drmfb-composer

drmfb-composer is a simple HIDL HAL implementation of [email protected] using Linux DRM legacy Kernel Mode Setting. It specifically does not make use of Atomic Mode Setting to support older hardware and kernels.

Note: For newer devices with stable Atomic Mode Setting support, prefer using drm_hwcomposer. It is a more complete implementation that can offload parts of the composition from the GPU to hardware. (See Comparison to drm_hwcomposer)

Introduction

drmfb-composer is mostly a proof of concept: it is a Graphics Composer HAL that works very much like the old Framebuffer (FB) HALs. This allows implementing additional features like multiple display support, while still supporting all old hardware.

The main difference from implementations like drm_hwcomposer is that it does not do any composition in hardware. It only handles displaying the results of client (GPU) composition in SurfaceFlinger using Kernel Mode Setting. That makes it less efficient, but avoids use of Atomic Mode Setting and multiple planes that sometimes cause issues on older hardware.

It has also turned out to be a great learning experience, especially to understand which features are supposed to be provided by a Graphics Composer HAL, and how they can be implemented using DRM/KMS.

Features

• Multiple displays and hotplug
  • Note: Although not limited in the Composer HAL, the current Android framework limits this to:
    • Only two displays working at the same time (one primary and one external)
    • Hotplugging the first (primary) display will result in crashes
• Exposes all available displays modes (e.g. possible lower resolutions or refresh rates)
• Hardware vertical sync (VSYNC) signals

Comparison to drm_hwcomposer (HWC2 HAL)

drm_hwcomposer is a more complete and efficient implementation of a HWC2 HAL implemented using Atomic Mode Setting. However, it does not run on older hardware without Atomic Mode Setting (e.g. radeon) in the kernel.

Features missing in drmfb-composer

• Atomic Mode Setting
• Hardware Composition
  • Currently, drmfb-composer lets SurfaceFlinger fall back to client composition using GLES on the GPU for all layers. Eventually, some simple composition could be implemented (e.g. cursor planes).
• Explicit Synchronization (e.g. Release Fences)
  • IN_FENCE_FD and OUT_FENCE_PTR only exist as properties for Atomic Mode Setting

Features missing in drm_hwcomposer

These are features that I would like to port to drm_hwcomposer eventually. (Still working on getting drm_hwcomposer working on my primary test device.)

• Multiple display support and hotplug: Already proposed as [WIP/RFC] External display hotplug support
• XBGR8888 fallback for ABGR8888 for primary planes on older Intel GPUs
• Runtime selection of libdrm or minigbm importer (currently a compile-time option in drm_hwcomposer)

Comparison to drm_gralloc (FB HAL)

drm_gralloc was one of the first Gralloc HAL implementations for DRM and used to implement a Framebuffer HAL that is still being used in Android-x86 today. drmfb-composer has a couple of advantages compared to drm_gralloc:

• Modern implementation of a Graphics Composer HAL instead of a legacy FB HAL
• PRIME file descriptors instead of GEM names
• Works fully binderized and with DRM GPU authentication enabled
• Multiple display support with different content instead of only screen mirroring

Comparison to drm_framebuffer (FB HAL)

drm_framebuffer was an earlier version of drmfb-composer implemented as a Framebuffer HAL (FB HAL). Implementing a proper Graphics Composer HAL has a couple of advantages:

• Modern implementation of a Graphics Composer HAL instead of a legacy FB HAL
• No need for changes in the Gralloc HAL (drm_framebuffer was a static library that required changes in the Gralloc HAL)
• Supports both libdrm gralloc handle and minigbm at the same time (detected at runtime)
• Multiple display support, hotplug and screen mirroring (by default extra screens are mirrored)

Requirements

• Linux kernel with a GPU supported by DRM (e.g. i915, radeon, ...)
• Android 9 (Pie) or newer
  • Necessary for properly working multiple display support and hotplugging
  • [email protected] (a header library drmfb-composer is built on) does not exist on previous Android versions
• Gralloc HAL: libdrm gralloc handle compatible (e.g. gbm_gralloc) or minigbm (detected at runtime)

Usage

Add drmfb-composer to your Android build tree and build [email protected].

drmfb-composer is a binderized HIDL HAL service. Ensure that the binary and init file is pushed to your device:

• /vendor/bin/hw/[email protected]
• /vendor/etc/init/[email protected]

SELinux Policy

sepolicy contains a simple SELinux Policy definition for drmfb-composer. You can include it in the build by adding the directory to BOARD_SEPOLICY_DIRS.

License

drmfb-composer is licensed under the Apache License, Version 2.0. Contributions welcome!