media: cec-pin-error-inj.rst: document CEC Pin Error Injection
The CEC Pin framework adds support for Error Injection. Document all the error injections commands and how to use it. Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
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Hans Verkuil
Mauro Carvalho Chehab
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@ -23,6 +23,7 @@ This part describes the CEC: Consumer Electronics Control
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cec-intro
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cec-funcs
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cec-pin-error-inj
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cec-header
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@ -0,0 +1,325 @@
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CEC Pin Framework Error Injection
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=================================
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The CEC Pin Framework is a core CEC framework for CEC hardware that only
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has low-level support for the CEC bus. Most hardware today will have
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high-level CEC support where the hardware deals with driving the CEC bus,
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but some older devices aren't that fancy. However, this framework also
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allows you to connect the CEC pin to a GPIO on e.g. a Raspberry Pi and
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you have now made a CEC adapter.
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What makes doing this so interesting is that since we have full control
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over the bus it is easy to support error injection. This is ideal to
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test how well CEC adapters can handle error conditions.
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Currently only the cec-gpio driver (when the CEC line is directly
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connected to a pull-up GPIO line) and the AllWinner A10/A20 drm driver
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support this framework.
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If ``CONFIG_CEC_PIN_ERROR_INJ`` is enabled, then error injection is available
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through debugfs. Specifically, in ``/sys/kernel/debug/cec/cecX/`` there is
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now an ``error-inj`` file.
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.. note::
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The error injection commands are not a stable ABI and may change in the
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future.
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With ``cat error-inj`` you can see both the possible commands and the current
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error injection status::
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$ cat /sys/kernel/debug/cec/cec0/error-inj
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# Clear error injections:
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# clear clear all rx and tx error injections
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# rx-clear clear all rx error injections
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# tx-clear clear all tx error injections
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# <op> clear clear all rx and tx error injections for <op>
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# <op> rx-clear clear all rx error injections for <op>
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# <op> tx-clear clear all tx error injections for <op>
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#
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# RX error injection:
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# <op>[,<mode>] rx-nack NACK the message instead of sending an ACK
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# <op>[,<mode>] rx-low-drive <bit> force a low-drive condition at this bit position
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# <op>[,<mode>] rx-add-byte add a spurious byte to the received CEC message
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# <op>[,<mode>] rx-remove-byte remove the last byte from the received CEC message
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# <op>[,<mode>] rx-arb-lost <poll> generate a POLL message to trigger an arbitration lost
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#
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# TX error injection settings:
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# tx-ignore-nack-until-eom ignore early NACKs until EOM
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# tx-custom-low-usecs <usecs> define the 'low' time for the custom pulse
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# tx-custom-high-usecs <usecs> define the 'high' time for the custom pulse
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# tx-custom-pulse transmit the custom pulse once the bus is idle
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#
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# TX error injection:
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# <op>[,<mode>] tx-no-eom don't set the EOM bit
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# <op>[,<mode>] tx-early-eom set the EOM bit one byte too soon
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# <op>[,<mode>] tx-add-bytes <num> append <num> (1-255) spurious bytes to the message
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# <op>[,<mode>] tx-remove-byte drop the last byte from the message
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# <op>[,<mode>] tx-short-bit <bit> make this bit shorter than allowed
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# <op>[,<mode>] tx-long-bit <bit> make this bit longer than allowed
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# <op>[,<mode>] tx-custom-bit <bit> send the custom pulse instead of this bit
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# <op>[,<mode>] tx-short-start send a start pulse that's too short
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# <op>[,<mode>] tx-long-start send a start pulse that's too long
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# <op>[,<mode>] tx-custom-start send the custom pulse instead of the start pulse
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# <op>[,<mode>] tx-last-bit <bit> stop sending after this bit
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# <op>[,<mode>] tx-low-drive <bit> force a low-drive condition at this bit position
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#
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# <op> CEC message opcode (0-255) or 'any'
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# <mode> 'once' (default), 'always', 'toggle' or 'off'
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# <bit> CEC message bit (0-159)
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# 10 bits per 'byte': bits 0-7: data, bit 8: EOM, bit 9: ACK
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# <poll> CEC poll message used to test arbitration lost (0x00-0xff, default 0x0f)
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# <usecs> microseconds (0-10000000, default 1000)
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clear
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You can write error injection commands to ``error-inj`` using
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``echo 'cmd' >error-inj`` or ``cat cmd.txt >error-inj``. The ``cat error-inj``
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output contains the current error commands. You can save the output to a file
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and use it as an input to ``error-inj`` later.
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Basic Syntax
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------------
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Leading spaces/tabs are ignored. If the next character is a ``#`` or the end
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of the line was reached, then the whole line is ignored. Otherwise a command
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is expected.
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The error injection commands fall in two main groups: those relating to
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receiving CEC messages and those relating to transmitting CEC messages. In
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addition, there are commands to clear existing error injection commands and
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to create custom pulses on the CEC bus.
