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

[Laura Abbott]

On 7/24/19 2:59 AM, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 10:04:06PM -0700, John Stultz wrote:
> > Apologies, I'm not sure I'm understanding your suggestion here.
> > dma_alloc_contiguous() does have some interesting optimizations
> > (avoiding allocating single page from cma), though its focus on
> > default area vs specific device area doesn't quite match up the usage
> > model for dma heaps.  Instead of allocating memory for a single
> > device, we want to be able to allow userland, for a given usage mode,
> > to be able to allocate a dmabuf from a specific heap of memory which
> > will satisfy the usage mode for that buffer pipeline (across multiple
> > devices).
> >
> > Now, indeed, the system and cma heaps in this patchset are a bit
> > simple/trivial (though useful with my devices that require contiguous
> > buffers for the display driver), but more complex ION heaps have
> > traditionally stayed out of tree in vendor code, in many cases making
> > incompatible tweaks to the ION core dmabuf exporter logic.
>
> So what would the more complicated heaps be?
>
> > That's why
> > dmabuf heaps is trying to destage ION in a way that allows heaps to
> > implement their exporter logic themselves, so we can start pulling
> > those more complicated heaps out of their vendor hidey-holes and get
> > some proper upstream review.
> >
> > But I suspect I just am confused as to what your suggesting. Maybe
> > could you expand a bit? Apologies for being a bit dense.
>
> My suggestion is to merge the system and CMA heaps.  CMA (at least
> the system-wide CMA area) is really just an optimization to get
> large contigous regions more easily.  We should make use of it as
> transparent as possible, just like we do in the DMA code.

It's not just an optimization for Ion though. Ion was designed to
let the callers choose between system and multiple CMA heaps. On other
systems there may be multiple CMA regions dedicated to a specific
purpose or placed at a specific address. The callers need to
be able to choose exactly whether they want a particular CMA region
or discontiguous regions.

Thanks,
Laura