4.5.2. Memory-to-memory Stateless Video Decoder Interface

A stateless decoder is a decoder that works without retaining any kind of state between processed frames. This means that each frame is decoded independently of any previous and future frames, and that the client is responsible for maintaining the decoding state and providing it to the decoder with each decoding request. This is in contrast to the stateful video decoder interface, where the hardware and driver maintain the decoding state and all the client has to do is to provide the raw encoded stream and dequeue decoded frames in display order.

This section describes how user-space ("the client") is expected to communicate with stateless decoders in order to successfully decode an encoded stream. Compared to stateful codecs, the decoder/client sequence is simpler, but the cost of this simplicity is extra complexity in the client which is responsible for maintaining a consistent decoding state.

Stateless decoders make use of the Request API. A stateless decoder must expose the V4L2_BUF_CAP_SUPPORTS_REQUESTS capability on its OUTPUT queue when VIDIOC_REQBUFS() or VIDIOC_CREATE_BUFS() are invoked.

Depending on the encoded formats supported by the decoder, a single decoded frame may be the result of several decode requests (for instance, H.264 streams with multiple slices per frame). Decoders that support such formats must also expose the V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF capability on their OUTPUT queue.

4.5.2.1. Querying capabilities

  1. To enumerate the set of coded formats supported by the decoder, the client calls VIDIOC_ENUM_FMT() on the OUTPUT queue.
    • The driver must always return the full set of supported OUTPUT formats, irrespective of the format currently set on the CAPTURE queue.
    • Simultaneously, the driver must restrain the set of values returned by codec-specific capability controls (such as H.264 profiles) to the set actually supported by the hardware.
  2. To enumerate the set of supported raw formats, the client calls VIDIOC_ENUM_FMT() on the CAPTURE queue.
    • The driver must return only the formats supported for the format currently active on the OUTPUT queue.
    • Depending on the currently set OUTPUT format, the set of supported raw formats may depend on the value of some codec-dependent controls. The client is responsible for making sure that these controls are set before querying the CAPTURE queue. Failure to do so will result in the default values for these controls being used, and a returned set of formats that may not be usable for the media the client is trying to decode.
  3. The client may use VIDIOC_ENUM_FRAMESIZES() to detect supported resolutions for a given format, passing desired pixel format in v4l2_frmsizeenum's pixel_format.
  4. Supported profiles and levels for the current OUTPUT format, if applicable, may be queried using their respective controls via VIDIOC_QUERYCTRL().

4.5.2.2. Initialization

  1. Set the coded format on the OUTPUT queue via VIDIOC_S_FMT().

    • Required fields:

      type

      a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.

      pixelformat

      a coded pixel format.

      width, height

      coded width and height parsed from the stream.

      other fields

      follow standard semantics.

    Note

    Changing the OUTPUT format may change the currently set CAPTURE format. The driver will derive a new CAPTURE format from the OUTPUT format being set, including resolution, colorimetry parameters, etc. If the client needs a specific CAPTURE format, it must adjust it afterwards.

  2. Call VIDIOC_S_EXT_CTRLS() to set all the controls (parsed headers, etc.) required by the OUTPUT format to enumerate the CAPTURE formats.

  3. Call VIDIOC_G_FMT() for CAPTURE queue to get the format for the destination buffers parsed/decoded from the bytestream.

    • Required fields:

      type

      a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

    • Returned fields:

      width, height

      frame buffer resolution for the decoded frames.

      pixelformat

      pixel format for decoded frames.

      num_planes (for _MPLANE type only)

      number of planes for pixelformat.

      sizeimage, bytesperline

      as per standard semantics; matching frame buffer format.

    Note

    The value of pixelformat may be any pixel format supported for the OUTPUT format, based on the hardware capabilities. It is suggested that the driver chooses the preferred/optimal format for the current configuration. For example, a YUV format may be preferred over an RGB format, if an additional conversion step would be required for RGB.

  4. [optional] Enumerate CAPTURE formats via VIDIOC_ENUM_FMT() on the CAPTURE queue. The client may use this ioctl to discover which alternative raw formats are supported for the current OUTPUT format and select one of them via VIDIOC_S_FMT().

    Note

    The driver will return only formats supported for the currently selected OUTPUT format and currently set controls, even if more formats may be supported by the decoder in general.

    For example, a decoder may support YUV and RGB formats for resolutions 1920x1088 and lower, but only YUV for higher resolutions (due to hardware limitations). After setting a resolution of 1920x1088 or lower as the OUTPUT format, VIDIOC_ENUM_FMT() may return a set of YUV and RGB pixel formats, but after setting a resolution higher than 1920x1088, the driver will not return RGB pixel formats, since they are unsupported for this resolution.

