netdev - [PATCH v5 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP

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Message-Id: <20251029080608.92428-4-chia-yu.chang@nokia-bell-labs.com>
Date: Wed, 29 Oct 2025 09:05:57 +0100
From: chia-yu.chang@...ia-bell-labs.com
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v5 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

No functional changes.

Co-developed-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/linux/skbuff.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a7cc3d1f4fd1..74d6a209e203 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -671,7 +671,12 @@ enum {
 	/* This indicates the skb is from an untrusted source. */
 	SKB_GSO_DODGY = 1 << 1,
 
-	/* This indicates the tcp segment has CWR set. */
+	/* For Tx, this indicates the first TCP segment has CWR set, and any
+	 * subsequent segment in the same skb has CWR cleared. This cannot be
+	 * used on Rx, because the connection to which the segment belongs is
+	 * not tracked to use RFC3168 or Accurate ECN, and using RFC3168 ECN
+	 * offload may corrupt AccECN signal of AccECN segments.
+	 */
 	SKB_GSO_TCP_ECN = 1 << 2,
 
 	__SKB_GSO_TCP_FIXEDID = 1 << 3,
@@ -706,6 +711,12 @@ enum {
 
 	SKB_GSO_FRAGLIST = 1 << 18,
 
+	/* For TX, this indicates the TCP segment uses the CWR flag as part of
+	 * AccECN signal, and the CWR flag is not modified in the skb. For RX,
+	 * any CWR flagged segment must use SKB_GSO_TCP_ACCECN. This is to
+	 * ensure the CWR flag is not cleared by any RFC3168 ECN offload, and
+	 * thus keeping AccECN signal of AccECN segments.
+	 */
 	SKB_GSO_TCP_ACCECN = 1 << 19,
 
 	/* These indirectly map onto the same netdev feature.
-- 
2.34.1