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+===================
+Userland interfaces
+===================
+
+The DRM core exports several interfaces to applications, generally
+intended to be used through corresponding libdrm wrapper functions. In
+addition, drivers export device-specific interfaces for use by userspace
+drivers & device-aware applications through ioctls and sysfs files.
+
+External interfaces include: memory mapping, context management, DMA
+operations, AGP management, vblank control, fence management, memory
+management, and output management.
+
+Cover generic ioctls and sysfs layout here. We only need high-level
+info, since man pages should cover the rest.
+
+libdrm Device Lookup
+====================
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
+   :doc: getunique and setversion story
+
+
+Primary Nodes, DRM Master and Authentication
+============================================
+
+.. kernel-doc:: drivers/gpu/drm/drm_auth.c
+   :doc: master and authentication
+
+.. kernel-doc:: drivers/gpu/drm/drm_auth.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_auth.h
+   :internal:
+
+Render nodes
+============
+
+DRM core provides multiple character-devices for user-space to use.
+Depending on which device is opened, user-space can perform a different
+set of operations (mainly ioctls). The primary node is always created
+and called card<num>. Additionally, a currently unused control node,
+called controlD<num> is also created. The primary node provides all
+legacy operations and historically was the only interface used by
+userspace. With KMS, the control node was introduced. However, the
+planned KMS control interface has never been written and so the control
+node stays unused to date.
+
+With the increased use of offscreen renderers and GPGPU applications,
+clients no longer require running compositors or graphics servers to
+make use of a GPU. But the DRM API required unprivileged clients to
+authenticate to a DRM-Master prior to getting GPU access. To avoid this
+step and to grant clients GPU access without authenticating, render
+nodes were introduced. Render nodes solely serve render clients, that
+is, no modesetting or privileged ioctls can be issued on render nodes.
+Only non-global rendering commands are allowed. If a driver supports
+render nodes, it must advertise it via the DRIVER_RENDER DRM driver
+capability. If not supported, the primary node must be used for render
+clients together with the legacy drmAuth authentication procedure.
+
+If a driver advertises render node support, DRM core will create a
+separate render node called renderD<num>. There will be one render node
+per device. No ioctls except PRIME-related ioctls will be allowed on
+this node. Especially GEM_OPEN will be explicitly prohibited. Render
+nodes are designed to avoid the buffer-leaks, which occur if clients
+guess the flink names or mmap offsets on the legacy interface.
+Additionally to this basic interface, drivers must mark their
+driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
+clients can use them. Driver authors must be careful not to allow any
+privileged ioctls on render nodes.
+
+With render nodes, user-space can now control access to the render node
+via basic file-system access-modes. A running graphics server which
+authenticates clients on the privileged primary/legacy node is no longer
+required. Instead, a client can open the render node and is immediately
+granted GPU access. Communication between clients (or servers) is done
+via PRIME. FLINK from render node to legacy node is not supported. New
+clients must not use the insecure FLINK interface.
+
+Besides dropping all modeset/global ioctls, render nodes also drop the
+DRM-Master concept. There is no reason to associate render clients with
+a DRM-Master as they are independent of any graphics server. Besides,
+they must work without any running master, anyway. Drivers must be able
+to run without a master object if they support render nodes. If, on the
+other hand, a driver requires shared state between clients which is
+visible to user-space and accessible beyond open-file boundaries, they
+cannot support render nodes.
+
+VBlank event handling
+=====================
+
+The DRM core exposes two vertical blank related ioctls:
+
+DRM_IOCTL_WAIT_VBLANK
+    This takes a struct drm_wait_vblank structure as its argument, and
+    it is used to block or request a signal when a specified vblank
+    event occurs.
+
+DRM_IOCTL_MODESET_CTL
+    This was only used for user-mode-settind drivers around modesetting
+    changes to allow the kernel to update the vblank interrupt after
+    mode setting, since on many devices the vertical blank counter is
+    reset to 0 at some point during modeset. Modern drivers should not
+    call this any more since with kernel mode setting it is a no-op.
+
+This second part of the GPU Driver Developer's Guide documents driver
+code, implementation details and also all the driver-specific userspace
+interfaces. Especially since all hardware-acceleration interfaces to
+userspace are driver specific for efficiency and other reasons these
+interfaces can be rather substantial. Hence every driver has its own
+chapter.