  5. [optional] Choose a different CAPTURE format than suggested via VIDIOC_S_FMT() on CAPTURE queue. It is possible for the client to choose a different format than selected/suggested by the driver in VIDIOC_G_FMT().

    • Required fields:

      type

      a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

      pixelformat

      a raw pixel format.

      width, height

      frame buffer resolution of the decoded stream; typically unchanged from what was returned with VIDIOC_G_FMT(), but it may be different if the hardware supports composition and/or scaling.

    After performing this step, the client must perform step 3 again in order to obtain up-to-date information about the buffers size and layout.

  6. Allocate source (bytestream) buffers via VIDIOC_REQBUFS() on OUTPUT queue.

    • Required fields:

      count

      requested number of buffers to allocate; greater than zero.

      type

      a V4L2_BUF_TYPE_* enum appropriate for OUTPUT.

      memory

      follows standard semantics.

    • Return fields:

      count

      actual number of buffers allocated.

    • If required, the driver will adjust count to be equal or bigger to the minimum of required number of OUTPUT buffers for the given format and requested count. The client must check this value after the ioctl returns to get the actual number of buffers allocated.

  7. Allocate destination (raw format) buffers via VIDIOC_REQBUFS() on the CAPTURE queue.

    • Required fields:

      count

      requested number of buffers to allocate; greater than zero. The client is responsible for deducing the minimum number of buffers required for the stream to be properly decoded (taking e.g. reference frames into account) and pass an equal or bigger number.

      type

      a V4L2_BUF_TYPE_* enum appropriate for CAPTURE.

      memory

      follows standard semantics. V4L2_MEMORY_USERPTR is not supported for CAPTURE buffers.

    • Return fields:

      count

      adjusted to allocated number of buffers, in case the codec requires more buffers than requested.

    • The driver must adjust count to the minimum of required number of CAPTURE buffers for the current format, stream configuration and requested count. The client must check this value after the ioctl returns to get the number of buffers allocated.

  8. Allocate requests (likely one per OUTPUT buffer) via

    MEDIA_IOC_REQUEST_ALLOC() on the media device.

  9. Start streaming on both OUTPUT and CAPTURE queues via

    VIDIOC_STREAMON().

4.5.2.3. Decoding

For each frame, the client is responsible for submitting at least one request to which the following is attached:

  • The amount of encoded data expected by the codec for its current configuration, as a buffer submitted to the OUTPUT queue. Typically, this corresponds to one frame worth of encoded data, but some formats may allow (or require) different amounts per unit.
  • All the metadata needed to decode the submitted encoded data, in the form of controls relevant to the format being decoded.

The amount of data and contents of the source OUTPUT buffer, as well as the controls that must be set on the request, depend on the active coded pixel format and might be affected by codec-specific extended controls, as stated in documentation of each format.

If there is a possibility that the decoded frame will require one or more decode requests after the current one in order to be produced, then the client must set the V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag on the OUTPUT buffer. This will result in the (potentially partially) decoded CAPTURE buffer not being made available for dequeueing, and reused for the next decode request if the timestamp of the next OUTPUT buffer has not changed.

A typical frame would thus be decoded using the following sequence:

  1. Queue an OUTPUT buffer containing one unit of encoded bytestream data for the decoding request, using VIDIOC_QBUF().

    • Required fields:

      index

      index of the buffer being queued.

      type

      type of the buffer.

      bytesused

      number of bytes taken by the encoded data frame in the buffer.

      flags

      the V4L2_BUF_FLAG_REQUEST_FD flag must be set. Additionally, if we are not sure that the current decode request is the last one needed to produce a fully decoded frame, then V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF must also be set.

      request_fd

      must be set to the file descriptor of the decoding request.

      timestamp

      must be set to a unique value per frame. This value will be propagated into the decoded frame's buffer and can also be used to use this frame as the reference of another. If using multiple decode requests per frame, then the timestamps of all the OUTPUT buffers for a given frame must be identical. If the timestamp changes, then the currently held CAPTURE buffer will be made available for dequeuing and the current request will work on a new CAPTURE buffer.

  2. Set the codec-specific controls for the decoding request, using VIDIOC_S_EXT_CTRLS().

    • Required fields:

      which

      must be V4L2_CTRL_WHICH_REQUEST_VAL.

      request_fd

      must be set to the file descriptor of the decoding request.

      other fields

      other fields are set as usual when setting controls. The controls array must contain all the codec-specific controls required to decode a frame.

