-Linux SocketCAN driver for ELM327
-==================================
+.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
+
+can327: ELM327 driver for Linux SocketCAN
+==========================================
+
+Upstream
+--------
+
+This driver has become an official part of Linux since v6.0:
+
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/can/can327.c?h=v6.0
+
+For documentation and how to use the driver, please refer to the
+documentation that comes with your Linux kernel version, since
+the details may change as the driver evolves. The documentation
+below applies ONLY to the downstream driver.
+
+ https://www.kernel.org/doc/html/latest/networking/device_drivers/can/can327.html
+
+Please send patches upstream to [linux-can] and the maintainers for
+this driver.
+
+I may or may not backport upstream patches to this downstream
+repository, and/or use it for further pre-upstream work.
+
+
+
+Out-of-tree version
+--------------------
+
+This is the non-upstreamed version of the can327 driver.
+Please see out-of-tree.rst for compilation/usage hints.
+
+
Authors
--------
-Max Staudt <elmcan@enpas.org>
+Max Staudt <max-linux@enpas.org>
Motivation
-----------
-CAN adapters are expensive, few, and far between.
-ELM327 interfaces are cheap and plentiful.
-
This driver aims to lower the initial cost for hackers interested in
working with CAN buses.
+CAN adapters are expensive, few, and far between.
+ELM327 interfaces are cheap and plentiful.
+Let's use ELM327s as CAN adapters.
+
Introduction
-------------
This driver is an effort to turn abundant ELM327 based OBD interfaces
-into full-fledged (as far as possible) CAN interfaces.
+into full fledged (as far as possible) CAN interfaces.
-Since the ELM327 was never meant to be a stand-alone CAN controller,
-the driver has to switch between its modes asa quickly as possible in
-order to approximate full-duplex operation.
+Since the ELM327 was never meant to be a stand alone CAN controller,
+the driver has to switch between its modes as quickly as possible in
+order to fake full-duplex operation.
-As such, elmcan is a best-effort driver. However, this is more than
+As such, can327 is a best effort driver. However, this is more than
enough to implement simple request-response protocols (such as OBD II),
and to monitor broadcast messages on a bus (such as in a vehicle).
+How to attach the line discipline
+----------------------------------
+
+Every ELM327 chip is factory programmed to operate at a serial setting
+of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
+
+If you have kept this default configuration, the line discipline can
+be attached on a command prompt as follows::
+
+ sudo ldattach \
+ --debug \
+ --speed 38400 \
+ --eightbits \
+ --noparity \
+ --onestopbit \
+ --iflag -ICRNL,INLCR,-IXOFF \
+ 29 \
+ /dev/ttyUSB0
+
+To change the ELM327's serial settings, please refer to its data
+sheet. This needs to be done before attaching the line discipline.
+
+Once the ldisc is attached, the CAN interface starts out unconfigured.
+Set the speed before starting it::
+
+ # The interface needs to be down to change parameters
+ sudo ip link set can0 down
+ sudo ip link set can0 type can bitrate 500000
+ sudo ip link set can0 up
+
+500000 bit/s is a common rate for OBD-II diagnostics.
+If you're connecting straight to a car's OBD port, this is the speed
+that most cars (but not all!) expect.
+
+After this, you can set out as usual with candump, cansniffer, etc.
+
+
+
How to check the controller version
------------------------------------
>
+Note that clones may claim to be any version they like.
+It is not indicative of their actual feature set.
-How to attach the line discipline
-----------------------------------
-Every ELM327 chip is factory programmed to operate at a serial setting
-of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
-The line discipline can be attached on a command prompt as follows::
+Communication example
+----------------------
- sudo ldattach \
- --debug \
- --speed 38400 \
- --eightbits \
- --noparity \
- --onestopbit \
- --iflag -ICRNL,INLCR,-IXOFF \
- 26 \
- /dev/ttyUSB0
+This is a short and incomplete introduction on how to talk to an ELM327.
