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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: CEC
+
+.. _CEC_TRANSMIT:
+.. _CEC_RECEIVE:
+
+***********************************
+ioctls CEC_RECEIVE and CEC_TRANSMIT
+***********************************
+
+Name
+====
+
+CEC_RECEIVE, CEC_TRANSMIT - Receive or transmit a CEC message
+
+Synopsis
+========
+
+.. c:macro:: CEC_RECEIVE
+
+``int ioctl(int fd, CEC_RECEIVE, struct cec_msg *argp)``
+
+.. c:macro:: CEC_TRANSMIT
+
+``int ioctl(int fd, CEC_TRANSMIT, struct cec_msg *argp)``
+
+Arguments
+=========
+
+``fd``
+ File descriptor returned by :c:func:`open()`.
+
+``argp``
+ Pointer to struct cec_msg.
+
+Description
+===========
+
+To receive a CEC message the application has to fill in the
+``timeout`` field of struct :c:type:`cec_msg` and pass it to
+:ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`.
+If the file descriptor is in non-blocking mode and there are no received
+messages pending, then it will return -1 and set errno to the ``EAGAIN``
+error code. If the file descriptor is in blocking mode and ``timeout``
+is non-zero and no message arrived within ``timeout`` milliseconds, then
+it will return -1 and set errno to the ``ETIMEDOUT`` error code.
+
+A received message can be:
+
+1. a message received from another CEC device (the ``sequence`` field will
+ be 0, ``tx_status`` will be 0 and ``rx_status`` will be non-zero).
+2. the transmit result of an earlier non-blocking transmit (the ``sequence``
+ field will be non-zero, ``tx_status`` will be non-zero and ``rx_status``
+ will be 0).
+3. the reply to an earlier non-blocking transmit (the ``sequence`` field will
+ be non-zero, ``tx_status`` will be 0 and ``rx_status`` will be non-zero).
+
+To send a CEC message the application has to fill in the struct
+:c:type:`cec_msg` and pass it to :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>`.
+The :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` is only available if
+``CEC_CAP_TRANSMIT`` is set. If there is no more room in the transmit
+queue, then it will return -1 and set errno to the ``EBUSY`` error code.
+The transmit queue has enough room for 18 messages (about 1 second worth
+of 2-byte messages). Note that the CEC kernel framework will also reply
+to core messages (see :ref:`cec-core-processing`), so it is not a good
+idea to fully fill up the transmit queue.
+
+If the file descriptor is in non-blocking mode then the transmit will
+return 0 and the result of the transmit will be available via
+:ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>` once the transmit has finished.
+If a non-blocking transmit also specified waiting for a reply, then
+the reply will arrive in a later message. The ``sequence`` field can
+be used to associate both transmit results and replies with the original
+transmit.
+
+Normally calling :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` when the physical
+address is invalid (due to e.g. a disconnect) will return ``ENONET``.
+
+However, the CEC specification allows sending messages from 'Unregistered' to
+'TV' when the physical address is invalid since some TVs pull the hotplug detect
+pin of the HDMI connector low when they go into standby, or when switching to
+another input.
+
+When the hotplug detect pin goes low the EDID disappears, and thus the
+physical address, but the cable is still connected and CEC still works.
+In order to detect/wake up the device it is allowed to send poll and 'Image/Text
+View On' messages from initiator 0xf ('Unregistered') to destination 0 ('TV').
+
+.. tabularcolumns:: |p{1.0cm}|p{3.5cm}|p{12.8cm}|
+
+.. c:type:: cec_msg
+
+.. cssclass:: longtable
+
+.. flat-table:: struct cec_msg
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 1 1 16
+
+ * - __u64
+ - ``tx_ts``
+ - Timestamp in ns of when the last byte of the message was transmitted.
+ The timestamp has been taken from the ``CLOCK_MONOTONIC`` clock. To access
+ the same clock from userspace use :c:func:`clock_gettime`.
+ * - __u64
+ - ``rx_ts``
+ - Timestamp in ns of when the last byte of the message was received.
+ The timestamp has been taken from the ``CLOCK_MONOTONIC`` clock. To access
+ the same clock from userspace use :c:func:`clock_gettime`.
