block device backed by DRM buffer object

[Christian König]

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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