Re: Vramfs: filesystem driver to utilize extra RAM on VGA devices

From: Jonathan Campbell
Date: Wed Jan 28 2009 - 01:37:24 EST

Well my expectation of vramfs is that it's not meant to be used for the heavy-duty 3D gaming-style rendering that GEM is built to handle. It's meant for lighter tasks, like simple 2D/3D compositing or GPU work where you know what your resources need, you don't need to take that much, and you need direct mmap() access because some of it involves video that you will handle later. Situations like this can work perfectly fine without the use of a swapping system.

My other concern is that GEM with a filesystem might be the best option for 3D gaming, but that it wouldn't work if the driver doesn't know the card. The DRI drivers, as far as I know, are tied to the GPU and chipset of the device (because they have to manage it, after all!). How exactly would GEM work for cards that it doesn't recognize, like one machine of mine with a weird ATI chipset nobody knows how to talk to? If GEM doesn't recognize it, it won't provide VRAM resources to use it, right?

This is where vramfs has it's advantage: it's not the absolute best solution for 3D graphics, but it's simple, it can serve as a starting point for GPU experiments from userspace, or if nothing else allows the use of the onboard video RAM on an otherwise unused and unrecognized video device. It's device-agnostic by design.
The way you want do that is using OpenGL to put your data in textures
and framebuffer objects, and render them. With KMS, we'll be able to
support EGL even on the console so you can do the work without having an
X environment set up.

The problem with vramfs as a basis for GPU offload is that most GPU
tasks end up at some point exceeding the size of available
aperture/VRAM. So you need code that manages loading buffer objects in
and out on demand, managing the execution pipeline and GPU and CPU
caches as required. We have that with GEM already.

Remember, writing data to an aperture isn't the hard part of offloading
to the GPU, programming the GPU is. That's why you use OpenGL or
another abstraction to do it.

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