+ * - __u32
+ - ``len``
+ - The length of the message. For :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` this is filled in
+ by the application. The driver will fill this in for
+ :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`. For :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` it will be
+ filled in by the driver with the length of the reply message if ``reply`` was set.
+ * - __u32
+ - ``timeout``
+ - The timeout in milliseconds. This is the time the device will wait
+ for a message to be received before timing out. If it is set to 0,
+ then it will wait indefinitely when it is called by :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`.
+ If it is 0 and it is called by :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>`,
+ then it will be replaced by 1000 if the ``reply`` is non-zero or
+ ignored if ``reply`` is 0.
+ * - __u32
+ - ``sequence``
+ - A non-zero sequence number is automatically assigned by the CEC framework
+ for all transmitted messages. It is used by the CEC framework when it queues
+ the transmit result for a non-blocking transmit. This allows the application
+ to associate the received message with the original transmit.
+
+ In addition, if a non-blocking transmit will wait for a reply (ii.e. ``timeout``
+ was not 0), then the ``sequence`` field of the reply will be set to the sequence
+ value of the original transmit. This allows the application to associate the
+ received message with the original transmit.
+ * - __u32
+ - ``flags``
+ - Flags. See :ref:`cec-msg-flags` for a list of available flags.
+ * - __u8
+ - ``msg[16]``
+ - The message payload. For :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` this is filled in by the
+ application. The driver will fill this in for :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`.
+ For :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` it will be filled in by the driver with
+ the payload of the reply message if ``timeout`` was set.
+ * - __u8
+ - ``reply``
+ - Wait until this message is replied. If ``reply`` is 0 and the
+ ``timeout`` is 0, then don't wait for a reply but return after
+ transmitting the message. Ignored by :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`.
+ The case where ``reply`` is 0 (this is the opcode for the Feature Abort
+ message) and ``timeout`` is non-zero is specifically allowed to make it
+ possible to send a message and wait up to ``timeout`` milliseconds for a
+ Feature Abort reply. In this case ``rx_status`` will either be set
+ to :ref:`CEC_RX_STATUS_TIMEOUT <CEC-RX-STATUS-TIMEOUT>` or
+ :ref:`CEC_RX_STATUS_FEATURE_ABORT <CEC-RX-STATUS-FEATURE-ABORT>`.
+
+ If the transmitter message is ``CEC_MSG_INITIATE_ARC`` then the ``reply``
+ values ``CEC_MSG_REPORT_ARC_INITIATED`` and ``CEC_MSG_REPORT_ARC_TERMINATED``
+ are processed differently: either value will match both possible replies.
+ The reason is that the ``CEC_MSG_INITIATE_ARC`` message is the only CEC
+ message that has two possible replies other than Feature Abort. The
+ ``reply`` field will be updated with the actual reply so that it is
+ synchronized with the contents of the received message.
+ * - __u8
+ - ``rx_status``
+ - The status bits of the received message. See
+ :ref:`cec-rx-status` for the possible status values.
+ * - __u8
+ - ``tx_status``
+ - The status bits of the transmitted message. See
+ :ref:`cec-tx-status` for the possible status values.
+ When calling :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` in non-blocking mode,
+ this field will be 0 if the transmit started, or non-0 if the transmit
+ result is known immediately. The latter would be the case when attempting
+ to transmit a Poll message to yourself. That results in a
+ :ref:`CEC_TX_STATUS_NACK <CEC-TX-STATUS-NACK>` without ever actually
+ transmitting the Poll message.
+ * - __u8
+ - ``tx_arb_lost_cnt``
+ - A counter of the number of transmit attempts that resulted in the
+ Arbitration Lost error. This is only set if the hardware supports
+ this, otherwise it is always 0. This counter is only valid if the
+ :ref:`CEC_TX_STATUS_ARB_LOST <CEC-TX-STATUS-ARB-LOST>` status bit is set.
+ * - __u8
+ - ``tx_nack_cnt``
+ - A counter of the number of transmit attempts that resulted in the
+ Not Acknowledged error. This is only set if the hardware supports
+ this, otherwise it is always 0. This counter is only valid if the
+ :ref:`CEC_TX_STATUS_NACK <CEC-TX-STATUS-NACK>` status bit is set.
