Re: [PATCHv2 09/12] vb2: add V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF

[Alexandre Courbot <acourbot@chromium.org>]

On Mon, Aug 12, 2019 at 8:06 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>
> This RFC patch adds support for the V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag.
> It also adds a new V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF capability and
> a v4l2_m2m_release_capture_buf() helper function.
>
> Drivers should set vb2_queue->subsystem_flags to VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF
> to indicate support for this flag.
>
> The device_run() function should look like this:
>
> if (v4l2_m2m_release_capture_buf(out_vb, cap_vb)) {
>         v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
>         v4l2_m2m_job_finish(...);
>         return;
> }
> cap_vb->is_held = out_vb->flags & V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
>
> ...
>
> v4l2_m2m_buf_done(out_vb, VB2_BUF_STATE_DONE);
> if (!cap_vb->is_held) {
>         v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
>         v4l2_m2m_job_finish(...);
> }
>
> In order to handle the corner case where V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF
> is always set for the output buffer, and you reached the last frame (so no
> new output buffer will be queued with a new timestamp), the driver should
> implement support for the V4L2_DEC_CMD_STOP command, and that should do:

I wonder if "the driver must implement..." would not make things
easier: what about drivers that only support formats for which we have
an exact 1:1 mapping (like VP8)? Should they also implement it? What
it permitted for a driver supporting this flag to not implement
V4L2_DEC_CMD_STOP?

Leaving this as an option may complicate user-space quite a bit. Since
the implementation of V4L2_DEC_CMD_STOP does not seem too complicated,
how about making it mandatory for all stateless decoders?


