7.10. ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

7.10.1. Name

VIDIOC_ENCODER_CMD - VIDIOC_TRY_ENCODER_CMD - Execute an encoder command

7.10.2. Synopsis

int ioctl(int fd, VIDIOC_ENCODER_CMD, struct v4l2_encoder_cmd *argp)
int ioctl(int fd, VIDIOC_TRY_ENCODER_CMD, struct v4l2_encoder_cmd *argp)

7.10.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_encoder_cmd.

7.10.4. Description

These ioctls control an audio/video (usually MPEG-) encoder. VIDIOC_ENCODER_CMD sends a command to the encoder, VIDIOC_TRY_ENCODER_CMD can be used to try a command without actually executing it.

To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and call VIDIOC_ENCODER_CMD or VIDIOC_TRY_ENCODER_CMD with a pointer to this structure.

The cmd field must contain the command code. Some commands use the flags field for additional information.

After a STOP command, read() calls will read the remaining data buffered by the driver. When the buffer is empty, read() will return zero and the next read() call will restart the encoder.

A read() or VIDIOC_STREAMON call sends an implicit START command to the encoder if it has not been started yet. Applies to both queues of mem2mem encoders.

A close() or VIDIOC_STREAMOFF call of a streaming file descriptor sends an implicit immediate STOP to the encoder, and all buffered data is discarded. Applies to both queues of mem2mem encoders.

These ioctls are optional, not all drivers may support them. They were introduced in Linux 2.6.21. They are, however, mandatory for stateful mem2mem encoders (as further documented in Memory-to-Memory Stateful Video Encoder Interface).

v4l2_encoder_cmd
struct v4l2_encoder_cmd

__u32

cmd

The encoder command, see Encoder Commands.

__u32

flags

Flags to go with the command, see Encoder Command Flags. If no flags are defined for this command, drivers and applications must set this field to zero.

__u32

data[8]

Reserved for future extensions. Drivers and applications must set the array to zero.

Encoder Commands

V4L2_ENC_CMD_START

0

Start the encoder. When the encoder is already running or paused, this command does nothing. No flags are defined for this command.

For a device implementing the Memory-to-Memory Stateful Video Encoder Interface, once the drain sequence is initiated with the V4L2_ENC_CMD_STOP command, it must be driven to completion before this command can be invoked. Any attempt to invoke the command while the drain sequence is in progress will trigger an EBUSY error code. See Memory-to-Memory Stateful Video Encoder Interface for more details.

V4L2_ENC_CMD_STOP

1

Stop the encoder. When the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise encoding will stop immediately. When the encoder is already stopped, this command does nothing.

For a device implementing the Memory-to-Memory Stateful Video Encoder Interface, the command will initiate the drain sequence as documented in Memory-to-Memory Stateful Video Encoder Interface. No flags or other arguments are accepted in this case. Any attempt to invoke the command again before the sequence completes will trigger an EBUSY error code.

V4L2_ENC_CMD_PAUSE

2

Pause the encoder. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already paused, this command does nothing. No flags are defined for this command.

V4L2_ENC_CMD_RESUME

3

Resume encoding after a PAUSE command. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already running, this command does nothing. No flags are defined for this command.

Encoder Command Flags

V4L2_ENC_CMD_STOP_AT_GOP_END

0x0001

Stop encoding at the end of the current Group Of Pictures, rather than immediately.

Does not apply to Memory-to-Memory Stateful Video Encoder Interface.

7.10.5. 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.

EBUSY

A drain sequence of a device implementing the Memory-to-Memory Stateful Video Encoder Interface is still in progress. It is not allowed to issue another encoder command until it completes.

EINVAL

The cmd field is invalid.

EPERM

The application sent a PAUSE or RESUME command when the encoder was not running.