Insanity Has A Name

Karol Herbst. At SGC, we know this man. We fear him. His photo is on the wall over a break-in-case-of-emergency glass panel which shields a button activating a subterranean escape route set to implode as soon as I sprint through.

Despite this, and despite all past evidence leading me to be wary of any idea he pitched, the madman got me again.

cl_khr_image2d_from_buffer. On the surface, an innocuous little extension used to access a buffer like a 2D image. Vulkan already has this support for 1D images in the form of VkBufferView, so why would adding a stride to that be any harder (aside from the fact that the API doesn’t support it)?

I was deep into otherworldly optimizations at this point, far beyond the point where I was able to differentiate between improvement and neutral, let alone sane or twisted. His words seemed so reasonable: why couldn’t I just throw a buffer to the GPU as a 2D image? I’d have to be an idiot not to be able to do something as simple as that. Wouldn’t I?

Dammit, Karol.

How to 2D a Buffer

You can’t. I mean, I can, but you? Vulkan won’t let you do it. There’s (currently) no extension that enables a 2D bufferview. Rumor has it some madman on a typewriter is preparing to fax over an extension specification to add this, but only time will tell whether Khronos accepts submissions in this format.

Here at SGC, we’re all smart HUMANS though, so there’s an obvious solution to this.

It’s not memory aliasing. Sure, rebinding buffer memory onto an image might work. But in reading the spec, the synchronization guarantees for buffer-image aliasing didn’t seem that strong. And also it’d be a whole bunch of code to track it, and maybe do weird layout stuff, and add some kind of synchronization on the buffer too, and pray the driver isn’t buggy, and doesn’t this sound a lot like the we-have-this-at-home version of another, better mechanism that zink already has incredible support for?

Yeah. What about these things? How do they wORK?

DMA Buffers: Totally Normal

A DMAbuf is basically a pipe. On one end you have memory. And if you yell TRIANGLE into the other end really loud, something unimaginable and ineffable that lurks deep withinthevoid will slitherand crawl its way up the pipeuntil it GAZES UPON YOU IN YOUR FLESHY MORTAL SHELL ATTEMPTING TO USURP THE POWERS OF THE OLD ONES. It’s a fun little experiment with absolutely no unwanted consequences. Try it at home!

The nice thing about dmabufs is I know they work. And I know they work in zink. That’s because in order to run an x̸̧̠͓̣̣͎͚̰͎̍̾s̶̡̢͙̞̙͍̬̝̠̩̱̞̮̩̣̑͂͊̎͆̒̓͐͛͊̒͆̄̋ȩ̶̡̨̳̭̲̹̲͎̪̜͒̓̈́̏r̶̩̗͖͙͖̬̟̞̜̠͙̠̎͑̉̌̎̍̑́̏̓̏̒̍͜͝v̶̞̠̰̘̞͖̙̯̩̯̝̂̃̕͜e̴̢̡͎̮͔̤͖̤͙̟̳̹͛̓͌̈̆̈́̽͘̕ŕ̶̫̾͐͘ or a Wayland compositor (e.g., Ŵ̶̢͍̜̙̺͈͉̼̩̯̺̗̰̰͕͍̱͊͊̓̈̀͛̾̒̂̚̕͝ͅḙ̵̛̬̜͔̲͕͖̜̱̻͊̌̾͊͘s̶̢̗̜͈̘͎̠̘̺͉͕̣̯̘̦͓͈̹̻͙̬̘̿͆̏̃̐̍̂̕ͅt̷̨͈̠͕͔̬̙̣͈̪͕̱͕̙̦͕̼̩͙̲͖͉̪̹̼͛̌͋̃̂̂̓̏̂́̔͠͝ͅơ̸̢̛̛̲̟͙͚̰͇̞̖̭̲͍͇̫̘̦̤̩̖͍̄̓́͑̉̿̅̀̉͒͋͒̂́̆̋̚͝ͅͅn̶̢̡̝̥̤̣͔̣͉͖̖̻̬̝̥̦͇͕̘͋͂͛̌̃͠ͅͅ, the reference compositor), dmabufs have to work. Zink can run both of those just fine, so I know there’s absolutely zero bugs. There can’t be any bugs. No. Not bugs again. NO MORE BUGS

Even better, I know that I can do imports and exports of dmabufs in any dimensionality thanks to that crazy CL-GL sharing extension Karol already suckered me into supporting at the expense of every Vulkan driver’s bug tracker. That KAROL HERBST guy, hah, he’s such a kidder!

So obviously–It’s just common sense at this point–Obviously I should just be able to hook up the pipes here. Export a buffer and then import a 2D image with whatever random CAUSALITY IS A LIE passes for stride. Right? Basically a day at the beach for me.

And of course it works perfectly with no problems whatsoever, giving Davinci Resolve a nice performance boost.

Stay sane, readers.