[PATH 4/4] doc-rst: Add virtual DRM documentation

[Tomohito Esaki <etom@igel.co.jp>]

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
---
 Documentation/gpu/drivers.rst |  1 +
 Documentation/gpu/vdrm.rst    | 51 +++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 Documentation/gpu/vdrm.rst

diff --git a/Documentation/gpu/drivers.rst b/Documentation/gpu/drivers.rst
index b4a0ed3ca961..bad0226de972 100644
--- a/Documentation/gpu/drivers.rst
+++ b/Documentation/gpu/drivers.rst
@@ -19,6 +19,7 @@ GPU Driver Documentation
    xen-front
    afbc
    komeda-kms
+   vdrm
 
 .. only::  subproject and html
 
diff --git a/Documentation/gpu/vdrm.rst b/Documentation/gpu/vdrm.rst
new file mode 100644
index 000000000000..2ab1699c2f42
--- /dev/null
+++ b/Documentation/gpu/vdrm.rst
@@ -0,0 +1,51 @@
+=============================
+ drm/vdrm virtual DRM driver
+=============================
+
+Virtual DRM splits the overlay planes of a display controller into multiple
+virtual devices to allow each plane to be accessed by each process.
+
+This makes it possible to overlay images output from multiple processes on a
+display. For example, one process displays the camera image without compositor
+while another process overlays the UI.
+
+The virtual DRM creates standalone virtual device and make DRM planes from a
+master device (e.g. card0) accessible via one or more virtual device. However,
+these plane are no longer accessible from the original device.
+Each virtual device (and plane) can be accessed via a separate device file.
+
+The virtual DRM driver doesn't directly control the display hardware and has
+no access to the physical bus. Instead, the virtual DRM driver issues requests
+to the standard DRM device driver ("master" driver) when the hardware needs to
+be controlled. The master driver is modified to notify the virtual DRM driver
+of interrupt events from the display hardware.
+
+Plane position and size
+=======================
+The initial position, size and z-position of the plane used in virtual DRM is
+specified in the device tree. The position and size of the planes are set as
+properties and can be updated. The name of each property is as
+follows:
+* vdrm_offset_x: x-coordinate of the left-top of the plane on the screen
+* vdrm_offset_y: y-coordinate of the left-top of the plane on the screen
+* vdrm_width: width of the plane
+* vdrm_height: height of the plane
+
+Virtual DRM Functions Reference
+===============================
+
+.. kernel-doc:: drivers/gpu/drm/vdrm/vdrm_api.h
+    :internal:
+
+.. kernel-doc:: drivers/gpu/drm/vdrm/vdrm_drv.c
+    :export:
+
+Driver limitations
+==================
+1. Each virtual DRM device only supports one plane per CRTC.
+
+2. Virtual DRM doesn't support hot plug connector.
+
+3. If virtual DRM has multiple CRTCs, the initial position and size of the
+   virtual DRM planes is the same for all planes, since they cannot be set
+   for each plane in the device tree.
-- 
2.25.1