After discussing xen-zcopy and hyper-dmabuf  approaches
it seems that xen-zcopy can be made not depend on DRM core any more
and be dma-buf centric (which it in fact is).
The DRM code was mostly there for dma-buf's FD import/export
with DRM PRIME UAPI and with DRM use-cases in mind, but it comes out that if
the proposed 2 IOCTLs (DRM_XEN_ZCOPY_DUMB_FROM_REFS and
are extended to also provide a file descriptor of the corresponding
PRIME stuff in the driver is not needed anymore.
That being said, xen-zcopy can safely be detached from DRM and moved from
drivers/gpu/drm/xen into drivers/xen/dma-buf-backend(?).
This driver then becomes a universal way to turn any shared buffer
and DomU(s) into a dma-buf, e.g. one can create a dma-buf from any grant
or represent a dma-buf as grant-references for export.
This way the driver can be used not only for DRM use-cases, but also for
use-cases which may require zero copying between domains.
For example, the use-cases we are about to work in the nearest future
V4L, e.g. we plan to support cameras, codecs etc. and all these will benefit
from zero copying much. Potentially, even block/net devices may benefit,
but this needs some evaluation.
I would love to hear comments for authors of the hyper-dmabuf
and Xen community, as well as DRI-Devel and other interested parties.
On 03/29/2018 04:19 PM, Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>
When using Xen PV DRM frontend driver then on backend side one will need
to do copying of display buffers' contents (filled by the
frontend's user-space) into buffers allocated at the backend side.
Taking into account the size of display buffers and frames per seconds
it may result in unneeded huge data bus occupation and performance loss.
This helper driver allows implementing zero-copying use-cases
when using Xen para-virtualized frontend display driver by
implementing a DRM/KMS helper driver running on backend's side.
It utilizes PRIME buffers API to share frontend's buffers with
physical device drivers on backend's side:
- a dumb buffer created on backend's side can be shared
with the Xen PV frontend driver, so it directly writes
into backend's domain memory (into the buffer exported from
DRM/KMS driver of a physical display device)
- a dumb buffer allocated by the frontend can be imported
into physical device DRM/KMS driver, thus allowing to
achieve no copying as well
For that reason number of IOCTLs are introduced:
This will create a DRM dumb buffer from grant references provided
by the frontend
This will grant references to a dumb/display buffer's memory provided
by the backend
This will block until the dumb buffer with the wait handle provided
With this helper driver I was able to drop CPU usage from 17% to 3%
on Renesas R-Car M3 board.
This was tested with Renesas' Wayland-KMS and backend running as DRM master.
Oleksandr Andrushchenko (1):
drm/xen-zcopy: Add Xen zero-copy helper DRM driver
Documentation/gpu/drivers.rst | 1 +
Documentation/gpu/xen-zcopy.rst | 32 +
drivers/gpu/drm/xen/Kconfig | 25 +
drivers/gpu/drm/xen/Makefile | 5 +
drivers/gpu/drm/xen/xen_drm_zcopy.c | 880 ++++++++++++++++++++++++++++
drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c | 154 +++++
drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h | 38 ++
include/uapi/drm/xen_zcopy_drm.h | 129 ++++
8 files changed, 1264 insertions(+)
create mode 100644 Documentation/gpu/xen-zcopy.rst
create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h
create mode 100644 include/uapi/drm/xen_zcopy_drm.h