From nobody Sun Feb 8 07:07:10 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B29F82741BC; Sun, 12 Oct 2025 11:24:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760268261; cv=none; b=fE+cgsOGafEfdqulSaBt0RfjzHVhQe0lNx2g5qBCQ9ionKvPkUAJ5QHyimEQKlzkjbBzPHcofiINmnd/fhwYrAwqFKJSCr86uU6MEVvtFdEmCSo+1Z7ZsrxTZUMie5WYNtiwq2S7Xz7kbnNHl2xb6pDIPiFCQWFH5KRnMNZew7M= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760268261; c=relaxed/simple; bh=hvsex+jt2rVqZK9V7K+FVvNoMhemQw6fzEbsRr0vloU=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=aqEn3G5xD/o02AUTTsfYipp/h+ppb1frBnVXYaZ7s0AzPLHRpL6dq3ZJCqCGxKynoHscH36rt8yi/eqY4w9azsY70IAXAeIvFql9NQ/brgDTLJ0CVbWmgr/ZHeLb+CtY0dG3PSFKPdGax08KgS276ZR1O/zmsYE49o0Ai3gjimA= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pAliAPLJ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="pAliAPLJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50B85C16AAE; Sun, 12 Oct 2025 11:24:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1760268261; bh=hvsex+jt2rVqZK9V7K+FVvNoMhemQw6fzEbsRr0vloU=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=pAliAPLJRpThSNxFmpN+NmrAVkND0J1sX7K2OZ+ryDhOMxOreCgXvYaomhTGtke/N iZ5URNaxaghTFp4JFsewKHtpTY95N6y+zBIMxWJiluWNG8Y+qcXC/J3r4HlUFyxmp8 uFo13O6HParDEertgarVhxSuz2OHYLbjR7WWJkoxburkB3eFhEwfj5uECGEfTFge/u nC08M84PqzzCQEkDouZw8QY1ubp8AufBnAETb1U1/mt1laoe1V1GQOYqFvC+9sZS1N eu7HOOWHwquWH9QzPOQIU46OcQ+BmjwbtWsaf1p+eLew+SXxD2C/3kpT4WwtLoOxYg T7hbnx58TCO+A== From: Vincent Mailhol Date: Sun, 12 Oct 2025 20:23:42 +0900 Subject: [PATCH 1/2] can: remove false statement about 1:1 mapping between DLC and length Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20251012-can-fd-doc-v1-1-86cc7d130026@kernel.org> References: <20251012-can-fd-doc-v1-0-86cc7d130026@kernel.org> In-Reply-To: <20251012-can-fd-doc-v1-0-86cc7d130026@kernel.org> To: Oliver Hartkopp , Marc Kleine-Budde Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Jonathan Corbet , Geert Uytterhoeven , linux-can@vger.kernel.org, netdev@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Vincent Mailhol X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=2180; i=mailhol@kernel.org; h=from:subject:message-id; bh=hvsex+jt2rVqZK9V7K+FVvNoMhemQw6fzEbsRr0vloU=; b=owGbwMvMwCV2McXO4Xp97WbG02pJDBmv+29HXbT6vNKmYIvV0QM/j7OEa6vP7XG8M7/20SK1z 9Zbt1/t6ChlYRDjYpAVU2RZVs7JrdBR6B126K8lzBxWJpAhDFycAjCRW0UM/2tE7z9zTGeKZD4o v5x3y7RtsXOzVNgW/i/UM/HylyrJNmJkuK4duv9/+r6nffNbHx6ZWcHQ/mry9UmZzVw/4zwzP+7 z4gUA X-Developer-Key: i=mailhol@kernel.org; a=openpgp; fpr=ED8F700574E67F20E574E8E2AB5FEB886DBB99C2 The CAN-FD section of can.rst still states that there is a 1:1 mapping between the Classical CAN DLC and its length. This is only true for the DLC values up to 8. Beyond that point, the length remains at 8. For reference, the mapping between the CAN DLC and the length is given in below table [1]: DLC value CBFF and CEFF FBFF and FEFF [decimal] [byte] [byte] ---------------------------------------------- 0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 9 8 12 10 8 16 11 8 20 12 8 24 13 8 32 14 8 48 15 8 64 Remove the erroneous statement. Instead just state that the length of a Classical CAN frame ranges from 0 to 8. [1] ISO 11898-1:2024, Table 5 -- DLC: coding of the four LSB Signed-off-by: Vincent Mailhol --- Documentation/networking/can.rst | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/Documentation/networking/can.rst b/Documentation/networking/ca= n.rst index f93049f03a37..58c026d51d94 100644 --- a/Documentation/networking/can.rst +++ b/Documentation/networking/can.rst @@ -1398,10 +1398,9 @@ second bit timing has to be specified in order to en= able the CAN FD bitrate. Additionally CAN FD capable CAN controllers support up to 64 bytes of payload. The representation of this length in can_frame.len and canfd_frame.len for userspace applications and inside the Linux network -layer is a plain value from 0 .. 