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I want to draw simple (2D bitmapped) graphics onto my screen (in response to (simple) external inputs) with the lowest latency possible (the order of tens of milliseconds) so I can empirically test the results of drawing to the screen a) in realtime, b) with the least overhead possible, c) with page flipping completely disabled (tearing is fine).

Then I can compare this with drawing to the screen in various stereotypical (and arguably pathological :) ) scenarios (eg X11, Wayland, Wayland+XWayland, Wayland+XWayland+xcompmgr, etc).

To this end, how might I modify Linux so that I can draw over the top of my existing X11/Wayland session? Worded differently, yes, I want to fiddle with DRM, a) from inside the kernel, b) while X11 owns the DRM master. :)

I suspect that a hardware overlay would be the easiest (and most hardware-accelerated!) way to keep X11/Wayland from scribbling over what I'm drawing. (I'm imagining alternatives involving implementing a shadow/write-through framebuffer cache, to avoid read-back... no thanks!)

I really, really want to confine my madness to a kernel module, and I don't want to have to fiddle with my graphics driver. :S (I'm incidentally using Intel graphics, albeit on an i915)

So, reading through https://dri.freedesktop.org/docs/drm/, I get the idea that maybe I want to try and hack together something that lets me create a dumb framebuffer, create a plane looking at that dumb framebuffer, and then double-maybe set up some kind of DMA-BUF... no, wait, if I've got a plane pointing at the framebuffer, it's already on the screen... I think?

My main question is, how do I play fast and loose with DRM such that it basically tells X it's the only thing talking to the screen, while behind the scenes I'm managing an extra plane?

Answers very greatly appreciated!


My current understanding of Wayland is that it is fundamentally based around compositing, and by design cannot function without buffering one or more entire video frames before releasing it/them to the video card.

While X11 does not have this restriction - COMPOSITE is an optional extension - it uses a stream-based drawing protocol, and I am not aware of any method to draw directly into a window. The closest thing I know of is the use of the MIT-SHM extension, and while this does allow the use of shared memory, that involves at least two memory copies (me->kernel, kernel->X11), and then the poking of an XShmPutImage down the X11 pipe to tell X11 to please flip. This means that, even if I were to make my process run in realtime... well, I'm too chicken to run X in realtime as well, so I'd still have to wait for X to be scheduled, decode and reach my request in its command queue, and finally flip.

Hence my throwing my hands up in the air and trying to see if I can just shove my graphics drawing code straight into the kernel, and hopefully make it all coexist somehow.

I can imagine all of this additional overhead really adds up, and I want to quantify exactly how - or, alternatively, concretely establish that my mental models are completely incorrect and that the speed of contemporary hardware makes these concerns moot.

I am extremely curious to see what the impact would be if I eliminated all the bottlenecks, and also what the difference would be like comparing older systems versus more modern hardware.

Incidentally, this question is related to another question I asked over here and over here, both of which sadly still have no answers, and which I am no further forward on.

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