    Note

    It is possible to specify the controls in different invocations of VIDIOC_S_EXT_CTRLS(), or to overwrite a previously set control, as long as request_fd and which are properly set. The controls state at the moment of request submission is the one that will be considered.

    Note

    The order in which steps 1 and 2 take place is interchangeable.

  3. Submit the request by invoking MEDIA_REQUEST_IOC_QUEUE() on the request FD.

    If the request is submitted without an OUTPUT buffer, or if some of the required controls are missing from the request, then MEDIA_REQUEST_IOC_QUEUE() will return -ENOENT. If more than one OUTPUT buffer is queued, then it will return -EINVAL. MEDIA_REQUEST_IOC_QUEUE() returning non-zero means that no CAPTURE buffer will be produced for this request.

CAPTURE buffers must not be part of the request, and are queued independently. They are returned in decode order (i.e. the same order as coded frames were submitted to the OUTPUT queue).

Runtime decoding errors are signaled by the dequeued CAPTURE buffers carrying the V4L2_BUF_FLAG_ERROR flag. If a decoded reference frame has an error, then all following decoded frames that refer to it also have the V4L2_BUF_FLAG_ERROR flag set, although the decoder will still try to produce (likely corrupted) frames.

4.5.2.4. Buffer management while decoding

Contrary to stateful decoders, a stateless decoder does not perform any kind of buffer management: it only guarantees that dequeued CAPTURE buffers can be used by the client for as long as they are not queued again. "Used" here encompasses using the buffer for compositing or display.

A dequeued capture buffer can also be used as the reference frame of another buffer.

A frame is specified as reference by converting its timestamp into nanoseconds, and storing it into the relevant member of a codec-dependent control structure. The v4l2_timeval_to_ns() function must be used to perform that conversion. The timestamp of a frame can be used to reference it as soon as all its units of encoded data are successfully submitted to the OUTPUT queue.

A decoded buffer containing a reference frame must not be reused as a decoding target until all the frames referencing it have been decoded. The safest way to achieve this is to refrain from queueing a reference buffer until all the decoded frames referencing it have been dequeued. However, if the driver can guarantee that buffers queued to the CAPTURE queue are processed in queued order, then user-space can take advantage of this guarantee and queue a reference buffer when the following conditions are met:

  1. All the requests for frames affected by the reference frame have been queued, and
  2. A sufficient number of CAPTURE buffers to cover all the decoded referencing frames have been queued.

When queuing a decoding request, the driver will increase the reference count of all the resources associated with reference frames. This means that the client can e.g. close the DMABUF file descriptors of reference frame buffers if it won't need them afterwards.

4.5.2.5. Seeking

In order to seek, the client just needs to submit requests using input buffers corresponding to the new stream position. It must however be aware that resolution may have changed and follow the dynamic resolution change sequence in that case. Also depending on the codec used, picture parameters (e.g. SPS/PPS for H.264) may have changed and the client is responsible for making sure that a valid state is sent to the decoder.

The client is then free to ignore any returned CAPTURE buffer that comes from the pre-seek position.

4.5.2.6. Pausing

In order to pause, the client can just cease queuing buffers onto the OUTPUT queue. Without source bytestream data, there is no data to process and the codec will remain idle.

4.5.2.7. Dynamic resolution change

If the client detects a resolution change in the stream, it will need to perform the initialization sequence again with the new resolution:

  1. If the last submitted request resulted in a CAPTURE buffer being held by the use of the V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag, then the last frame is not available on the CAPTURE queue. In this case, a V4L2_DEC_CMD_FLUSH command shall be sent. This will make the driver dequeue the held CAPTURE buffer.
  2. Wait until all submitted requests have completed and dequeue the corresponding output buffers.
  3. Call VIDIOC_STREAMOFF() on both the OUTPUT and CAPTURE queues.
  4. Free all CAPTURE buffers by calling VIDIOC_REQBUFS() on the CAPTURE queue with a buffer count of zero.
  5. Perform the initialization sequence again (minus the allocation of OUTPUT buffers), with the new resolution set on the OUTPUT queue. Note that due to resolution constraints, a different format may need to be picked on the CAPTURE queue.

4.5.2.8. Drain

If the last submitted request resulted in a CAPTURE buffer being held by the use of the V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag, then the last frame is not available on the CAPTURE queue. In this case, a V4L2_DEC_CMD_FLUSH command shall be sent. This will make the driver dequeue the held CAPTURE buffer.

After that, in order to drain the stream on a stateless decoder, the client just needs to wait until all the submitted requests are completed.