+It is here to guide understanding of the controller's and the driver's
+limitation (listed below) as well as manual testing.
-To change the ELM327's serial settings, please refer to its data
-sheet. This needs to be done before attaching the line discipline.
+
+The ELM327 has two modes:
+
+- Command mode
+- Reception mode
+
+In command mode, it expects one command per line, terminated by CR.
+By default, the prompt is a "``>``", after which a command can be
+entered::
+
+ >ATE1
+ OK
+ >
+
+The init script in the driver switches off several configuration options
+that are only meaningful in the original OBD scenario the chip is meant
+for, and are actually a hindrance for can327.
+
+
+When a command is not recognized, such as by an older version of the
+ELM327, a question mark is printed as a response instead of OK::
+
+ >ATUNKNOWN
+ ?
+ >
+
+At present, can327 does not evaluate this response. See the section
+below on known limitations for details.
+
+
+When a CAN frame is to be sent, the target address is configured, after
+which the frame is sent as a command that consists of the data's hex
+dump::
+
+ >ATSH123
+ OK
+ >DEADBEEF12345678
+ OK
+ >
+
+The above interaction sends the SFF frame "``DE AD BE EF 12 34 56 78``"
+with (11 bit) CAN ID ``0x123``.
+For this to function, the controller must be configured for SFF sending
+mode (using "``AT PB``", see code or datasheet).
+
+
+Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
+configured on ``listen-only=off``), or when the reply timeout expires
+and the driver sets the controller into monitoring mode (``ATMA``),
+the ELM327 will send one line for each received CAN frame, consisting
+of CAN ID, DLC, and data::
+
+ 123 8 DEADBEEF12345678
+
+For EFF (29 bit) CAN frames, the address format is slightly different,
+which can327 uses to tell the two apart::
+
+ 12 34 56 78 8 DEADBEEF12345678
+
+The ELM327 will receive both SFF and EFF frames - the current CAN
+config (``ATPB``) does not matter.
+
+
+If the ELM327's internal UART sending buffer runs full, it will abort
+the monitoring mode, print "BUFFER FULL" and drop back into command
+mode. Note that in this case, unlike with other error messages, the
+error message may appear on the same line as the last (usually
+incomplete) data frame::
+
+ 12 34 56 78 8 DEADBEEF123 BUFFER FULL
Known limitations of the controller
------------------------------------
+- Clone devices ("v1.5" and others)
+
+ Sending RTR frames is not supported and will be dropped silently.
+
+ Receiving RTR with DLC 8 will appear to be a regular frame with
+ the last received frame's DLC and payload.
+
+ "``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is
+ not supported, and is hard coded to ON. Thus, frames are not ACKed
+ while listening: "``AT MA``" (Monitor All) will always be "silent".
+ However, immediately after sending a frame, the ELM327 will be in
+ "receive reply" mode, in which it *does* ACK any received frames.
+ Once the bus goes silent, or an error occurs (such as BUFFER FULL),
+ or the receive reply timeout runs out, the ELM327 will end reply
+ reception mode on its own and can327 will fall back to "``AT MA``"
+ in order to keep monitoring the bus.
+
+ Other limitations may apply, depending on the clone and the quality
+ of its firmware.
+
+
- All versions
- No automatic full duplex operation is supported. The driver will
- switch between input/output mode as quickly as possible.
+ No full duplex operation is supported. The driver will switch
+ between input/output mode as quickly as possible.
The length of outgoing RTR frames cannot be set. In fact, some
clones (tested with one identifying as "``v1.5``") are unable to
- Versions prior to 1.3
- These chips cannot be used at all with elmcan. They do not support
- the "``AT D1``", which is necessary to avoid parsing conflicts on
- incoming data, as well as distinction of RTR frame lengths.
+ These chips cannot be used at all with can327. They do not support
+ the "``AT D1``" command, which is necessary to avoid parsing conflicts
+ on incoming data, as well as distinction of RTR frame lengths.
