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Mon, 1 Jun 2026 22:09:32 +0000 Message-ID: Date: Mon, 1 Jun 2026 15:09:30 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] net: ibm: emac: Reserve VLAN header in MJS limit To: Paolo Abeni , Rosen Penev , CC: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , open list References: <20260526202247.13823-1-rosenp@gmail.com> <625ecceb-3b0a-45a8-817d-337b37df7335@intel.com> <0b71d385-7504-4a5b-8e21-dcf7c692d456@redhat.com> Content-Language: en-US From: Jacob Keller In-Reply-To: <0b71d385-7504-4a5b-8e21-dcf7c692d456@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0127.namprd03.prod.outlook.com (2603:10b6:303:8c::12) To DS0PR11MB7381.namprd11.prod.outlook.com (2603:10b6:8:134::14) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7381:EE_|PH8PR11MB9605:EE_ X-MS-Office365-Filtering-Correlation-Id: 7171ac9c-6834-4346-cbf2-08dec02a754e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|4143699003|22082099003|18002099003|56012099006|11063799006|6133799003|5023799004; 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The hardware sizes the threshold >>> against the L2 frame minus the ethernet header, but does not >>> discount the 802.1Q tag, so a frame carrying a VLAN tag and a full >>> 1500-byte payload exceeds MJS by exactly 4 bytes and is dropped. >>> >>> This is normally hidden because JPSM (and therefore the MJS check) >>> only engages when the MTU is raised above ETH_DATA_LEN. With the >>> qca8k DSA tagger the conduit MTU is bumped by QCA_HDR_LEN to 1502 >>> during dsa_conduit_setup(), which is enough to enable JPSM and >>> expose the off-by-VLAN-tag in the limit. >>> >>> Pad MJS by VLAN_HLEN so a VLAN-tagged full-MTU frame passes. >>> >>> Reported on Meraki MX60 (qca8k switch): tagged VLAN >>> traffic drops at 1500-byte payload, while 1496 bytes works >>> and untagged 1500 bytes works. >>> >>> Assisted-by: Claude:Opus-4.7 >>> Signed-off-by: Rosen Penev >> >> If this results in dropped packets, that seems like a user visible bug.. >> Should this have a Fixes tag and target the net-next tree? I guess its >> not a "regression" since the driver has always behaved this way, but it >> still seems like a bug to me. > > Given it never worked and we are trying to keep the amount of changes > into net under control, I tend to consider this suitable for net-next > i.e. enable DSA support. > > /P > Makes sense. Reviewed-by: Jacob Keller