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Mon, 01 Jun 2026 06:35:18 -0700 (PDT) Received: from [192.168.88.32] ([169.155.232.197]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-490b079c7d4sm67795e9.7.2026.06.01.06.35.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 01 Jun 2026 06:35:17 -0700 (PDT) Message-ID: <0b71d385-7504-4a5b-8e21-dcf7c692d456@redhat.com> Date: Mon, 1 Jun 2026 15:35:16 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] net: ibm: emac: Reserve VLAN header in MJS limit To: Jacob Keller , Rosen Penev , netdev@vger.kernel.org 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> From: Paolo Abeni Content-Language: en-US In-Reply-To: <625ecceb-3b0a-45a8-817d-337b37df7335@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/30/26 1:18 AM, Jacob Keller wrote: > On 5/26/2026 1:22 PM, Rosen Penev wrote: >> The IBM EMAC programs its Maximum Jumbo Size (MJS) drop >> threshold from ndev->mtu directly. 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