Re: [Linaro-mm-sig] [RFC 0/4] dma-buf Constraints-Enabled Allocation helpers

[Laura Abbott <lauraa@codeaurora.org>]

On 10/10/2014 1:07 PM, Sumit Semwal wrote:
> Hi,
>
> Why:
> ====
>   While sharing buffers using dma-buf, currently there's no mechanism to let
> devices share their memory access constraints with each other to allow for
> delayed allocation of backing storage.
>
> This RFC attempts to introduce the idea of memory constraints of a device,
> and how these constraints can be shared and used to help allocate buffers that
> can satisfy requirements of all devices attached to a particular dma-buf.
>
> How:
> ====
>   A constraints_mask is added to dma_parms of the device, and at the time of
> each device attachment to a dma-buf, the dma-buf uses this constraints_mask
> to calculate the access_mask for the dma-buf.
>
> Allocators can be defined for each of these constraints_masks, and then helper
> functions can be used to allocate the backing storage from the matching
> allocator satisfying the constraints of all devices interested.
>
> A new miscdevice, /dev/cenalloc [1] is created, which acts as the dma-buf
> exporter to make this transparent to the devices.
>
> More details in the patch description of "cenalloc: Constraint-Enabled
> Allocation helpers for dma-buf".
>
>
> At present, the constraint_mask is only a bitmask, but it should be possible to
> change it to a struct and adapt the constraint_mask calculation accordingly,
> based on discussion.
>
>
> Important requirement:
> ======================
>   Of course, delayed allocation can only work if all participating devices
> will wait for other devices to have 'attached' before mapping the buffer
> for the first time.
>
> As of now, users of dma-buf(drm prime, v4l2 etc) call the attach() and then
> map_attachment() almost immediately after it. This would need to be changed if
> they were to benefit from constraints.
>
>
> What 'cenalloc' is not:
> =======================
> - not 'general' allocator helpers - useful only for constraints-enabled
>    devices that share buffers with others using dma-buf.
> - not a replacement for existing allocation mechanisms inside various
>    subsystems; merely a possible alternative.
> - no page-migration - it would be very complementary to the delayed allocation
>     suggested here.
>
> TODOs:
> ======
> - demonstration test cases
> - vma helpers for allocators
> - more sample allocators
> - userspace ioctl (It should be a simple one, and we have one ready, but wanted
>     to agree on the kernel side of things first)
>
>

I'm interested to see the userspace ioctl. The mask based approach of
Ion does not scale well to a userspace ABI so I'm curious if cenalloc
does better.

Thanks,
Laura

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