64 instead of the CAN 'data length code'. -The data length code was a 1:1 mapping to the payload length in the Classi= cal -CAN frames anyway. The payload length to the bus-relevant DLC mapping is -only performed inside the CAN drivers, preferably with the helper +layer is a plain value from 0 .. 64 instead of the Classical CAN length +which ranges from 0 to 8. The payload length to the bus-relevant DLC mappi= ng +is only performed inside the CAN drivers, preferably with the helper functions can_fd_dlc2len() and can_fd_len2dlc(). =20 The CAN netdevice driver capabilities can be distinguished by the network --=20 2.49.1 From nobody Sun Feb 8 07:07:10 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 80E59274B40; Sun, 12 Oct 2025 11:24:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760268263; cv=none; b=dWvhztkVX30mvmEA1UxMklMTTq6KbLWqYtpczgbKPa890xQvASU9i/qwu+Qkxnysvk0uGIqJ7FtQQ0AGGFE54kgmXwpGAk4dfGEgkX8Y6N9GtdxRTkzbnSbyoVFgmEYzgsnglxpsq2w3p/RUWwLdrz5XIMu9mMVBIuSZlSJrmJ8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760268263; c=relaxed/simple; bh=IF07Mt8rYmUujU1hE7dfnLqFNnP79CelssmnQbjwpiA=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=FeskOalqQbOULBKUp14ujBU22+bj85/IRX/i95YFhO76R6kETTxMAMupAoXntnSipR9sK5xCqtVAFFWbNu/6+9wlgV1O02LuX+X6WwuhVH/e5YDNCFlBsXrB6VLLAjPPa1TmLl5l5xPYLPrRg0Vocf0LnCQGVkJtvx3wG6u3jO8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qV9gDa84; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="qV9gDa84" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 80A0CC116D0; Sun, 12 Oct 2025 11:24:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1760268263; bh=IF07Mt8rYmUujU1hE7dfnLqFNnP79CelssmnQbjwpiA=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=qV9gDa849luF9pS3ep8osjNW551NYhmIU8gnYWTEuQRwQ/xuGF+YZD5XX8UQfNEQ9 smM9sw/fkzd7SIxqr4jlS5WnlB6omrILMlXKXIy6xMjOtLYHRY+XPKg3+c95O7YlvR gjTmuBGcq5t4X3Z5LPtuR4J16jYYpuLEBJ6ZrS2tT6Xm8oGaQlwjXuWej0D3//nCV7 R11k/0vbDT7q8sPUqcA/f4++RFyyROIYXQDOXTwzINx26ev9J8L/25Q7VhJg241VAI AC/psM2DDbubL9SDNUQqfXrQcvQ0XSuYAXW3X3U9ZMEKDC84cr0U5ym2t9wB3mFqde jeOLkUl5aydCA== From: Vincent Mailhol Date: Sun, 12 Oct 2025 20:23:43 +0900 Subject: [PATCH 2/2] can: add Transmitter Delay Compensation (TDC) documentation Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20251012-can-fd-doc-v1-2-86cc7d130026@kernel.org> References: <20251012-can-fd-doc-v1-0-86cc7d130026@kernel.org> In-Reply-To: <20251012-can-fd-doc-v1-0-86cc7d130026@kernel.org> To: Oliver Hartkopp , Marc Kleine-Budde Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Jonathan Corbet , Geert Uytterhoeven , linux-can@vger.kernel.org, netdev@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Vincent Mailhol X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=4110; i=mailhol@kernel.org; h=from:subject:message-id; bh=IF07Mt8rYmUujU1hE7dfnLqFNnP79CelssmnQbjwpiA=; b=owGbwMvMwCV2McXO4Xp97WbG02pJDBmv++/OuMc0vahLVXWS8lWtGQqlwn+FpEOEorQ+69k0P 1jzi3VKRykLgxgXg6yYIsuyck5uhY5C77BDfy1h5rAygQxh4OIUgInsVWL4Z3t+z4eZ882/+cZv ui/dtP7XrZwq5gnNR/7cXFQUc2xPRSojw52tB64m8jL/k6ry+HUit2Wrx2G7yCdzXNIlxGtOnJU pZQIA X-Developer-Key: i=mailhol@kernel.org; a=openpgp; fpr=ED8F700574E67F20E574E8E2AB5FEB886DBB99C2 Back in 2021, support for CAN TDC was added to the kernel in series [1] and in iproute2 in series [2]. However, the documentation was never updated. Add a new sub-section