tcp: clarify tcp_congestion_ops functions comments

The optional and required hints in the tcp_congestion_ops are information
for the user of this interface to signalize its importance when
implementing these functions.

However, cong_avoid comment incorrectly tells that it is required,
in reality congestion control must provide one of either cong_avoid or
cong_control.

In addition, min_tso_segs has not had any comment optional/required
hints. So mark it as optional since it is used only in BBR.

Co-developed-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Daniel Sedlak <daniel.sedlak@cdn77.com>
Link: https://patch.msgid.link/20260105115533.1151442-1-daniel.sedlak@cdn77.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0deb5e9dd9114641468f26696b086b8261a67f60..ef0fee58fde82620399df49a35c7ecae8e34068e 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1243,12 +1243,27 @@ struct rate_sample {
 struct tcp_congestion_ops {
 /* fast path fields are put first to fill one cache line */
 
+       /* A congestion control (CC) must provide one of either:
+        *
+        * (a) a cong_avoid function, if the CC wants to use the core TCP
+        *     stack's default functionality to implement a "classic"
+        *     (Reno/CUBIC-style) response to packet loss, RFC3168 ECN,
+        *     idle periods, pacing rate computations, etc.
+        *
+        * (b) a cong_control function, if the CC wants custom behavior and
+        *      complete control of all congestion control behaviors.
+        */
+       /* (a) "classic" response: calculate new cwnd.
+        */
+       void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked);
+       /* (b) "custom" response: call when packets are delivered to update
+        * cwnd and pacing rate, after all the ca_state processing.
+        */
+       void (*cong_control)(struct sock *sk, u32 ack, int flag, const struct rate_sample *rs);
+
        /* return slow start threshold (required) */
        u32 (*ssthresh)(struct sock *sk);
 
-       /* do new cwnd calculation (required) */
-       void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked);
-
        /* call before changing ca_state (optional) */
        void (*set_state)(struct sock *sk, u8 new_state);
 
@@ -1261,15 +1276,9 @@ struct tcp_congestion_ops {
        /* hook for packet ack accounting (optional) */
        void (*pkts_acked)(struct sock *sk, const struct ack_sample *sample);
 
-       /* override sysctl_tcp_min_tso_segs */
+       /* override sysctl_tcp_min_tso_segs (optional) */
        u32 (*min_tso_segs)(struct sock *sk);
 
-       /* call when packets are delivered to update cwnd and pacing rate,
-        * after all the ca_state processing. (optional)
-        */
-       void (*cong_control)(struct sock *sk, u32 ack, int flag, const struct rate_sample *rs);
-
-
        /* new value of cwnd after loss (required) */
        u32  (*undo_cwnd)(struct sock *sk);
        /* returns the multiplier used in tcp_sndbuf_expand (optional) */