[PATCH v4 1/3] media: v4l2-mem2mem: Add a kref to the v4l2_m2m_dev structure

[Ulf Hansson]

On Fri, 5 Dec 2025 at 02:55, <ming.qian at oss.nxp.com> wrote:
>
> From: Nicolas Dufresne <nicolas.dufresne at collabora.com>
>
> Adding a reference count to the v4l2_m2m_dev structure allow safely
> sharing it across multiple hardware nodes. This can be used to prevent
> running jobs concurrently on m2m cores that have some internal resource
> sharing.
>
> Signed-off-by: Nicolas Dufresne <nicolas.dufresne at collabora.com>
> Signed-off-by: Ming Qian <ming.qian at oss.nxp.com>

I certainly don't have the complete picture for how this needs to work.

However, I was thinking that rather than using a kref and having to
share two specific functions to update it (v4l2_m2m_get|put), couldn't
we just use a device-link instead?

Kind regards
Uffe

> ---
> v4
> - Add my Signed-off-by
>
>  drivers/media/v4l2-core/v4l2-mem2mem.c | 23 +++++++++++++++++++++++
>  include/media/v4l2-mem2mem.h           | 21 +++++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> index fec93c1a9231..ae0de54d4c3e 100644
> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> @@ -90,6 +90,7 @@ static const char * const m2m_entity_name[] = {
>   * @job_work:          worker to run queued jobs.
>   * @job_queue_flags:   flags of the queue status, %QUEUE_PAUSED.
>   * @m2m_ops:           driver callbacks
> + * @kref:              device reference count
>   */
>  struct v4l2_m2m_dev {
>         struct v4l2_m2m_ctx     *curr_ctx;
> @@ -109,6 +110,8 @@ struct v4l2_m2m_dev {
>         unsigned long           job_queue_flags;
>
>         const struct v4l2_m2m_ops *m2m_ops;
> +
> +       struct kref kref;
>  };
>
>  static struct v4l2_m2m_queue_ctx *get_queue_ctx(struct v4l2_m2m_ctx *m2m_ctx,
> @@ -1200,6 +1203,7 @@ struct v4l2_m2m_dev *v4l2_m2m_init(const struct v4l2_m2m_ops *m2m_ops)
>         INIT_LIST_HEAD(&m2m_dev->job_queue);
>         spin_lock_init(&m2m_dev->job_spinlock);
>         INIT_WORK(&m2m_dev->job_work, v4l2_m2m_device_run_work);
> +       kref_init(&m2m_dev->kref);
>
>         return m2m_dev;
>  }
> @@ -1211,6 +1215,25 @@ void v4l2_m2m_release(struct v4l2_m2m_dev *m2m_dev)
>  }
>  EXPORT_SYMBOL_GPL(v4l2_m2m_release);
>
> +void v4l2_m2m_get(struct v4l2_m2m_dev *m2m_dev)
> +{
> +       kref_get(&m2m_dev->kref);
> +}
> +EXPORT_SYMBOL_GPL(v4l2_m2m_get);
> +
> +static void v4l2_m2m_release_from_kref(struct kref *kref)
> +{
> +       struct v4l2_m2m_dev *m2m_dev = container_of(kref, struct v4l2_m2m_dev, kref);
> +
> +       v4l2_m2m_release(m2m_dev);
> +}
> +
> +void v4l2_m2m_put(struct v4l2_m2m_dev *m2m_dev)
> +{
> +       kref_put(&m2m_dev->kref, v4l2_m2m_release_from_kref);
> +}
> +EXPORT_SYMBOL_GPL(v4l2_m2m_put);
> +
>  struct v4l2_m2m_ctx *v4l2_m2m_ctx_init(struct v4l2_m2m_dev *m2m_dev,
>                 void *drv_priv,
>                 int (*queue_init)(void *priv, struct vb2_queue *src_vq, struct vb2_queue *dst_vq))
> diff --git a/include/media/v4l2-mem2mem.h b/include/media/v4l2-mem2mem.h
> index bf6a09a04dcf..ca295c660c7f 100644
> --- a/include/media/v4l2-mem2mem.h
> +++ b/include/media/v4l2-mem2mem.h
> @@ -547,6 +547,27 @@ v4l2_m2m_register_media_controller(struct v4l2_m2m_dev *m2m_dev,
>   */
>  void v4l2_m2m_release(struct v4l2_m2m_dev *m2m_dev);
>
> +/**
> + * v4l2_m2m_get() - take a reference to the m2m_dev structure
> + *
> + * @m2m_dev: opaque pointer to the internal data to handle M2M context
> + *
> + * This is used to share the M2M device across multiple devices. This
> + * can be used to avoid scheduling two hardware nodes concurrently.
> + */
> +void v4l2_m2m_get(struct v4l2_m2m_dev *m2m_dev);
> +
> +/**
> + * v4l2_m2m_put() - remove a reference to the m2m_dev structure
> + *
> + * @m2m_dev: opaque pointer to the internal data to handle M2M context
> + *
> + * Once the M2M device have no more references, v4l2_m2m_realse() will be
> + * called automatically. Users of this method should never call
> + * v4l2_m2m_release() directly. See v4l2_m2m_get() for more details.
> + */
> +void v4l2_m2m_put(struct v4l2_m2m_dev *m2m_dev);
> +
>  /**
>   * v4l2_m2m_ctx_init() - allocate and initialize a m2m context
>   *
> --
> 2.52.0
>