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Subject[PATCH v4 0/2] drm/xen-front: Add support for Xen PV display frontend
From: Oleksandr Andrushchenko <>


1. Boris, I put your R-b tag as I almost didn't change Xen part
of the driver (see below). Please let me know if this is not acceptable,
so I remove the tag.
2. With this patch series I am also adding a patch from Noralf Trønnes [12]
to enable critical sections for unplugabble devices as agreed in [13].
This can be applied without any respect to Xen PV DRM frontend driver.

This patch series adds support for Xen [1] para-virtualized
frontend display driver. It implements the protocol from
include/xen/interface/io/displif.h [2].
Accompanying backend [3] is implemented as a user-space application
and its helper library [4], capable of running as a Weston client
or DRM master.
Configuration of both backend and frontend is done via
Xen guest domain configuration options [5].

* Driver limitations
1. Configuration options 1.1 (contiguous display buffers) and 2 (backend
allocated buffers) below are not supported at the same time.

2. Only primary plane without additional properties is supported.

3. Only one video mode supported which resolution is configured via XenStore.

4. All CRTCs operate at fixed frequency of 60Hz.

* Driver modes of operation in terms of display buffers used
Depending on the requirements for the para-virtualized environment, namely
requirements dictated by the accompanying DRM/(v)GPU drivers running in both
host and guest environments, number of operating modes of para-virtualized
display driver are supported:
- display buffers can be allocated by either frontend driver or backend
- display buffers can be allocated to be contiguous in memory or not

Note! Frontend driver itself has no dependency on contiguous memory for
its operation.

* 1. Buffers allocated by the frontend driver.

The below modes of operation are configured at compile-time via
frontend driver's kernel configuration.

1.1. Front driver configured to use GEM CMA helpers
This use-case is useful when used with accompanying DRM/vGPU driver in
guest domain which was designed to only work with contiguous buffers,
e.g. DRM driver based on GEM CMA helpers: such drivers can only import
contiguous PRIME buffers, thus requiring frontend driver to provide
such. In order to implement this mode of operation para-virtualized
frontend driver can be configured to use GEM CMA helpers.

1.2. Front driver doesn't use GEM CMA
If accompanying drivers can cope with non-contiguous memory then, to
lower pressure on CMA subsystem of the kernel, driver can allocate
buffers from system memory.

Note! If used with accompanying DRM/(v)GPU drivers this mode of operation
may require IOMMU support on the platform, so accompanying DRM/vGPU
hardware can still reach display buffer memory while importing PRIME
buffers from the frontend driver.

* 2. Buffers allocated by the backend

This mode of operation is run-time configured via guest domain configuration
through XenStore entries.

For systems which do not provide IOMMU support, but having specific
requirements for display buffers it is possible to allocate such buffers
at backend side and share those with the frontend.
For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting
physically contiguous memory, this allows implementing zero-copying

I would like to thank at least, but not at last the following
people/communities who helped this driver to happen ;)

1. My team at EPAM for continuous support
2. Xen community for answering tons of questions on different
modes of operation of the driver with respect to virtualized
3. Rob Clark for "GEM allocation for para-virtualized DRM driver" [6]
4. Maarten Lankhorst for "Atomic driver and old remove FB behavior" [7]
5. Ville Syrjälä for "Questions on page flips and atomic modeset" [8]

Changes since v3:
- no changes to Xen related code (shared buffer handling, event channels etc.),
but minor changes to xenbus_driver state machine due to re-worked unplug
implementation: additional state checks added
- re-worked dumb creation code to fix race condition (drm_gem_handle_create)
- use drm_dev_{enter|exit} to protect code which must not run when unplugged
- re-work unplug code to fully support "zombie" DRM devices on backend
- implement a dedicated page flip time-out worker, remove logic from connector
detect callback
- move mode_valid from drm_connector_helper_funcs to drm_simple_display_pipe_funcs
- use drm_gem_object_put_unlocked instead of obsolete drm_gem_object_unreference_unlocked
- minor cleanups

Changes since v2:
- no changes to Xen related code (shared buffer handling, event channels etc.)
- rework DRM driver release with hotplug (Daniel)
- squash xen_drm_front and xen_drm_front_drv as they depend on each
other too heavily now
- remove platform driver and instantiate DRM device
from xenbus driver directly
- have serializing mutex per connector, not a single one,
so we don't introduce a bottle neck for multiple connectors
- minor comments addressed (Daniel)

Changes since v1:
- use SPDX license identifier, set license to GPLv2 OR MIT
- changed midlayers to direct function calls, removed:
- front_ops
- gem_ops
- renamed xenbus_driver callbacks to align with exisitng PV drivers
- re-worked backend error handling with connector hotplug uevents
- removed vblank handling so user-space doesn't have an impression
we really support that
- directly use front's mode_set in display enable/disable
- removed BUG_ON, error handling implemented
- moved driver documentation into Documentation/gpu
- other comments from Xen community addressed (Boris and Juergen)
- squashed Xen and DRM patches for better interrconnection visibility
- for your convenience driver is available at [11]

Thank you,
Oleksandr Andrushchenko


Noralf Trønnes (1):
drm: Use srcu to protect drm_device.unplugged

Oleksandr Andrushchenko (1):
drm/xen-front: Add support for Xen PV display frontend

Documentation/gpu/drivers.rst | 1 +
Documentation/gpu/xen-front.rst | 43 ++
drivers/gpu/drm/Kconfig | 2 +
drivers/gpu/drm/Makefile | 1 +
drivers/gpu/drm/drm_drv.c | 54 +-
drivers/gpu/drm/xen/Kconfig | 30 +
drivers/gpu/drm/xen/Makefile | 16 +
drivers/gpu/drm/xen/xen_drm_front.c | 880 ++++++++++++++++++++++++++++
drivers/gpu/drm/xen/xen_drm_front.h | 189 ++++++
drivers/gpu/drm/xen/xen_drm_front_cfg.c | 77 +++
drivers/gpu/drm/xen/xen_drm_front_cfg.h | 37 ++
drivers/gpu/drm/xen/xen_drm_front_conn.c | 115 ++++
drivers/gpu/drm/xen/xen_drm_front_conn.h | 27 +
drivers/gpu/drm/xen/xen_drm_front_evtchnl.c | 382 ++++++++++++
drivers/gpu/drm/xen/xen_drm_front_evtchnl.h | 81 +++
drivers/gpu/drm/xen/xen_drm_front_gem.c | 309 ++++++++++
drivers/gpu/drm/xen/xen_drm_front_gem.h | 41 ++
drivers/gpu/drm/xen/xen_drm_front_gem_cma.c | 78 +++
drivers/gpu/drm/xen/xen_drm_front_kms.c | 371 ++++++++++++
drivers/gpu/drm/xen/xen_drm_front_kms.h | 27 +
drivers/gpu/drm/xen/xen_drm_front_shbuf.c | 432 ++++++++++++++
drivers/gpu/drm/xen/xen_drm_front_shbuf.h | 72 +++
include/drm/drm_device.h | 9 +-
include/drm/drm_drv.h | 15 +-
24 files changed, 3279 insertions(+), 10 deletions(-)
create mode 100644 Documentation/gpu/xen-front.rst
create mode 100644 drivers/gpu/drm/xen/Kconfig
create mode 100644 drivers/gpu/drm/xen/Makefile
create mode 100644 drivers/gpu/drm/xen/xen_drm_front.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_cfg.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_cfg.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_conn.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_conn.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_evtchnl.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_evtchnl.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem_cma.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_kms.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_kms.h
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_shbuf.c
create mode 100644 drivers/gpu/drm/xen/xen_drm_front_shbuf.h


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