+ * - __u8
+ - ``tx_low_drive_cnt``
+ - A counter of the number of transmit attempts that resulted in the
+ Arbitration Lost error. This is only set if the hardware supports
+ this, otherwise it is always 0. This counter is only valid if the
+ :ref:`CEC_TX_STATUS_LOW_DRIVE <CEC-TX-STATUS-LOW-DRIVE>` status bit is set.
+ * - __u8
+ - ``tx_error_cnt``
+ - A counter of the number of transmit errors other than Arbitration
+ Lost or Not Acknowledged. This is only set if the hardware
+ supports this, otherwise it is always 0. This counter is only
+ valid if the :ref:`CEC_TX_STATUS_ERROR <CEC-TX-STATUS-ERROR>` status bit is set.
+
+.. tabularcolumns:: |p{6.2cm}|p{1.0cm}|p{10.1cm}|
+
+.. _cec-msg-flags:
+
+.. flat-table:: Flags for struct cec_msg
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+ * .. _`CEC-MSG-FL-REPLY-TO-FOLLOWERS`:
+
+ - ``CEC_MSG_FL_REPLY_TO_FOLLOWERS``
+ - 1
+ - If a CEC transmit expects a reply, then by default that reply is only sent to
+ the filehandle that called :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>`. If this
+ flag is set, then the reply is also sent to all followers, if any. If the
+ filehandle that called :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` is also a
+ follower, then that filehandle will receive the reply twice: once as the
+ result of the :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>`, and once via
+ :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>`.
+
+ * .. _`CEC-MSG-FL-RAW`:
+
+ - ``CEC_MSG_FL_RAW``
+ - 2
+ - Normally CEC messages are validated before transmitting them. If this
+ flag is set when :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` is called,
+ then no validation takes place and the message is transmitted as-is.
+ This is useful when debugging CEC issues.
+ This flag is only allowed if the process has the ``CAP_SYS_RAWIO``
+ capability. If that is not set, then the ``EPERM`` error code is
+ returned.
+
+.. tabularcolumns:: |p{5.6cm}|p{0.9cm}|p{10.8cm}|
+
+.. _cec-tx-status:
+
+.. flat-table:: CEC Transmit Status
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 16
+
+ * .. _`CEC-TX-STATUS-OK`:
+
+ - ``CEC_TX_STATUS_OK``
+ - 0x01
+ - The message was transmitted successfully. This is mutually
+ exclusive with :ref:`CEC_TX_STATUS_MAX_RETRIES <CEC-TX-STATUS-MAX-RETRIES>`.
+ Other bits can still be set if earlier attempts met with failure before
+ the transmit was eventually successful.
+ * .. _`CEC-TX-STATUS-ARB-LOST`:
+
+ - ``CEC_TX_STATUS_ARB_LOST``
+ - 0x02
+ - CEC line arbitration was lost, i.e. another transmit started at the
+ same time with a higher priority. Optional status, not all hardware
+ can detect this error condition.
+ * .. _`CEC-TX-STATUS-NACK`:
+
+ - ``CEC_TX_STATUS_NACK``
+ - 0x04
+ - Message was not acknowledged. Note that some hardware cannot tell apart
+ a 'Not Acknowledged' status from other error conditions, i.e. the result
+ of a transmit is just OK or FAIL. In that case this status will be
+ returned when the transmit failed.
+ * .. _`CEC-TX-STATUS-LOW-DRIVE`:
+
+ - ``CEC_TX_STATUS_LOW_DRIVE``
+ - 0x08
+ - Low drive was detected on the CEC bus. This indicates that a
+ follower detected an error on the bus and requests a
+ retransmission. Optional status, not all hardware can detect this
+ error condition.
+ * .. _`CEC-TX-STATUS-ERROR`:
+
+ - ``CEC_TX_STATUS_ERROR``
+ - 0x10
+ - Some error occurred. This is used for any errors that do not fit
+ ``CEC_TX_STATUS_ARB_LOST`` or ``CEC_TX_STATUS_LOW_DRIVE``, either because
+ the hardware could not tell which error occurred, or because the hardware
+ tested for other conditions besides those two. Optional status.
