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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-490b0e20a24sm122157465e9.5.2026.06.03.01.13.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Jun 2026 01:13:55 -0700 (PDT) Date: Wed, 3 Jun 2026 09:13:54 +0100 From: David Laight To: Andrew Lunn Cc: Selvamani Rajagopal , Piergiorgio Beruto , "parthiban.veerasooran@microchip.com" , "andrew+netdev@lunn.ch" , "davem@davemloft.net" , "edumazet@google.com" , "kuba@kernel.org" , "pabeni@redhat.com" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH net-next v3 08/14] net: ethernet: oa_tc6: Remove FCS size in RX frame Message-ID: <20260603091354.62b65ec3@pumpkin> In-Reply-To: References: <20260602150712.5b28d58b@pumpkin> <20260602223807.1ac76456@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 2 Jun 2026 23:47:23 +0200 Andrew Lunn wrote: > > The frames are padded to a minimum size so that collision detect works > > on maximal length 10M coax segments. > > (Or rather coax+link+coax+link+coax acting as a single collision domain.) > > The same minimum frame length is used on modern networks which are all > > physically point-to-point and all 'hubs' are store+forward where a minimum > > packet length isn't strictly needed. > > You need to be careful here. This is automotive T1 system. They do > actually have old school CSMA/CD. Did i remember the acronym correct, > Carrier Sensing, Multiple Access, Collision Detection? That's what I'm remembering it being. Somewhere I lost (or failed to steal) the hardback (IIRC) 8802.11 book that explained it all (I doubt it was even 100 pages). The max length collision domain is a 1.5km coax segment linked by a 1.5km bridge to a second coax segment and then onto a third coax one. The minimum packet length has to be greater than the round trip time from one end to the other. I doubt a car has that much cable in it. It might have a 10/100m HDX TP hub that creates a collision domain. Under load it should retransmit rather than discard. -- David > > Andrew