under CAN-FD driver support to document how to configure the TDC using the "ip tool". [1] add the netlink interface for CAN-FD Transmitter Delay Compensation (TD= C) Link: https://lore.kernel.org/all/20210918095637.20108-1-mailhol.vincent@wa= nadoo.fr/ [2] iplink_can: cleaning, fixes and adding TDC support Link: https://lore.kernel.org/all/20211103164428.692722-1-mailhol.vincent@w= anadoo.fr/ Signed-off-by: Vincent Mailhol --- Documentation/networking/can.rst | 60 ++++++++++++++++++++++++++++++++++++= ++++ 1 file changed, 60 insertions(+) diff --git a/Documentation/networking/can.rst b/Documentation/networking/ca= n.rst index 58c026d51d94..de9e7549859f 100644 --- a/Documentation/networking/can.rst +++ b/Documentation/networking/can.rst @@ -1464,6 +1464,66 @@ Example when 'fd-non-iso on' is added on this switch= able CAN FD adapter:: can state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart= -ms 0 =20 =20 +Transmitter Delay Compensation +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +At high bit rates, the propagation delay from the TX pin to the RX pin of +the transceiver might become greater than the actual bit time causing +measurement errors: the RX pin would still be measuring the previous bit. + +The Transmitter Delay Compensation (thereafter, TDC) resolves this problem +by introducing a Secondary Sample Point (SSP) equal to the distance, in +minimum time quantum, from the start of the bit time on the TX pin to the +actual measurement on the RX pin. The SSP is calculated as the sum of two +configurable values: the TDC Value (TDCV) and the TDC offset (TDCO). + +TDC, if supported by the device, can be configured together with CAN-FD +using the ip tool's "tdc-mode" argument as follow:: + +- **omitted**: when no "tdc-mode" option is provided, the kernel will + automatically decide whether TDC should be turned on, in which case it + will calculate a default TDCO and use the TDCV as measured by the + device. This is the recommended method to use TDC. + +- **"tdc-mode off"**: TDC is explicitly disabled. + +- **"tdc-mode auto"**: the user must provide the "tdco" argument. The TDCV + will be automatically calculated by the device. This option is only + available if the device supports the TDC-AUTO CAN controller mode. + +- **"tdc-mode manual"**: the user must provide both the "tdco" and "tdcv" + arguments. This option is only available if the device supports the + TDC-MANUAL CAN controller mode. + +Note that some devices may offer an additional parameter: "tdcf" (TDC Filt= er +window). If supported by your device, this can be added as an optional +argument to either "tdc-mode auto" or "tdc-mode manual". + +Example configuring a 500 kbit/s arbitration bitrate, a 5 Mbit/s data +bitrate, a TDCO of 15 minimum time quantum and a TDCV automatically measur= ed +by the device:: + + $ ip link set can0 up type can bitrate 500000 \ + fd on dbitrate 4000000 \ + tdc-mode auto tdco 15 + $ ip -details link show can0 + 5: can0: mtu 72 qdisc pfifo_fast state UP \ + mode DEFAULT group default qlen 10 + link/can promiscuity 0 allmulti 0 minmtu 72 maxmtu 72 + can state ERROR-ACTIVE restart-ms 0 + bitrate 500000 sample-point 0.875 + tq 12 prop-seg 69 phase-seg1 70 phase-seg2 20 sjw 10 brp 1 + ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512= \ + brp_inc 1 + dbitrate 4000000 dsample-point 0.750 + dtq 12 dprop-seg 7 dphase-seg1 7 dphase-seg2 5 dsjw 2 dbrp 1 + tdco 15 tdcf 0 + ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 \ + dbrp_inc 1 + tdco 0..127 tdcf 0..127 + clock 80000000 + + Supported CAN Hardware ---------------------- =20 --=20 2.49.1