2.11. ioctls CEC_RECEIVE and CEC_TRANSMIT

2.11.1. Name

CEC_RECEIVE, CEC_TRANSMIT - Receive or transmit a CEC message

2.11.2. Synopsis

CEC_RECEIVE

int ioctl(int fd, CEC_RECEIVE, struct cec_msg *argp)

CEC_TRANSMIT

int ioctl(int fd, CEC_TRANSMIT, struct cec_msg *argp)

2.11.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct cec_msg.

2.11.4. Description

To receive a CEC message the application has to fill in the timeout field of struct cec_msg and pass it to ioctl 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 cec_msg and pass it to ioctl CEC_TRANSMIT. The ioctl 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 Core Message 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 ioctl 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 ioctl 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’).

type cec_msg

struct cec_msg

__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 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 clock_gettime().
__u32	len	The length of the message. For ioctl CEC_TRANSMIT this is filled in by the application. The driver will fill this in for ioctl CEC_RECEIVE. For ioctl 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 ioctl CEC_RECEIVE. If it is 0 and it is called by ioctl 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 Flags for struct cec_msg for a list of available flags.
__u8	msg[16]	The message payload. For ioctl CEC_TRANSMIT this is filled in by the application. The driver will fill this in for ioctl CEC_RECEIVE. For ioctl 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 ioctl 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 CEC_RX_STATUS_TIMEOUT or 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 CEC Receive Status for the possible status values.
__u8	tx_status	The status bits of the transmitted message. See CEC Transmit Status for the possible status values. When calling ioctl 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 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 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 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 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 CEC_TX_STATUS_ERROR status bit is set.

Flags for struct cec_msg

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 ioctl CEC_TRANSMIT. If this flag is set, then the reply is also sent to all followers, if any. If the filehandle that called ioctl CEC_TRANSMIT is also a follower, then that filehandle will receive the reply twice: once as the result of the ioctl CEC_TRANSMIT, and once via ioctl CEC_RECEIVE.
CEC_MSG_FL_RAW	2	Normally CEC messages are validated before transmitting them. If this flag is set when ioctl 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.

CEC Transmit Status

CEC_TX_STATUS_OK	0x01	The message was transmitted successfully. This is mutually exclusive with 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	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	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	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	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	0x20	The transmit failed after one or more retries. This status bit is mutually exclusive with CEC_TX_STATUS_OK. Other bits can still be set to explain which failures were seen.
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	0x80	The transmit timed out. This should not normally happen and this indicates a driver problem.

CEC Receive Status

CEC_RX_STATUS_OK	0x01	The message was received successfully.
CEC_RX_STATUS_TIMEOUT	0x02	The reply to an earlier transmitted message timed out.
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	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 CEC_TRANSMIT that waited for a reply was interrupted.

2.11.5. 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 Generic Error Codes chapter.

The ioctl 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 ioctl 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. ioctl 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. ioctl 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 cec_msg is invalid.

ERESTARTSYS

The wait for a successful transmit was interrupted (e.g. by Ctrl-C).