[RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

[FUJITA Tomonori]

On Wed, 21 Jul 2010 21:30:34 -0700
Zach Pfeffer <zpfeffer at codeaurora.org> wrote:

> On Wed, Jul 21, 2010 at 10:44:37AM +0900, FUJITA Tomonori wrote:
> > On Tue, 20 Jul 2010 15:20:01 -0700
> > Zach Pfeffer <zpfeffer at codeaurora.org> wrote:
> > 
> > > > I'm not saying that it's reasonable to pass (or even allocate) a 1MB
> > > > buffer via the DMA API.
> > > 
> > > But given a bunch of large chunks of memory, is there any API that can
> > > manage them (asked this on the other thread as well)?
> > 
> > What is the problem about mapping a 1MB buffer with the DMA API?
> > 
> > Possibly, an IOMMU can't find space for 1MB but it's not the problem
> > of the DMA API.
> 
> This goes to the nub of the issue. We need a lot of 1 MB physically
> contiguous chunks. The system is going to fragment and we'll never get
> our 12 1 MB chunks that we'll need, since the DMA API allocator uses
> the system pool it will never succeed. For this reason we reserve a
> pool of 1 MB chunks (and 16 MB, 64 KB etc...) to satisfy our
> requests. This same use case is seen on most embedded "media" engines
> that are getting built today.

We don't need a new abstraction to reserve some memory.

If you want pre-allocated memory pool per device (and share them with
some), the DMA API can for coherent memory (see
dma_alloc_from_coherent). You can extend the DMA API if necessary.