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a=ed25519-sha256; c=relaxed/relaxed; t=1757494139; s=strato-dkim-0003; d=hartkopp.net; h=In-Reply-To:From:References:To:Subject:Date:Message-ID:Cc:Date:From: Subject:Sender; bh=15VvcYXnCDgh2YQH3JtlRJ9B6SfYbrICCZZ0atxzCkQ=; b=fXxQwMebfQmscxX6e/wmwCR8eqhTFA/xcZLS+RUarneHk3+HYsuj1FcTjoV9mhYXiJ JbJtJd6nr1VpMb4W+QCw== X-RZG-AUTH: ":P2MHfkW8eP4Mre39l357AZT/I7AY/7nT2yrDxb8mjH4JKvMdQv2tTUsMrZpkO3Mw3lZ/t54cFxeEQ7s8bGWj0Q==" Received: from [IPV6:2a00:6020:4a38:6810::9f3] by smtp.strato.de (RZmta 52.1.2 AUTH) with ESMTPSA id K5d36118A8mxorw (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256 bits)) (Client did not present a certificate); Wed, 10 Sep 2025 10:48:59 +0200 (CEST) Message-ID: <3a963548-faa9-4611-a3cf-e41b425968a8@hartkopp.net> Date: Wed, 10 Sep 2025 10:48:54 +0200 Precedence: bulk X-Mailing-List: linux-can@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v5 2/2] can: reject CAN FD content when disabled on CAN XL interfaces To: Vincent Mailhol , linux-can@vger.kernel.org References: <20250909092433.30546-1-socketcan@hartkopp.net> <20250909092433.30546-2-socketcan@hartkopp.net> <7578f44d-d85c-473e-8e7a-65d1fc974e68@kernel.org> <20c5c885-0bab-4c42-82c6-e98571a5d19d@hartkopp.net> <552631f3-15fe-4bb3-a512-1eaca57be5ca@kernel.org> Content-Language: en-US From: Oliver Hartkopp In-Reply-To: <552631f3-15fe-4bb3-a512-1eaca57be5ca@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10.09.25 09:40, Vincent Mailhol wrote: > On 10/09/2025 at 16:27, Oliver Hartkopp wrote: >>>     /* CAN XL is allowed on virtual interfaces if it fits the MTU */ >>>        if (!priv) >>>            return dev->mtu == CANXL_MTU; >> >>         return can_is_canxl_dev_mtu(mtu); >> >> The MTU of CAN XL interfaces might vary. > > Maybe this is something that we discussed before, I do not remember, but how is > it that the MTU can vary? > > MTU is the *Maximum* Transmission Unit. I understand that the size of a CAN XL > frame is variable, but the MTU should be constant, right? Why can it vary? Depending on the realtime requirements the length of the CAN frames (and therefore the time the bus is blocked) the MTU can be reduced. This is (like the bitrate settings) a network architects decision which is enforced by setting the MTU accordingly. Best regards, Oliver