Specifically, this allows for easy distinction of SFF and EFF
frames, and to check whether frames are complete. While it is possible
helpful here, however SocketCAN does not currently provide a facility
to make use of such hardware features.
-- No BUS-OFF state and automatic restart
-
- We currently reset the ELM327 and generate error frames manually.
- In the future, we may be able to use ``can_bus_off()`` and its siblings.
+Rationale behind the chosen configuration
+------------------------------------------
-Communication example
-----------------------
+``AT E1``
+ Echo on
-This is a short and incomplete introduction on how to talk to an ELM327.
+ We need this to be able to get a prompt reliably.
+``AT S1``
+ Spaces on
-The ELM327 has two modes:
-
-- Command mode
-- Reception mode
-
-In command mode, it expects one command per line, terminated by CR.
-By default, the prompt is a "``>``", after which a command can be
-entered::
-
- >ATE1
- OK
- >
-
-The init script in the driver switches off several configuration options
-that are only meaningful in the original OBD scenario the chip is meant
-for, and are actually a hindrance for elmcan.
-
-
-When a command is not recognized, such as by an older version of the
-ELM327, a question mark is printed as a response instead of OK::
-
- >ATUNKNOWN
- ?
- >
-
-At present, elmcan does not evaluate this response and silently assumes
-that all commands are recognized. It is structured such that it will
-degrade gracefully when a command is unknown. See the sections above on
-known limitations for details.
-
-
-When a CAN frame is to be sent, the target address is configured, after
-which the frame is sent as a command that consists of the data's hex
-dump::
-
- >ATSH123
- OK
- >DEADBEEF12345678
- OK
- >
-
-The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
-the 11 bit CAN ID ``0x123``.
-For this to function, the controller must be configured for 11 bit CAN
-ID sending mode (using "``AT PB``", see code or datasheet).
-
-
-Once a frame has been sent and wait-for-reply mode is on (ATR1,
-configured on listen-only=off), or when the reply timeout expires and
-the driver sets the controller into monitoring mode (``ATMA``), the ELM327
-will send one line for each received CAN frame, consisting of CAN ID,
-DLC, and data::
-
- 123 8 DEADBEEF12345678
-
-For 29 bit CAN frames, the address format is slightly different, which
-elmcan uses to tell the two apart::
-
- 12 34 56 78 8 DEADBEEF12345678
-
-The ELM327 will receive both 11 and 29 bit frames - the current CAN
-config (``ATPB``) does not matter.
-
-
-If the ELM327's internal UART sending buffer runs full, it will abort
-the monitoring mode, print "BUFFER FULL" and drop back into command
-mode. Note that in this case, unlike with other error messages, the
-error message may appear on the same line as the last (usually
-incomplete) data frame::
-
- 12 34 56 78 8 DEADBEEF123 BUFFER FULL
-
-
-
-To Do list for future development
-----------------------------------
+ We need this to distinguish 11/29 bit CAN addresses received.
-- Handle ``write()`` error
+ Note:
+ We can usually do this using the line length (odd/even),
+ but this fails if the line is not transmitted fully to
+ the host (BUFFER FULL).
-- Rename ``elm327_panic()``
+``AT D1``
+ DLC on
-- No auto-restart in ``elm327_panic()``?
+ We need this to tell the "length" of RTR frames.
-- Stop current function when in ``elm327_panic()``
-- ``if (!elm)``: Race with ``ldisc_close()``
-- DMA capable rx/tx buffers
+A note on CAN bus termination
+------------------------------
-- fixup constants, constant for '``>``'
+Your adapter may have resistors soldered in which are meant to terminate
+the bus. This is correct when it is plugged into a OBD-II socket, but
+not helpful when trying to tap into the middle of an existing CAN bus.
-- flushing of ``tx_work`` is too late in ``ldisc_close()``
+If communications don't work with the adapter connected, check for the
+termination resistors on its PCB and try removing them.
projects / elmcan.git / blobdiff