Writing DRI Engine Driver for Your GPU

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Writing a driver of the DRI engine for your GPU.

Table of Contents

Overview
Example

Overview

In order to support modern graphics card or GPU, we introduced a new NEWGAL engine of dri. The developer can use this engine to run MiniGUI apps on a Linux box on which the DRI (Direct Rendering Infrastructure) is enabled.

The dri engine uses libdrm developed by Free Desktop project:

https://dri.freedesktop.org/wiki/

Libdrm is a user-space library implements the Direct Rendering Manager. MiniGUI mainly uses this library to support the dumb frame buffer (no hardware acceleration). However, you can write a driver for your graphics card or GPU to implement the hardware accelerated features.

To avoid modifying the MiniGUI source code when supporting a new GPU, the dri engine has adopted a scalable design:

You can directly use the dri engine to run MiniGUI on a GPU which supports dumb frame buffer.
When you want to take advantage of the hardware acceleration of your GPU, you can write some code for your GPU as a sub driver of dri engine outside MiniGUI.

This document describes how to enable dri engine and write a hardware-accelerated driver for your own GPU.

Note that, generally, the driver will be implemented by the GPU or SoC vendors. The MiniGUI app developers do not need to care about this.

Compile-time configuration

There are two configure options related to the dri engine:

--enable-videodri enables the dri engine, and --disable-videodri disables the dri engine. Note that the dri engine is only available on Linux, and you need to install the libdrm 2.4 or later first.
--with-targetname=external. When you configure MiniGUI with this option. MiniGUI will use the external function __dri_ex_driver_get to initialize the DRI driver. If you do not implement this function, the link will fail.

Run-time configuration

For dri engine, we introduce a new section in MiniGUI runtime configuration:

[dri]
defaultmode=1024x768-32bpp
dpi=96
pixelformat=XR24
device=/dev/dri/card0

You can use the key dri.device to specify your DRI device.

You can use the key dri.pixelformat to specify the DRM pixel format for the screen. We use DRM fourcc code to defined the pixel format of the screen surface. For more information, please see <drm/drm_fourcc.h> header file. Note that only 8/16/24/32 bpp RGB formats are supported. For example, XR24 means X8R8G8B8 pixel format.

Implement DRI driver

The header file <minigui/exstubs.h> defines the operators (a set of callback functions) you need to implement for your GPU externally.

To exploit the GPU's accelerated rendering capabilities, a MiniGUI app can use cairo and/or OpenGL ES to assist in rendering 2D/3D graphics when using the dri engine. We will provide some samples in mg-tests or mg-samples for this purpose.

Restrictions

In version 4.0.0, the dri NEWGAL engine does not provide support for MiniGUI-Processes run-time mode. We will enhance this in the subsequent version of MiniGUI.

Also note that when you use the hardware accelerated sub driver, MiniGUI app may need the root privilege to call drmSetMaster to set the video mode. However, under MiniGUI-Processes run-time mode, only the server (mginit) will need this privilege when you use the future dri engine.

Example

As an example, we implement the sub driver for i915 graphics chard in mg-tests/dri-engine/. Please refer to mg-tests repository:

https://github.com/VincentWei/mg-tests/tree/master/dri-engine