The display controller and render device are completely distinct logical devices, even though they are often grouped in a "GPU". On mobile architectures they are quite far separated, leading to annoying problems surrounding what we on Linux call "split drm devices".

Updating plane properties such as to move the cursor plane around or disable it would by itself not block on render activities, as they are completely distinct blocks.

The render hardware could be powered down, but I doubt powering it up and compositing the cursor would take long enough to complete to cause any noticable lag.

Under the Linux APIs, updates to the display controller are done through KMS atomic commits, and one mistake you could do display-server side would be to provide a fence in this atomic commit that the scheduler will use to wait on long-running GPU work before using the provided graphics buffers. Under this API, none of the changes - including mouse movements - would then be applied until that fence is signalled. Changing plane associations can lead to resource reallocations that can be a bit heavy.

Not sure if the kernel driver in macOS works anything remotely similar to this, and the driver could also just be dumb and block on unrelated things ("let's just wait another vblank to see this apply....", "as we only need one plane now let's power down hardware and wait for that to settle..."). It could also just be windowserver that waits for work to finish on its own, not providing any cursor updates in the meantime.

The reality is that it will take reverse engineering or looking at actual code to know what's going on.

But wouldn't the software cursor operations also go in the queue? I don't see the problem.

This was a really informative and interesting reply articulated in simple enough terms that I am now interested in GPUs, thanks

This is plausible to me as well. A couple years ago we were trying to make dynamic memory allocation in Vello more robust and explored using async readback of a status buffer. In that case, the async task doesn't wake until the command buffer completes and signals a fence back to the CPU.

Long story short, performance was disappointing and we abandoned the approach. It's easy to believe it's a real problem especially when there are other factors including GPU being clocked down to save power.

Same caveat as parent, I have no direct knowledge of MacBook Neo or this specific issue.

how do hardware cursors work in a composited desktop?

the cursor could just be another small rectangle texture you position on top of the other surfaces. there is no need to read the framebuffer/write into it, its just a z-stack of 3d surfaces now

>A software cursor is manually drawn by the graphics stack, which saves what was under it, draws the cursor, and then whenever it needs to be moved, writes the original data back, saves the data at the new position, and then draws the cursor there.

If a hardware layer is not being used the cursor layer will be treated like any other layer in the compositor. Modern compositors don't try and save and write pixels like that. It will just rerender it.

>(which is normally done per mouse interrupt);

It's normally done every frame the compositor makes.

>or it may end up drawing over what software wants to draw

The compositor composites everything at that will be shown on the next refresh of the display. Things don't indepently step on each others toes since it's just the compositor rendering and synchronizing all hardware layers (planes).

Terrible fix but it's a fix that's minimally-invasive and addresses a bug that causes a disproportionate annoyance to the fix. I can imagine your cursor lagging is something that is extremely annoying over time.

You could just put it in your calendar. 'Check if that hacky bugfix at ~/.dirtyhack.sh is still required with chmod -x ~/.dirtyhack.sh'

Relevant quote from https://www.folklore.org/Shut_Up.html:

We showed [Gates] how the Macintosh mouse cursor moved smoothly, in a flicker-free fashion.

"What kind of hardware do you use to draw the cursor?", he asked. Many current personal computers had special hardware to draw small bitmaps called "sprites", and he thought we might be doing something similar.

  % Overlay plane compatibility hack for cg4 frame buffer.
  % This is a nebulous layer abstracted from a messy program, to make it run 
  % on generic NeWS servers. It should be rethought and rewritten. Repent!
  % 
  % Requires the devices /dev/cgfour0, /dev/cgfour0_ovl, and /dev/cgfour0_ove
  % (which can all be major 39 minor 0, or whatever), and the following patch 
  % to the NeWS 1.1 server sources (but X11/NeWS doesn't need to be patched!),
  % in order to take advantage of a cg4 under NeWS 1.1 (Otherwise it falls back
  % to using exclusive-or). 

  % Damn damn damn! X11/NeWS Version 1.0 FCS on a cg4 can open up the
  % enable plane, but there's a bug that trashes the enable plane color map,
  % so we can draw in gray scales but we can't draw in white (black?).

[flagged]

No

guard is an inverted if statement, with the additional requirement that the branch must exit the parent scope. It's useful sometimes for readability, particularly for avoiding the "pyramid of doom" when you have a lot of preconditions that need to be checked:

    if fooOK 
        if barOK {
            if bazOK {
                // do something
            }
        }
    }

can be written as:

    guard fooOK else { return }
    guard barOK else { return }
    guard bazOK else { return }
    // do something

Obviously there are other options (like writing a negated if), but sometimes guard is more readable. It's a style thing.

The more important use case for guard is that 'guard let' statements can pattern-match and introduce bindings that are valid for the rest of the scope:

    guard let foo = someOptional else { return }
    print(foo);

This is useful enough that Rust copied it in the form of 'let ... else {}' statements (but did not bring over boolean guard statements).

guard has two advantages: the compiler ensures that you exit the current scope if the condition does not hold (via return, break, continue, etc), and bindings established in the guard clause (e.g. let foo = optionalBar) remaining in scope after the guard block, rather than inside it like an if block.

That was like 20 years ago. My computer insists on downloading gigabytes of videos at random (correction: specifically when I use the hotspot) because maybe I have a video screensaver enabled.

I had to set up a daemon to kill the downloader every 10 seconds.

What's the big deal?

It's just a simple one-liner

curl github.com/trustworthyguy/fix_everything | sh

And it all works. Not like recompiling entire kernel to play an mp3.

/s obviously