[RFC] Experimental DMA-BUF Device Heaps

[Ezequiel Garcia]

On Mon, 2020-08-17 at 20:49 -0700, James Jones wrote:
> On 8/17/20 8:18 AM, Brian Starkey wrote:
> > Hi Ezequiel,
> > 
> > On Sun, Aug 16, 2020 at 02:22:46PM -0300, Ezequiel Garcia wrote:
> > > This heap is basically a wrapper around DMA-API dma_alloc_attrs,
> > > which will allocate memory suitable for the given device.
> > > 
> > > The implementation is mostly a port of the Contiguous Videobuf2
> > > memory allocator (see videobuf2/videobuf2-dma-contig.c)
> > > over to the DMA-BUF Heap interface.
> > > 
> > > The intention of this allocator is to provide applications
> > > with a more system-agnostic API: the only thing the application
> > > needs to know is which device to get the buffer for.
> > > 
> > > Whether the buffer is backed by CMA, IOMMU or a DMA Pool
> > > is unknown to the application.
> > > 
> > > I'm not really expecting this patch to be correct or even
> > > a good idea, but just submitting it to start a discussion on DMA-BUF
> > > heap discovery and negotiation.
> > > 
> > 
> > My initial reaction is that I thought dmabuf heaps are meant for use
> > to allocate buffers for sharing across devices, which doesn't fit very
> > well with having per-device heaps.
> > 
> > For single-device allocations, would using the buffer allocation
> > functionality of that device's native API be better in most
> > cases? (Some other possibly relevant discussion at [1])
> > 
> > I can see that this can save some boilerplate for devices that want
> > to expose private chunks of memory, but might it also lead to 100
> > aliases for the system's generic coherent memory pool?
> > 
> > I wonder if a set of helpers to allow devices to expose whatever they
> > want with minimal effort would be better.
> 
> I'm rather interested on where this goes, as I was toying with using 
> some sort of heap ID as a basis for a "device-local" constraint in the 
> memory constraints proposals Simon and I will be discussing at XDC this 
> year.  It would be rather elegant if there was one type of heap ID used 
> universally throughout the kernel that could provide a unique handle for 
> the shared system memory heap(s), as well as accelerator-local heaps on 
> fancy NICs, GPUs, NN accelerators, capture devices, etc. so apps could 
> negotiate a location among themselves.  This patch seems to be a step 
> towards that in a way, but I agree it would be counterproductive if a 
> bunch of devices that were using the same underlying system memory ended 
> up each getting their own heap ID just because they used some SW 
> framework that worked that way.
> 
> Would appreciate it if you could send along a pointer to your BoF if it 
> happens!
> 

Here is it:

https://linuxplumbersconf.org/event/7/contributions/818/

It would be great to see you there and discuss this,
given I was hoping we could talk about how to meet a
userspace allocator library expectations as well.

Thanks,
Ezequiel

> Thanks,
> -James
> 
> > Cheers,
> > -Brian
> > 
> > 1. https://lore.kernel.org/dri-devel/57062477-30e7-a3de-6723-a50d03a402c4@kapsi.fi/
> > 
> > > Given Plumbers is just a couple weeks from now, I've submitted
> > > a BoF proposal to discuss this, as perhaps it would make
> > > sense to discuss this live?
> > > 
> > > Not-signed-off-by: Ezequiel Garcia <ezequiel at collabora.com>