DRM display framework as I understand it

What is? DRM：

DRM The full name is DirectRenderingManager, yes linux A mainstream display framework , Support multi-layer synthesis , Provide unified for user layers API（libdrm）, To visit GPU, Achieve unified management . It is to solve the problem of multiple programs on video card Access to collaborative resource usage .DRM It can be simply divided into three modules ：

• libdrm： It's a library , Provide general to the upper layer API, It's for all kinds of bottom ioctl To encapsulate , Used for image display and image processing .  Of course with drm Of ioctl Or with framebuffer You can also operate the display image .
• GEM： memory management , Mainly responsible for displaying buffer Distribution and release of 、 Sharing and processing of video memory CPU、GPU Memory synchronization
• KMS：Kernel Mode Setting, Used to update the screen and parameter settings , Complete the configuration of the graphics card , And hardware gpu The operation is encapsulated by a layer of interface . Can be abstracted as ：Framebuffer、CRTC、Plane、Encoder、Connector Equal module .

DRM Frame diagram ：

KMS Frame composition ：

• Framebuffer： A memory area , Used to store displayed image data , Both driver and application layers can access , And save the resolution 、 Color pattern （yuv、rgb） Etc .
• Crtc： Display control , As for display timings,resolution Other configuration , Scan read Framebuffer The data of , It's superimposing Plane The content of is transmitted to Encoder. stay vop（Video Output Processor） in , That is to say lcdc controller .
• Plane： and Framebuffer The same is a piece of memory , It is a hardware image layer , An image layer consists of one or more plane form . The superposition of different mobile phone interfaces is this principle . There are three types of plane：
  • DRM_PLANE_TYPE_PRIMARY: Main layer , Display the background or image content
  • DRM_PLANE_TYPE_OVERLAY： Used to display overlay 、 The zoom
  • DRM_PLANE_TYPE_CURSOR： Used to display the mouse
• Encoder： Encoder , Convert the received data into different signals ：mipi、edp、vga etc. .
• Connector： Specific external interfaces , Such as edp, hdmi, mipi etc. , Connect to the physical display output device , It stores information about the output device , Such as ：EDID、 Video mode, etc .

         The general process is that the user fills the displayed data in Framebuffer, And then through libdrm Interface notification VOP equipment ,（crtc）vop Drive will Framebuffer and plane The data is superimposed and then converted into standard LCDC timing sequential , And then through  Encoder Translate into concrete mipi、edp、vga Equal signal , To Connector display output .

KMS Frame diagram ：