Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps

From: Benjamin Gaignard
Date: Thu Jul 25 2019 - 14:07:25 EST

Le jeu. 25 juil. 2019 Ã 16:33, Christoph Hellwig <hch@xxxxxxxxxxxxx> a Ãcrit :
> On Thu, Jul 25, 2019 at 03:20:11PM +0200, Benjamin Gaignard wrote:
> > > But that just means we need a flag that memory needs to be contiguous,
> > > which totally makes sense at the API level. But CMA is not the only
> > > source of contigous memory, so we should not conflate the two.
> >
> > We have one file descriptor per heap to be able to add access control
> > on each heap.
> > That wasn't possible with ION because the heap was selected given the
> > flags in ioctl
> > structure and we can't do access control based on that. If we put flag
> > to select the
> > allocation mechanism (system, CMA, other) in ioctl we come back to ION status.
> > For me one allocation mechanism = one heap.
> Well, I agree with your split for different fundamentally different
> allocators. But the point is that CMA (at least the system cma area)
> fundamentally isn't a different allocator. The per-device CMA area
> still are kinda the same, but you can just have one fd for each
> per-device CMA area to make your life simple.

Form my point of view default cma area is a specific allocator because
it could migrate page to give them in priority to contigous allocation. It is
also one of the last region where system page are going to be allocated.
I don't think that system allocator does have the same features, either we
would have use it rather develop CMA years ago ;-). In short CMA had
solved lot of problems on our side so it had to have it own heap.