2.10. ioctls CEC_RECEIVE and CEC_TRANSMIT¶

2.10.1. Name¶

CEC_RECEIVE, CEC_TRANSMIT - Receive or transmit a CEC message

2.10.2. Synopsis¶

int ioctl(int fd, int request, struct cec_msg *argp)¶

2.10.3. Arguments¶

fd: File descriptor returned by open().
request: CEC_RECEIVE, CEC_TRANSMIT

argp

2.10.4. Description¶

Note

This documents the proposed CEC API. This API is not yet finalized and is currently only available as a staging kernel module.

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:

a message received from another CEC device (the sequence field will be 0).
the result of an earlier non-blocking transmit (the sequence field 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 :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 ioctl CEC_RECEIVE once the transmit has finished (including waiting for a reply, if requested).

The sequence field is filled in for every transmit and this can be checked against the received messages to find the corresponding transmit result.

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(2)()`.
__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(2)()`.
__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 (when transmit was called in non-blocking mode). This allows the application to associate the received message with the original transmit.
__u32	`flags`	Flags. No flags are defined yet, so set this to 0.
__u8	`tx_status`	The status bits of the transmitted message. See CEC Transmit Status for the possible status values. It is 0 if this messages was received, not transmitted.
__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.
__u8	`rx_status`	The status bits of the received message. See CEC Receive Status for the possible status values. It is 0 if this message was transmitted, not received, unless this is the reply to a transmitted message. In that case both `rx_status` and `tx_status` are set.
__u8	`tx_status`	The status bits of the transmitted message. See CEC Transmit Status for the possible status values. It is 0 if this messages was received, not transmitted.
__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.

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.
`CEC_TX_STATUS_NACK`	0x04	Message was not acknowledged.
`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.
`CEC_TX_STATUS_ERROR`	0x10	Some error occurred. This is used for any errors that do not fit the previous two, either because the hardware could not tell which error occurred, or because the hardware tested for other conditions besides those two.
`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 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.

2.10.5. 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.