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diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst new file mode 100644 index 000000000000..536bf3eaadd4 --- /dev/null +++ b/Documentation/gpu/drm-uapi.rst @@ -0,0 +1,111 @@ +=================== +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. |