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Most error injection commands can be executed for specific CEC opcodes or for
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all opcodes (``any``). Each command also has a 'mode' which can be ``off``
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(can be used to turn off an existing error injection command), ``once``
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(the default) which will trigger the error injection only once for the next
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received or transmitted message, ``always`` to always trigger the error
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injection and ``toggle`` to toggle the error injection on or off for every
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transmit or receive.
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So '``any rx-nack``' will NACK the next received CEC message,
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'``any,always rx-nack``' will NACK all received CEC messages and
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'``0x82,toggle rx-nack``' will only NACK if an Active Source message was
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received and do that only for every other received message.
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After an error was injected with mode ``once`` the error injection command
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is cleared automatically, so ``once`` is a one-time deal.
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All combinations of ``<op>`` and error injection commands can co-exist. So
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this is fine::
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0x9e tx-add-bytes 1
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0x9e tx-early-eom
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0x9f tx-add-bytes 2
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any rx-nack
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All four error injection commands will be active simultaneously.
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However, if the same ``<op>`` and command combination is specified,
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but with different arguments::
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0x9e tx-add-bytes 1
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0x9e tx-add-bytes 2
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Then the second will overwrite the first.
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Clear Error Injections
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----------------------
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``clear``
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Clear all error injections.
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``rx-clear``
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Clear all receive error injections
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``tx-clear``
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Clear all transmit error injections
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``<op> clear``
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Clear all error injections for the given opcode.
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``<op> rx-clear``
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Clear all receive error injections for the given opcode.
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``<op> tx-clear``
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Clear all transmit error injections for the given opcode.
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Receive Messages
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----------------
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``<op>[,<mode>] rx-nack``
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NACK broadcast messages and messages directed to this CEC adapter.
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Every byte of the message will be NACKed in case the transmitter
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keeps transmitting after the first byte was NACKed.
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``<op>[,<mode>] rx-low-drive <bit>``
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Force a Low Drive condition at this bit position. If <op> specifies
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a specific CEC opcode then the bit position must be at least 18,
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otherwise the opcode hasn't been received yet. This tests if the
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transmitter can handle the Low Drive condition correctly and reports
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the error correctly. Note that a Low Drive in the first 4 bits can also
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be interpreted as an Arbitration Lost condition by the transmitter.
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This is implementation dependent.
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``<op>[,<mode>] rx-add-byte``
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Add a spurious 0x55 byte to the received CEC message, provided
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the message was 15 bytes long or less. This is useful to test
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the high-level protocol since spurious bytes should be ignored.
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``<op>[,<mode>] rx-remove-byte``
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Remove the last byte from the received CEC message, provided it
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was at least 2 bytes long. This is useful to test the high-level
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protocol since messages that are too short should be ignored.
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``<op>[,<mode>] rx-arb-lost <poll>``
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Generate a POLL message to trigger an Arbitration Lost condition.
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This command is only allowed for ``<op>`` values of ``next`` or ``all``.
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As soon as a start bit has been received the CEC adapter will switch
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to transmit mode and it will transmit a POLL message. By default this is
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0x0f, but it can also be specified explicitly via the ``<poll>`` argument.
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This command can be used to test the Arbitration Lost condition in
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the remote CEC transmitter. Arbitration happens when two CEC adapters
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start sending a message at the same time. In that case the initiator
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with the most leading zeroes wins and the other transmitter has to
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stop transmitting ('Arbitration Lost'). This is very hard to test,
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except by using this error injection command.
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This does not work if the remote CEC transmitter has logical address
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0 ('TV') since that will always win.
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Transmit Messages
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-----------------
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``tx-ignore-nack-until-eom``
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This setting changes the behavior of transmitting CEC messages. Normally
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as soon as the receiver NACKs a byte the transmit will stop, but the
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specification also allows that the full message is transmitted and only
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at the end will the transmitter look at the ACK bit. This is not
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recommended behavior since there is no point in keeping the CEC bus busy
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for longer than is strictly needed. Especially given how slow the bus is.
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This setting can be used to test how well a receiver deals with
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transmitters that ignore NACKs until the very end of the message.
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``<op>[,<mode>] tx-no-eom``
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Don't set the EOM bit. Normally the last byte of the message has the EOM
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(End-Of-Message) bit set. With this command the transmit will just stop
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without ever sending an EOM. This can be used to test how a receiver
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handles this case. Normally receivers have a time-out after which
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they will go back to the Idle state.
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``<op>[,<mode>] tx-early-eom``
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Set the EOM bit one byte too soon. This obviously only works for messages
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of two bytes or more. The EOM bit will be set for the second-to-last byte
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and not for the final byte. The receiver should ignore the last byte in
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this case. Since the resulting message is likely to be too short for this
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same reason the whole message is typically ignored. The receiver should be
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in Idle state after the last byte was transmitted.
