-Xd7mf9PBUQKL8YMKhB81Jkkw@mail.gmail.com>
Date: Tue, 20 Jan 2026 12:18:02 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: chia-yu.chang@...ia-bell-labs.com
Cc: pabeni@...hat.com, parav@...dia.com, linux-doc@...r.kernel.org, 
	corbet@....net, horms@...nel.org, dsahern@...nel.org, kuniyu@...gle.com, 
	bpf@...r.kernel.org, netdev@...r.kernel.org, dave.taht@...il.com, 
	jhs@...atatu.com, kuba@...nel.org, stephen@...workplumber.org, 
	xiyou.wangcong@...il.com, jiri@...nulli.us, davem@...emloft.net, 
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	liuhangbin@...il.com, shuah@...nel.org, linux-kselftest@...r.kernel.org, 
	ij@...nel.org, ncardwell@...gle.com, koen.de_schepper@...ia-bell-labs.com, 
	g.white@...lelabs.com, ingemar.s.johansson@...csson.com, 
	mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at, 
	Jason_Livingood@...cast.com, vidhi_goel@...le.com
Subject: Re: [PATCH v9 net-next 13/15] tcp: accecn: add tcpi_ecn_mode and
 tcpi_option2 in tcp_info

On Mon, Jan 19, 2026 at 7:59 PM <chia-yu.chang@...ia-bell-labs.com> wrote:
>
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
> Add 2-bit tcpi_ecn_mode feild within tcp_info to indicate which ECN
> mode is negotiated: ECN_MODE_DISABLED, ECN_MODE_RFC3168, ECN_MODE_ACCECN,
> or ECN_MODE_PENDING. This is done by utilizing available bits from
> tcpi_accecn_opt_seen (reduced from 16 bits to 2 bits) and
> tcpi_accecn_fail_mode (reduced from 16 bits to 4 bits).
>
> Also, an extra 24-bit tcpi_options2 field is identified to represent
> newer options and connection features, as all 8 bits of tcpi_options
> field have been used.
>
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> Co-developed-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

Are you sure Neal Cardwell really is ok with this patch, saw it and
gave his SOB ?

> ---

>  struct tcp_info {
>         __u8    tcpi_state;
>         __u8    tcpi_ca_state;
> @@ -316,15 +334,17 @@ struct tcp_info {
>                                          * in milliseconds, including any
>                                          * unfinished recovery.
>                                          */
> -       __u32   tcpi_received_ce;    /* # of CE marks received */
> +       __u32   tcpi_ecn_mode:2,
> +               tcpi_accecn_opt_seen:2,
> +               tcpi_accecn_fail_mode:4,
> +               tcpi_options2:24;
> +       __u32   tcpi_received_ce;    /* # of CE marked segments received */
>         __u32   tcpi_delivered_e1_bytes;  /* Accurate ECN byte counters */
>         __u32   tcpi_delivered_e0_bytes;
>         __u32   tcpi_delivered_ce_bytes;
>         __u32   tcpi_received_e1_bytes;
>         __u32   tcpi_received_e0_bytes;
>         __u32   tcpi_received_ce_bytes;
> -       __u16   tcpi_accecn_fail_mode;
> -       __u16   tcpi_accecn_opt_seen;
>  };

tcp_info is ABI.

We can not add/remove fields in the middle.

You must add fields at the end of it only.