一、DRM 简介
In computing, the Direct Rendering Manager (DRM), a subsystem of the Linux kernel, interfaces with the GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU, and to perform operations such as configuring the mode setting of the display. DRM was first developed as the kernel space component of the X Server's Direct Rendering Infrastructure,[1] but since then it has been used by other graphic stack alternatives such as Wayland.
User-space programs can use the DRM API to command the GPU to do hardware-accelerated 3D rendering and video decoding as well as GPGPU computing.
The Linux Kernel already had an API called fbdev allowing to manage the framebuffer of a graphics adapter,[2] but it couldn't be used to handle the needs of modern 3D accelerated GPU based video cards. These type of cards usually require setting and managing a command queue in the card's memory (Video RAM) to dispatch commands to the GPU, and also they need a proper management of the buffers and free space of the Video RAM itself.[3] Initially user space programs (such as the X Server) directly managed these resources, but these programs usually acted as if they were the only ones with access to the card's resources. When two or more programs tried to control the same video card at the same time, and set its resources each one in its own way, most times they ended catastrophically.[3]
When the Direct Rendering Manager was first created, the purpose was that multiple programs using resources from the video card can cooperate through it.[4] The DRM gets an exclusive access to the video card, and it's responsible for initializing and maintaining the command queue, the VRAM and any other hardware resource. The programs that want to use the GPU send their requests to DRM, which acts as an arbitrator and takes care to avoid possible conflicts.
Since then, the scope of DRM has been expanded over the years to cover more functionality previously handled by user space programs, such as framebuffer managing and mode setting, memory sharing objects and memory synchronization.[5][6] Some of these expansions had carried their own specific names, such as Graphics Execution Manager (GEM) or Kernel Mode-Setting (KMS), and the terminology prevails when the functionality they provide is specifically alluded. But they are really parts of the whole kernel DRM subsystem.
The trend to include two GPUs in a computer —a discrete GPU and an integrated one— led to new problems such as GPU switching that should be also solved at the DRM layer. In order to match the Nvidia Optimus technology, DRM was provided with GPU offloading abilities, called PRIME.[7]
二、DRM 架构
参考链接： https://en.wikipedia.org/wiki/Direct_Rendering_Manager