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``<op>[,<mode>] tx-add-bytes <num>``
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Append ``<num>`` (1-255) spurious bytes to the message. The extra bytes
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have the value of the byte position in the message. So if you transmit a
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two byte message (e.g. a Get CEC Version message) and add 2 bytes, then
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the full message received by the remote CEC adapter is
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``0x40 0x9f 0x02 0x03``.
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This command can be used to test buffer overflows in the receiver. E.g.
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what does it do when it receives more than the maximum message size of 16
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bytes.
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``<op>[,<mode>] tx-remove-byte``
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Drop the last byte from the message, provided the message is at least
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two bytes long. The receiver should ignore messages that are too short.
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``<op>[,<mode>] tx-short-bit <bit>``
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Make this bit period shorter than allowed. The bit position cannot be
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an Ack bit. If <op> specifies a specific CEC opcode then the bit position
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must be at least 18, otherwise the opcode hasn't been received yet.
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Normally the period of a data bit is between 2.05 and 2.75 milliseconds.
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With this command the period of this bit is 1.8 milliseconds, this is
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done by reducing the time the CEC bus is high. This bit period is less
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than is allowed and the receiver should respond with a Low Drive
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condition.
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This command is ignored for 0 bits in bit positions 0 to 3. This is
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because the receiver also looks for an Arbitration Lost condition in
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those first four bits and it is undefined what will happen if it
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sees a too-short 0 bit.
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``<op>[,<mode>] tx-long-bit <bit>``
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Make this bit period longer than is valid. The bit position cannot be
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an Ack bit. If <op> specifies a specific CEC opcode then the bit position
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must be at least 18, otherwise the opcode hasn't been received yet.
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Normally the period of a data bit is between 2.05 and 2.75 milliseconds.
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With this command the period of this bit is 2.9 milliseconds, this is
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done by increasing the time the CEC bus is high.
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Even though this bit period is longer than is valid it is undefined what
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a receiver will do. It might just accept it, or it might time out and
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return to Idle state. Unfortunately the CEC specification is silent about
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this.
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This command is ignored for 0 bits in bit positions 0 to 3. This is
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because the receiver also looks for an Arbitration Lost condition in
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those first four bits and it is undefined what will happen if it
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sees a too-long 0 bit.
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``<op>[,<mode>] tx-short-start``
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Make this start bit period shorter than allowed. Normally the period of
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a start bit is between 4.3 and 4.7 milliseconds. With this command the
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period of the start bit is 4.1 milliseconds, this is done by reducing
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the time the CEC bus is high. This start bit period is less than is
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allowed and the receiver should return to Idle state when this is detected.
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``<op>[,<mode>] tx-long-start``
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Make this start bit period longer than is valid. Normally the period of
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a start bit is between 4.3 and 4.7 milliseconds. With this command the
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period of the start bit is 5 milliseconds, this is done by increasing
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the time the CEC bus is high. This start bit period is more than is
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valid and the receiver should return to Idle state when this is detected.
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Even though this start bit period is longer than is valid it is undefined
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what a receiver will do. It might just accept it, or it might time out and
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return to Idle state. Unfortunately the CEC specification is silent about
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this.
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``<op>[,<mode>] tx-last-bit <bit>``
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Just stop transmitting after this bit. If <op> specifies a specific CEC
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opcode then the bit position must be at least 18, otherwise the opcode
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hasn't been received yet. This command can be used to test how the receiver
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reacts when a message just suddenly stops. It should time out and go back
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to Idle state.
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``<op>[,<mode>] tx-low-drive <bit>``
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Force a Low Drive condition at this bit position. If <op> specifies a
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specific CEC opcode then the bit position must be at least 18, otherwise
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the opcode hasn't been received yet. This can be used to test how the
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receiver handles Low Drive conditions. Note that if this happens at bit
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positions 0-3 the receiver can interpret this as an Arbitration Lost
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condition. This is implementation dependent.
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Custom Pulses
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-------------
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``tx-custom-low-usecs <usecs>``
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This defines the duration in microseconds that the custom pulse pulls
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the CEC line low. The default is 1000 microseconds.
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``tx-custom-high-usecs <usecs>``
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This defines the duration in microseconds that the custom pulse keeps the
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CEC line high (unless another CEC adapter pulls it low in that time).
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The default is 1000 microseconds. The total period of the custom pulse is
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``tx-custom-low-usecs + tx-custom-high-usecs``.
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``<op>[,<mode>] tx-custom-bit <bit>``
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Send the custom bit instead of a regular data bit. The bit position cannot
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be an Ack bit. If <op> specifies a specific CEC opcode then the bit
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position must be at least 18, otherwise the opcode hasn't been received yet.
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``<op>[,<mode>] tx-custom-start``
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Send the custom bit instead of a regular start bit.
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``tx-custom-pulse``
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Transmit a single custom pulse as soon as the CEC bus is idle.
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