>
> struct vb2_v4l2_buffer *out_vb = v4l2_m2m_last_src_buf(m2m_ctx);
> struct vb2_v4l2_buffer *cap_vb = v4l2_m2m_last_dst_buf(m2m_ctx);
>
> if (out_vb) {
>         out_vb->flags &= ~V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
> } else if (cap_vb && cap_vb->is_held) {
>         v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
>         v4l2_m2m_job_finish(...);
> }
>
> At least, I think so. Comments on this are very welcome. We definitely
> need better support in v4l2-mem2mem.c for such situations (same for
> stateful codecs) since it's too complex for drivers to get right IMHO.
>
> Regards,
>
>         Hans
>
> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> ---
>  Documentation/media/uapi/v4l/buffer.rst       | 13 ++++++
>  .../media/uapi/v4l/vidioc-reqbufs.rst         |  6 +++
>  .../media/common/videobuf2/videobuf2-v4l2.c   |  8 +++-
>  include/media/v4l2-mem2mem.h                  | 42 +++++++++++++++++++
>  include/media/videobuf2-core.h                |  3 ++
>  include/media/videobuf2-v4l2.h                |  5 +++
>  include/uapi/linux/videodev2.h                | 13 +++---
>  7 files changed, 84 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index 1cbd9cde57f3..afb03906ead9 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -607,6 +607,19 @@ Buffer Flags
>         applications shall use this flag for output buffers if the data in
>         this buffer has not been created by the CPU but by some
>         DMA-capable unit, in which case caches have not been used.
> +    * .. _`V4L2-BUF-FLAG-M2M-HOLD-CAPTURE-BUF`:
> +
> +      - ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF``
> +      - 0x00000200
> +      - Only valid if ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF`` is
> +        set. It is typically used with stateless decoders where multiple
> +       output buffers each decode to a slice of the decoded frame.
> +       Applications can set this flag when queueing the output buffer
> +       to prevent the driver from dequeueing the capture buffer after
> +       the output buffer has been decoded (i.e. the capture buffer is
> +       'held'). If the timestamp of this output buffer differs from that
> +       of the previous output buffer, then that indicates the start of a
> +       new frame and the previously held capture buffer is dequeued.
>      * .. _`V4L2-BUF-FLAG-LAST`:
>
>        - ``V4L2_BUF_FLAG_LAST``
> diff --git a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> index d7faef10e39b..d0c643db477a 100644
> --- a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> @@ -125,6 +125,7 @@ aborting or finishing any DMA in progress, an implicit
>  .. _V4L2-BUF-CAP-SUPPORTS-DMABUF:
>  .. _V4L2-BUF-CAP-SUPPORTS-REQUESTS:
>  .. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
> +.. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
>
>  .. cssclass:: longtable
>
> @@ -150,6 +151,11 @@ aborting or finishing any DMA in progress, an implicit
>        - The kernel allows calling :ref:`VIDIOC_REQBUFS` while buffers are still
>          mapped or exported via DMABUF. These orphaned buffers will be freed
>          when they are unmapped or when the exported DMABUF fds are closed.
> +    * - ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF``
> +      - 0x00000020
> +      - Only valid for stateless decoders. If set, then userspace can set the
> +        ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
> +       capture buffer until the OUTPUT timestamp changes.
>
>  Return Value
>  ============
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index 5a9ba3846f0a..699787f48f46 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -50,7 +50,8 @@ module_param(debug, int, 0644);
>                                  V4L2_BUF_FLAG_TIMESTAMP_MASK)
>  /* Output buffer flags that should be passed on to the driver */
>  #define V4L2_BUFFER_OUT_FLAGS  (V4L2_BUF_FLAG_PFRAME | V4L2_BUF_FLAG_BFRAME | \
> -                                V4L2_BUF_FLAG_KEYFRAME | V4L2_BUF_FLAG_TIMECODE)
> +                                V4L2_BUF_FLAG_KEYFRAME | V4L2_BUF_FLAG_TIMECODE | \
> +                                V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF)
>
>  /*
>   * __verify_planes_array() - verify that the planes array passed in struct
> @@ -194,6 +195,7 @@ static int vb2_fill_vb2_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b
>         }
>         vbuf->sequence = 0;
>         vbuf->request_fd = -1;
> +       vbuf->is_held = false;
>
>         if (V4L2_TYPE_IS_MULTIPLANAR(b->type)) {
>                 switch (b->memory) {
> @@ -321,6 +323,8 @@ static int vb2_fill_vb2_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b
>                  */
>                 vbuf->flags &= ~V4L2_BUF_FLAG_TIMECODE;
>                 vbuf->field = b->field;
> +               if (!(q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF))
> +                       vbuf->flags &= ~V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
>         } else {
>                 /* Zero any output buffer flags as this is a capture buffer */
>                 vbuf->flags &= ~V4L2_BUFFER_OUT_FLAGS;
> @@ -654,6 +658,8 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
>                 *caps |= V4L2_BUF_CAP_SUPPORTS_USERPTR;
>         if (q->io_modes & VB2_DMABUF)
>                 *caps |= V4L2_BUF_CAP_SUPPORTS_DMABUF;
> +       if (q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF)
> +               *caps |= V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF;
>  #ifdef CONFIG_MEDIA_CONTROLLER_REQUEST_API
>         if (q->supports_requests)
>                 *caps |= V4L2_BUF_CAP_SUPPORTS_REQUESTS;
> diff --git a/include/media/v4l2-mem2mem.h b/include/media/v4l2-mem2mem.h
> index 0b9c3a287061..724858efded2 100644
> --- a/include/media/v4l2-mem2mem.h
