[RFC] Experimental DMA-BUF Device Heaps

[Ezequiel Garcia]

Hi Brian,

Thanks a lot for reviewing this patch.

I'm happy my humble proof-of-concept
is able to spark some interest and raise
some questions.

On Mon, 2020-08-17 at 16:18 +0100, 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.
> 

That's true.

> 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])
> 

That may be true for existing subsystems.

However, how about a new subsystem/API wanting to
leverage the heap API and avoid exposing yet another
allocator API?

And then, also, if we have an allocator API, perhaps
we could imagine a future where applications would only
need to worry about that, and not about each per-framework
allocator.

> 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?
> 

The idea (even if PoC) was to let drivers decide if they are special
enough to add themselves (via dev_dma_heap_add).

Aliasing the heap would be ceratainly too much (although trying
it for every device was a fun experiment to watch). 

Thanks,
Ezequiel

> I wonder if a set of helpers to allow devices to expose whatever they
> want with minimal effort would be better.
> 
> 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>