Many people seem to get confused with the actual function of GPUs used in embedded (ARM / MIPS) SoC, and I can often read comments similar to “with lima drivers we should get video decoding in XBMc soon”, and I’ve just received any email reading “My main task is to build a full hd media player based on ffmpeg with hardware decoding acceleration for Linux. Is it possible with mali400mp4?”. So I’ve decided to write a short post about it to make things a bit more clear. Contrary to GPUs in the PC world, embedded GPUs only take care of 3D, and sometimes 2D graphics, and leave video encoding and/or decoding to another block called Video Processing Unit (VPU). There’s at least one exception with Broadcom Videocore IV GPU as found in the processor used in the Raspberry Pi that apparently takes care of 2D & 3D graphics as well as hardware video decoding & encoding, but this is not the norm.
Let’s take an example with Freescale i.MX6 Quad SoC.
In the multimedia section in the middle of the block diagram above you’ll see hardware graphics accelerators, and video codecs:
- 3D via Vivante GC2000 GPU
- 2D via Vivante GC320 GPU
- Vector Graphics (OpenVG 1.1) via Vivante GC355 GPU
- 1080p30 Enc/Dec via a Video Processing Unit (VPU)
Freescale SoC is using one GPU for 3D, two separate GPUs for 2D composition and vector graphics, and a VPU to handle video by hardware. That means Vivante GC2000 has nothing to do with video hardware decoding for example.
Let’s give another short example. AllWinner A20 features a Mali-400 (MP2) GPU with 3D graphics and OpenVG support, a separate 2D engine, and CedarX VPU for hardware video processing. So please, don’t come to ask me if it is possible to use Mali-400 hardware video decoder in Linux. 🙂
Where it gets a little confusing, is that some of the GPU capabilities can be used to decode video codecs that are not supported by the Video Processing Unit. For example, the Raspberry Pi guys used some features of the VideoCore IV GPU, but not the hardware codecs, to implemented VP6, VP8, MJPEG decoding in standard resolution. More recent GPUs comes with Renderscript and OpenCL support, which allows 1080p HEVC (H.265) video decoding using the CPU and GPU. That’s called GPU compute, and although it works, it won’t be as power efficient as video hardware decoding in the VPU.