> +++ b/include/media/v4l2-mem2mem.h
> @@ -644,6 +644,48 @@ void v4l2_m2m_buf_copy_metadata(const struct vb2_v4l2_buffer *out_vb,
>                                 struct vb2_v4l2_buffer *cap_vb,
>                                 bool copy_frame_flags);
>
> +/**
> + * v4l2_m2m_release_capture_buf() - check if the capture buffer should be
> + * released
> + *
> + * @out_vb: the output buffer
> + * @cap_vb: the capture buffer
> + *
> + * This helper function returns true if the current capture buffer should
> + * be released to vb2. This is the case if the output buffer specified that
> + * the capture buffer should be held (i.e. not returned to vb2) AND if the
> + * timestamp of the capture buffer differs from the output buffer timestamp.
> + *
> + * This helper is to be called at the start of the device_run callback:
> + *
> + * .. code-block:: c
> + *
> + *     if (v4l2_m2m_release_capture_buf(out_vb, cap_vb)) {
> + *             v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
> + *             v4l2_m2m_job_finish(...);
> + *             return;
> + *     }
> + *     cap_vb->is_held = out_vb->flags & V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
> + *
> + *     ...
> + *
> + *     v4l2_m2m_buf_done(out_vb, VB2_BUF_STATE_DONE);
> + *     if (!cap_vb->is_held) {
> + *             v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
> + *             v4l2_m2m_job_finish(...);
> + *     }
> + *
> + * This allows for multiple output buffers to be used to fill in a single
> + * capture buffer. This is typically used by stateless decoders where
> + * multiple e.g. H.264 slices contribute to a single decoded frame.
> + */
> +static inline bool v4l2_m2m_release_capture_buf(const struct vb2_v4l2_buffer *out_vb,
> +                                               const struct vb2_v4l2_buffer *cap_vb)
> +{
> +       return cap_vb->is_held && cap_vb->vb2_buf.copied_timestamp &&
> +              out_vb->vb2_buf.timestamp != cap_vb->vb2_buf.timestamp;
> +}
> +
>  /* v4l2 request helper */
>
>  void v4l2_m2m_request_queue(struct media_request *req);
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 640aabe69450..a2b2208b02da 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -505,6 +505,8 @@ struct vb2_buf_ops {
>   * @buf_ops:   callbacks to deliver buffer information.
>   *             between user-space and kernel-space.
>   * @drv_priv:  driver private data.
> + * @subsystem_flags: Flags specific to the subsystem (V4L2/DVB/etc.). Not used
> + *             by the vb2 core.
>   * @buf_struct_size: size of the driver-specific buffer structure;
>   *             "0" indicates the driver doesn't want to use a custom buffer
>   *             structure type. for example, ``sizeof(struct vb2_v4l2_buffer)``
> @@ -571,6 +573,7 @@ struct vb2_queue {
>         const struct vb2_buf_ops        *buf_ops;
>
>         void                            *drv_priv;
> +       u32                             subsystem_flags;
>         unsigned int                    buf_struct_size;
>         u32                             timestamp_flags;
>         gfp_t                           gfp_flags;
> diff --git a/include/media/videobuf2-v4l2.h b/include/media/videobuf2-v4l2.h
> index 8a10889dc2fd..13ab101864aa 100644
> --- a/include/media/videobuf2-v4l2.h
> +++ b/include/media/videobuf2-v4l2.h
> @@ -33,6 +33,7 @@
>   * @timecode:  frame timecode.
>   * @sequence:  sequence count of this frame.
>   * @request_fd:        the request_fd associated with this buffer
> + * @is_held:   if true, then this buffer was held
>   * @planes:    plane information (userptr/fd, length, bytesused, data_offset).
>   *
>   * Should contain enough information to be able to cover all the fields
> @@ -46,9 +47,13 @@ struct vb2_v4l2_buffer {
>         struct v4l2_timecode    timecode;
>         __u32                   sequence;
>         __s32                   request_fd;
> +       bool                    is_held;
>         struct vb2_plane        planes[VB2_MAX_PLANES];
>  };
>
> +/* VB2 V4L2 flags as set in vb2_queue.subsystem_flags */
> +#define VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF (1 << 0)
> +
>  /*
>   * to_vb2_v4l2_buffer() - cast struct vb2_buffer * to struct vb2_v4l2_buffer *
>   */
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 530638dffd93..4fa9f543742d 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -915,11 +915,12 @@ struct v4l2_requestbuffers {
>  };
>
>  /* capabilities for struct v4l2_requestbuffers and v4l2_create_buffers */
> -#define V4L2_BUF_CAP_SUPPORTS_MMAP     (1 << 0)
> -#define V4L2_BUF_CAP_SUPPORTS_USERPTR  (1 << 1)
> -#define V4L2_BUF_CAP_SUPPORTS_DMABUF   (1 << 2)
> -#define V4L2_BUF_CAP_SUPPORTS_REQUESTS (1 << 3)
> -#define V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS (1 << 4)
> +#define V4L2_BUF_CAP_SUPPORTS_MMAP                     (1 << 0)
> +#define V4L2_BUF_CAP_SUPPORTS_USERPTR                  (1 << 1)
> +#define V4L2_BUF_CAP_SUPPORTS_DMABUF                   (1 << 2)
> +#define V4L2_BUF_CAP_SUPPORTS_REQUESTS                 (1 << 3)
> +#define V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS            (1 << 4)
> +#define V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF     (1 << 5)
>
>  /**
>   * struct v4l2_plane - plane info for multi-planar buffers
> @@ -1041,6 +1042,8 @@ static inline __u64 v4l2_timeval_to_ns(const struct timeval *tv)
>  #define V4L2_BUF_FLAG_IN_REQUEST               0x00000080
>  /* timecode field is valid */
>  #define V4L2_BUF_FLAG_TIMECODE                 0x00000100
> +/* Don't return the capture buffer until OUTPUT timestamp changes */
> +#define V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF     0x00000200
>  /* Buffer is prepared for queuing */
>  #define V4L2_BUF_FLAG_PREPARED                 0x00000400
>  /* Cache handling flags */
> --
> 2.20.1
>