The DRM (direct rendering manager, not the content protection stuff) graphics subsystem in the linux kernel does not have a generic 2D accelaration API. Despite an awful lot of of GPUs having more or less featureful blitter units. And many systems need them for a lot of use-cases, because the 3D engine is a bit too slow or too power hungry for just rendering desktops.
It’s a FAQ why this doesn’t exist and why it won’t get added, so I figured I’ll answer this once and for all.
Bit of nomeclatura upfront: A 2D engine (or blitter) is a bit of hardware that can copy stuff with some knowledge of the 2D layout usually used for pixel buffers. Some blitters also can do more like basic blending, converting color spaces or stretching/scaling. A 3D engine on the other hand is the fancy bit of high performance compute block, which run small programs (called shaders) on a massively parallel archicture. Generally with huge memory bandwidth and a dedicated controller to feed this beast through an asynchronous command buffer. 3D engines happen to be really good at rendering the pixels for 3D action games, among other things.
There’s no 2D Acceleration Standard
3D has it easy: There’s OpenGL and Vulkan and DirectX that require a certain feature set. And huge market forces that make sure if you use these features like a game would, rendering is fast.
Aside: This means the 2D engine in a browser actually needs to work like a 3D action game, or the GPU will crawl. The impendence mismatch compared to traditional 2D rendering designs is huge.
On the 2D side there’s no such thing: Every blitter engine is its own bespoke thing, with its own features, limitations and performance characteristics. There’s also no standard benchmarks that would drive common performance characteristics - today blitters are neeeded mostly in small systems, with very specific use cases. Anything big enough to run more generic workloads will have a 3D rendering block anyway. These systems still have blitters, but mostly just to help move data in and out of VRAM for the 3D engine to consume.
Now the huge problem here is that you need to fill these gaps in various hardware 2D engines using CPU side software rendering. The crux with any 2D render design is that transferring buffers and data too often between the GPU and CPU will kill performance. Usually the cliff is so steep that pure CPU rendering using only software easily beats any simplistic 2D acceleration design.
The only way to fix this is to be really careful when moving data between the CPU and GPU for different rendering operations. Sticking to one side, even if it’s a bit slower, tends to be an overall win. But these decisions highly depend upon the exact features and performance characteristics of your 2D engine. Putting a generic abstraction layer in the middle of this stack, where it’s guaranteed to be if you make it a part of the kernel/userspace interface, will not result in actual accelaration.
So either you make your 2D rendering look like it’s a 3D game, using 3D interfaces like OpenGL or Vulkan. Or you need a software stack that’s bespoke to your use-case and the specific hardware you want to run on.
2D Accelaration is Really Hard
This is the primary reason really. If you don’t believe that, look at all the tricks a browser employs to render CSS and HTML and text really fast, while still animating all that stuff smoothly. Yes, a web-browser is the pinnacle of current 2D acceleration tech, and you really need all the things in there for decent performance: Scene graphs, clever render culling, massive batching and huge amounts of pains to make sure you don’t have to fall back to CPU based software rendering at the wrong point in a rendering pipeline. Plus managing all kinds of assorted caches to balance reuse against running out of memory.
Unfortunately lots of people assume 2D must be a lot simpler than 3D rendering, and therefore they can design a 2D API that’s fast enough for everyone. No one jumps in and suggests we’ll have a generic 3D interface at the kernel level, because the lessons there are very clear:
The real application interface is fairly high level, and in userspace.
There’s a huge industry group doing really hard work to specify these interfaces.
The actual kernel to userspace interfaces ends up being highly specific to the hardware and architecture of the userspace driver (which contains most of the magic). Any attempt at a generic interface leaves lots of hardware specific tricks and hence performance on the floor.
3D APIs like OpenGL or Vulkan have all the batching and queueing and memory management issues covered in one way or another.
There are a bunch of DRM drivers which have a support for 2D render engines exposed to userspace. But they all use highly hardware specific interfaces, fully streamlined for the specific engine. And they all require a decently sized chunk of driver code in userspace to translate from a generic API to the hardware formats. This is what DRM maintainers will recommend you to do, if you submit a patch to add a generic 2D acceleration API.
Exactly like a 3D driver.
If All Else Fails, There’s Options
Now if you don’t care about the last bit of performance, and your use-case is limited, and your blitter engine is limited, then there’s already options:
You can take whatever pixel buffer you have, export it as a dma-buf, and then import it into some other subsystem which already has some kind of limited 2D accelaration support. Depending upon your blitter engine, a v4l2 mem2m device, or for simpler things there’s also dmaengines.
On top, the DRM subsystem does allow you to implement the traditional accelaration methods exposed by the fbdev subsystem. In case you have userspace that really insists on using these; it’s not recommended for anything new.
What about KMS?
The above is kinda a lie, since the KMS (kernel modesetting) IOCTL userspace API is a fairly full-featured 2D rendering interface. The aim of course is to render different pixel buffers onto a screen. With the recently added writeback support operations targetting memory are now possible. This could be used to expose a traditional blitter, if you only expose writeback support and no other outputs in your KMS driver.
There’s a few downsides:
KMS is highly geared for compositing just a few buffers (hardware usually has a very limited set of planes). For accelerated text rendering you want to do a composite operation for each character, which means this has rather limited use.
KMS only needs to run at 60Hz, or whatever the refresh rate of your monitor is. It’s not optimized for efficiency at higher throughput at all.
So all together this isn’t the high-speed 2D accelaration API you’re looking for either. It is a valid alternative to the options above though, e.g. instead of a v4l2 mem2m device.
FAQ for the FAQ, or: OpenVG?
OpenVG isn’t the standard you’re looking for either. For one it’s a userspace API, like OpenGL. All the same reasons for not implementing a generic OpenGL interface at the kernel/userspace apply to OpenVG, too.
Second, the Mesa3D userspace library did support OpenVG once. Didn’t gain traction, got canned. Just because it calls itself a standard doesn’t make it a widely adopted industry default. Unlike OpenGL/Vulkan/DirectX on the 3D side.
Thanks to Dave Airlie and Daniel Stone for reading and commenting on drafts of this text.