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Comment by pcwalton | Hacker News Reader

For the most part that's true, but simple 2D compositing is a bit of a different beast, because it can sometimes be done at scanout time, saving a blit. Last I checked, (non-Android) Linux rarely makes use of this except for the mouse cursor. But in general you can save a good bit of energy and memory bandwidth on HiDPI displays if you try to use 2D hardware layers where you can. You can virtually never use them for the UI itself, because they're far too limited, but the windowing system can often use them to composite windows together. It'd be nice if Wayland compositors made more use of this, e.g. to avoid having to blit the foreground window every frame.

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I used to work on mobile graphics and the android HWC stack.

The scanout-time hardware was often less useful that you might think - only in dynamic scenes where the GPU is otherwise idle (like playing video possibly with a static UI overlay was the premier use case).

For static scenes it's more efficient to render out to a buffer (using the GPU as the scanout overlay pipes often had limited feedback capability) and just output that using overlays disabled. It didn't take many frames for that to be worth it.

For apps that were animating or otherwise updating it's window, most UI toolkits used the GPU for widget rendering. And often the scanout pipes didn't hook into the (relatively large) system caches like the GPU did, so there were times it was again faster to composite the screen on the GPU to a single scanout buffer than flush already cached data, the get the scanout hardware to read it back from the memory bus.

And there weren't as cheap as people thought - one stat I remember was that the total area of the GPU on the omap4 platform was smaller than the display pipes. Though that is now a pretty old chip, and always had a bit of focus on "multimedia".

I think your information is quite outdated. The HWC overlay planes are heavily used, you can see this trivially just doing a 'dumpsys SurfaceFlinger' or grabbing a systrace/perfetto trace. When it falls back to GPU composition it's very obvious as there's a significant hit to latency and more GPU contention.

The overlay capabilities of the modern Snapdragons are also quite absurd. They support like upwards of a dozen overlays now and even have FP16 extended sRGB support. Some HWCs (like the one in the steam deck) even have per plane 3D LUTs for HDR tone mapping (ex https://github.com/ValveSoftware/gamescope/blob/master/src/d... )

The composition is bandwidth heavy of course, but for static scenes there's a cache after the HWC in the form of panel self refresh.

CRTC planes and scanout-time compositing makes sense, and Wayland compositors do use them, even for non-cursor surfaces. It's simply not something an application can use general-purpose and guarantee (though see the recent GtkSurfaceOffload stuff for the latest attempt at it).

Personally, I don't see it as a "2D drawing API", it doesn't accelerate anything special about 2D, only blits and transforms, which a 3D API will eat for breakfast.

what happened with these VESA "2d accelerated" API that was on every SVGA card in the middle of the 90s ? They make a huge difference and was well supported on Windows and X11

That stuff has been obsolete for quite a while as the general 3D capabilities are more than enough to saturate all the GPU's memory bandwidth.

If it's a VESA standard and still supported it might be useful as a fallback for hardware that doesn't have its own driver.

Edit: But actually, I couldn't find references to anything similar besides VBE/AF which even when current got almost no support directly in hardware, so folks had to resort to hardware-specific DOS TSR's. I'm not sure if there's anything newer than that.

GPU manufacturers stopped putting 2D functionality in their chips.

The basic display interface used by UEFI and low level boot loaders these days it's called GOP - Graphics Output Protocol. It replaced VESA.

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