+ * .. _`CEC-TX-STATUS-MAX-RETRIES`:
+
+ - ``CEC_TX_STATUS_MAX_RETRIES``
+ - 0x20
+ - The transmit failed after one or more retries. This status bit is
+ mutually exclusive with :ref:`CEC_TX_STATUS_OK <CEC-TX-STATUS-OK>`.
+ Other bits can still be set to explain which failures were seen.
+ * .. _`CEC-TX-STATUS-ABORTED`:
+
+ - ``CEC_TX_STATUS_ABORTED``
+ - 0x40
+ - The transmit was aborted due to an HDMI disconnect, or the adapter
+ was unconfigured, or a transmit was interrupted, or the driver
+ returned an error when attempting to start a transmit.
+ * .. _`CEC-TX-STATUS-TIMEOUT`:
+
+ - ``CEC_TX_STATUS_TIMEOUT``
+ - 0x80
+ - The transmit timed out. This should not normally happen and this
+ indicates a driver problem.
+
+.. tabularcolumns:: |p{5.6cm}|p{0.9cm}|p{10.8cm}|
+
+.. _cec-rx-status:
+
+.. flat-table:: CEC Receive Status
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 16
+
+ * .. _`CEC-RX-STATUS-OK`:
+
+ - ``CEC_RX_STATUS_OK``
+ - 0x01
+ - The message was received successfully.
+ * .. _`CEC-RX-STATUS-TIMEOUT`:
+
+ - ``CEC_RX_STATUS_TIMEOUT``
+ - 0x02
+ - The reply to an earlier transmitted message timed out.
+ * .. _`CEC-RX-STATUS-FEATURE-ABORT`:
+
+ - ``CEC_RX_STATUS_FEATURE_ABORT``
+ - 0x04
+ - The message was received successfully but the reply was
+ ``CEC_MSG_FEATURE_ABORT``. This status is only set if this message
+ was the reply to an earlier transmitted message.
+ * .. _`CEC-RX-STATUS-ABORTED`:
+
+ - ``CEC_RX_STATUS_ABORTED``
+ - 0x08
+ - The wait for a reply to an earlier transmitted message was aborted
+ because the HDMI cable was disconnected, the adapter was unconfigured
+ or the :ref:`CEC_TRANSMIT <CEC_RECEIVE>` that waited for a
+ reply was interrupted.
+
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.
+
+The :ref:`ioctl CEC_RECEIVE <CEC_RECEIVE>` can return the following
+error codes:
+
+EAGAIN
+ No messages are in the receive queue, and the filehandle is in non-blocking mode.
+
+ETIMEDOUT
+ The ``timeout`` was reached while waiting for a message.
+
+ERESTARTSYS
+ The wait for a message was interrupted (e.g. by Ctrl-C).
+
+The :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` can return the following
+error codes:
+
+ENOTTY
+ The ``CEC_CAP_TRANSMIT`` capability wasn't set, so this ioctl is not supported.
+
+EPERM
+ The CEC adapter is not configured, i.e. :ref:`ioctl CEC_ADAP_S_LOG_ADDRS <CEC_ADAP_S_LOG_ADDRS>`
+ has never been called, or ``CEC_MSG_FL_RAW`` was used from a process that
+ did not have the ``CAP_SYS_RAWIO`` capability.
+
+ENONET
+ The CEC adapter is not configured, i.e. :ref:`ioctl CEC_ADAP_S_LOG_ADDRS <CEC_ADAP_S_LOG_ADDRS>`
+ was called, but the physical address is invalid so no logical address was claimed.
+ An exception is made in this case for transmits from initiator 0xf ('Unregistered')
+ to destination 0 ('TV'). In that case the transmit will proceed as usual.
+
+EBUSY
+ Another filehandle is in exclusive follower or initiator mode, or the filehandle
+ is in mode ``CEC_MODE_NO_INITIATOR``. This is also returned if the transmit
+ queue is full.
+
+EINVAL
+ The contents of struct :c:type:`cec_msg` is invalid.
+
+ERESTARTSYS
+ The wait for a successful transmit was interrupted (e.g. by Ctrl-C).