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In general a rather interesting idea
and actually shouldn't be to hard to implement.
The crux is that allocating memory is device driver dependent, so there isn't a general purpose API for doing so. Additional to that the CPU invisible VRAM is only accessible by the GPU so you at least need to be able to submit DMA commands to copy the data back and forth. For an experiment I suggest you do this with Radeon first and if that works generalize from there. Regards, Christian. On 21.09.2015 13:33, Steven Newbury wrote: I have a mostly* headless server containing a Radeon discrete GPU. It occured to me that having a GiB or two of high speed memory sitting unused is pretty wasteful. Not an original thought; indeed there's a Gentoo wiki which describes how to map the memory as a mtd device: http://www.gentoo-wiki.info/TIP_Use_memory_on_video_card_as_swap There's also a driver (cudaram) written by Piotr Jaroszyński which allocates memory through CUDA and maps it to a block device: http://blog.piotrj.org/2011/03/cudaram-block-device-exposing-nvidia.htm l The Gentoo method is extremely limited, and of course isn't exactly safe, although using pci-stub would probably help. There's no arbitration between the DRM driver and the mtd phram driver. Most of all, it can only expose the memory mapped into the PCI aperture, likely somewhat less than the full VMEM. The second method obviously requires the proprietary NVidia driver and an NVidia gfx card, but it's good proof of concept at utilising a driver managed buffer object as a block device. I'm looking into creating a volatile block device driver which allocates VMEM from DRM, what if any is the right API to use? The DRM userspace API is well documented, but I'd like to make this a kernel module, so using a userspace API would seem inappropriate at best. Ideally I'd like to create something like the bcache flash_vol_create interface except expose a /sys/fs/drm-block/card0/vol_create (where drm-block is a placeholder for the proposed driver name) for each DRM device in the system, and likewise persistence through udev. So is there an API I can (mis-)use for this? Any suggestions? * There's a screen attached for maintainence, but it usually turned off. |
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