Hi Philipp,
On Tue, Nov 13, 2018 at 04:06:21PM +0100, Philipp Zabel wrote:
> From: John Sheu <sheu@xxxxxxxxxxxx>
>
> Videobuf2 presently does not allow VIDIOC_REQBUFS to destroy outstanding
> buffers if the queue is of type V4L2_MEMORY_MMAP, and if the buffers are
> considered "in use". This is different behavior than for other memory
> types and prevents us from deallocating buffers in following two cases:
>
> 1) There are outstanding mmap()ed views on the buffer. However even if
> we put the buffer in reqbufs(0), there will be remaining references,
> due to vma .open/close() adjusting vb2 buffer refcount appropriately.
> This means that the buffer will be in fact freed only when the last
> mmap()ed view is unmapped.
>
> 2) Buffer has been exported as a DMABUF. Refcount of the vb2 buffer
> is managed properly by VB2 DMABUF ops, i.e. incremented on DMABUF
> get and decremented on DMABUF release. This means that the buffer
> will be alive until all importers release it.
>
> Considering both cases above, there does not seem to be any need to
> prevent reqbufs(0) operation, because buffer lifetime is already
> properly managed by both mmap() and DMABUF code paths. Let's remove it
> and allow userspace freeing the queue (and potentially allocating a new
> one) even though old buffers might be still in processing.
>
> To let userspace know that the kernel now supports orphaning buffers
> that are still in use, add a new V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS
> to be set by reqbufs and create_bufs.
>
> Signed-off-by: John Sheu <sheu@xxxxxxxxxxxx>
> Reviewed-by: Pawel Osciak <posciak@xxxxxxxxxxxx>
> Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> [p.zabel@xxxxxxxxxxxxxx: moved __vb2_queue_cancel out of the mmap_lock
> and added V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS]
> Signed-off-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
This lets the user to allocate lots of mmap'ed buffers that are pinned in
physical memory. Considering that we don't really have a proper mechanism
to limit that anyway,
Acked-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
That said, the patch must be accompanied by the documentation change in
Documentation/media/uapi/v4l/vidioc-reqbufs.rst .
I wonder what Hans thinks.
> ---
> .../media/common/videobuf2/videobuf2-core.c | 26 +------------------
> .../media/common/videobuf2/videobuf2-v4l2.c | 2 +-
> include/uapi/linux/videodev2.h | 1 +
> 3 files changed, 3 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 975ff5669f72..608459450c1e 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -553,20 +553,6 @@ bool vb2_buffer_in_use(struct vb2_queue *q, struct vb2_buffer *vb)
> }
> EXPORT_SYMBOL(vb2_buffer_in_use);
>
> -/*
> - * __buffers_in_use() - return true if any buffers on the queue are in use and
> - * the queue cannot be freed (by the means of REQBUFS(0)) call
> - */
> -static bool __buffers_in_use(struct vb2_queue *q)
> -{
> - unsigned int buffer;
> - for (buffer = 0; buffer < q->num_buffers; ++buffer) {
> - if (vb2_buffer_in_use(q, q->bufs[buffer]))
> - return true;
> - }
> - return false;
> -}
> -
> void vb2_core_querybuf(struct vb2_queue *q, unsigned int index, void *pb)
> {
> call_void_bufop(q, fill_user_buffer, q->bufs[index], pb);
> @@ -674,23 +660,13 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>
> if (*count == 0 || q->num_buffers != 0 ||
> (q->memory != VB2_MEMORY_UNKNOWN && q->memory != memory)) {
> - /*
> - * We already have buffers allocated, so first check if they
> - * are not in use and can be freed.
> - */
> - mutex_lock(&q->mmap_lock);
> - if (q->memory == VB2_MEMORY_MMAP && __buffers_in_use(q)) {
> - mutex_unlock(&q->mmap_lock);
> - dprintk(1, "memory in use, cannot free\n");
> - return -EBUSY;
> - }
> -
> /*
> * Call queue_cancel to clean up any buffers in the
> * QUEUED state which is possible if buffers were prepared or
> * queued without ever calling STREAMON.
> */
> __vb2_queue_cancel(q);
> + mutex_lock(&q->mmap_lock);
> ret = __vb2_queue_free(q, q->num_buffers);
> mutex_unlock(&q->mmap_lock);
> if (ret)
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index a17033ab2c22..f02d452ceeb9 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -624,7 +624,7 @@ EXPORT_SYMBOL(vb2_querybuf);
>
> static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
> {
> - *caps = 0;
> + *caps = V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS;
> if (q->io_modes & VB2_MMAP)
> *caps |= V4L2_BUF_CAP_SUPPORTS_MMAP;
> if (q->io_modes & VB2_USERPTR)
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index c8e8ff810190..2a223835214c 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -879,6 +879,7 @@ struct v4l2_requestbuffers {
> #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)
>
> /**
> * struct v4l2_plane - plane info for multi-planar buffers
--
Kind regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx
