[PATCH RESEND v2 2/2] drm/xen-front: Provide kernel documentation
- Date: Tue, 13 Mar 2018 18:21:06 +0200
- From: Oleksandr Andrushchenko <andr2000@xxxxxxxxx>
- Subject: [PATCH RESEND v2 2/2] drm/xen-front: Provide kernel documentation
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Provide kernel documentation for the Xen para-virtualized
frontend DRM driver.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Documentation/gpu/index.rst | 1 +
Documentation/gpu/xen-front.rst | 77 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 78 insertions(+)
create mode 100644 Documentation/gpu/xen-front.rst
diff --git a/Documentation/gpu/index.rst b/Documentation/gpu/index.rst
index c36586dad29d..e31684af0a20 100644
@@ -20,6 +20,7 @@ Linux GPU Driver Developer's Guide
.. only:: subproject and html
diff --git a/Documentation/gpu/xen-front.rst b/Documentation/gpu/xen-front.rst
new file mode 100644
@@ -0,0 +1,77 @@
+Xen para-virtualized frontend driver
+This frontend driver implements Xen para-virtualized display
+according to the display protocol described at
+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
+Buffers allocated by the frontend driver
+The below modes of operation are configured at compile-time via
+frontend driver's kernel configuration:
+With 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.
+Without GEM CMA helpers
+ 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.
+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
+Note, while using this scenario the following should be considered:
+#. If guest domain dies then pages/grants received from the backend
+ cannot be claimed back
+#. Misbehaving guest may send too many requests to the
+ backend exhausting its grant references and memory
+ (consider this from security POV).
+#. Only primary plane without additional properties is supported.
+#. Only one video mode per connector supported which is configured via XenStore.
+#. All CRTCs operate at fixed frequency of 60Hz.