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Comment by ta3401 | Hacker News Reader

Thank you. So, if I get this right, the cursor position, which is what the video card needs to position the mouse pointer picture on the screen as an overlay to the actual framebuffer, isn't updated asynchronously to the screen update (ie. whenever the mouse is moved), but instead each time a frame is being rendered, and thus the pointer is only moved at these times, which may avoid tearing (though I don't see why) and other nasty effects, yet introduces a small rendering lag.

I don't know however if the mouse pointer picture is still handled the VESA way, or if GPUs video cards nowadays have a more generic API, or what.

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There really isn't such a thing as "the actual framebuffer". Instead the display hardware can do composition during scanout from a set of buffers at a set of positions with varying capabilities. These buffers then just being arbitrary dmabufs.

It doesn't give a damn if you give it 2 buffers and one contains a mouse cursor and the other everything else or if you give it 2 buffers and one is everything including the mouse and the other is a video, allowing complete power collapse of the GPU rendering units.

Often they support more than 2 of these as well, and with color conversions, 1D & 3D LUTs, and a handful of other useful properties. Mobile SoCs in particular, like your typical mid/high end snapdragon, actually have upwards of a dozen overlay planes. This is how Android manages to almost never hit GPU composition at all.

On desktop linux all of these go through the drm/kms APIs.

Well, GPUs from the big players do give a damn as they tend to have surprisingly limited plane count and capabilities. It is often just a single primary, cursor and overlay plane, sometimes the latter is shared across all outputs, and sometimes what the plane can do depends on what the plane it overlaps with is doing.

Mobile chips are as you mention far ahead in this space, with some having outright arbitrary plane counts.

Although desktop GPU plane counts are much more limited, it's not as restricted as you're portraying. Here's the AMD SoC in the Steam Deck, for example: https://github.com/ValveSoftware/gamescope/blob/master/src/d...

Even though it's only 3 planes, they are relatively feature-rich still. In a typical desktop UI that would indeed be primary, cursor, and video planes. But if the system cursor is hidden, such as in a game, that frees up a plane that can be used for something else - such as the aforementioned game.

What you are showing is just a standard color pipeline, which is the bare minimum for color management.

On AMD in particular, the cursor plane must match several aspects of any plane it overlaps with, including transform and color pipeline IIRC.

The AMD SoC in my laptop (much newer than the steam deck) only exposes two overlay planes to share among all 4 display controllers. Intel used to have a single overlay plane per display.

The Raspberry Pi 5 on the other hand intentionally limited the exposed overlay planes to "just" 48, as it can be as many as you have memory for.

You can peek at the reported capabilities of various devices here: https://drmdb.emersion.fr/

> which may avoid tearing (though I don't see why)

What I meant here is that I didn't see why asynchronous updates may introduce tearing; but my tired brain couldn't quite formulate it properly. And to answer that, it's clear to me know that an update of the pointer position while the pointer sprite is being drawn would introduce a shift somewhere within that sprite, which is, I suppose, the tearing discussed (and not a whole frame tearing).

> I don't know however if the mouse pointer picture is still handled the VESA way, or if GPUs video cards nowadays have a more generic API, or what.

Also, the VESA interface doesn't seem to handle mouse pointers, it's something that was available in the VGA BIOS, to provide a uniform support for this feature, as each vendor most likely did it their own way.

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