Hello,

Let me introduce the new ioctls, which are intended to allow gntdev to
map scatter-gather table on top of the existing dmabuf, referenced by
file descriptor.

When using dma-buf exporter to create dma-buf with backing storage and
map it to the grant refs, provided from the domain, we've met a problem,
that several HW (i.MX8 gpu in our case) do not support external buffer
and requires backing storage to be created using it's native tools.
That's why new ioctls were added to be able to pass existing dma-buffer
fd as input parameter and use it as backing storage to export to refs.

This is a pretty big NAK from my side to this approach.

If you need to replace a file descriptor number local to your process then you can simply use dup2() from userspace.

If your intention here is to replace the backing store of the fd on all processes which currently have it open then please just completely forget that. This will *NEVER* ever work correctly.

Regards,

Christian.

Following calls were added:
IOCTL_GNTDEV_DMABUF_MAP_REFS_TO_BUF - map existing buffer as the backing
storage and export it to the provided grant refs;
IOCTL_GNTDEV_DMABUF_MAP_RELEASE - detach buffer from the grant table and
set notification to unmap grant refs before releasing the external
buffer. After this call the external buffer should be detroyed.
IOCTL_GNTDEV_DMABUF_MAP_WAIT_RELEASED - wait for timeout until buffer is
completely destroyed and gnt refs unmapped so domain could free grant
pages. Should be called after buffer was destoyed.

Our setup is based on IMX8QM board. We're trying to implement zero-copy
support for DomU graphics using Wayland zwp_linux_dmabuf_v1_interface
implementation.

For dma-buf exporter we used i.MX8 gpu native tools to create backing
storage grant-refs, received from DomU. Buffer for the backing storage was
allocated using gbm_bo_create call because gpu do not support external
buffer and requires backing storage to be created using it's native tools
(eglCreateImageKHR returns EGL_NO_IMAGE_KHR for buffers, which were not
created using gbm_bo_create).

This behaviour was also tested on Qemu setup using
DRM_IOCTL_MODE_CREATE_DUMB call to create backing storage buffer.

---
Oleksii Moisieiev (3):
  xen/grant-table: save page_count on map and use if during async
    unmapping
  dma-buf: add dma buffer release notifier callback
  xen/grant-table: add new ioctls to map dmabuf to existing fd

 drivers/dma-buf/dma-buf.c   |  44 ++++
 drivers/xen/gntdev-common.h |   8 +-
 drivers/xen/gntdev-dmabuf.c | 416 +++++++++++++++++++++++++++++++++++-
 drivers/xen/gntdev-dmabuf.h |   7 +
 drivers/xen/gntdev.c        | 101 ++++++++-
 drivers/xen/grant-table.c   |  73 +++++--
 include/linux/dma-buf.h     |  15 ++
 include/uapi/xen/gntdev.h   |  62 ++++++
 include/xen/grant_table.h   |   8 +
 9 files changed, 703 insertions(+), 31 deletions(-)

--
2.25.1