Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for single pages

[Nicolin Chen]

Hi Ira,

On Fri, May 24, 2019 at 09:16:19AM -0700, Ira Weiny wrote:
> On Thu, May 23, 2019 at 09:06:33PM -0700, Nicolin Chen wrote:
> > The addresses within a single page are always contiguous, so it's
> > not so necessary to always allocate one single page from CMA area.
> > Since the CMA area has a limited predefined size of space, it may
> > run out of space in heavy use cases, where there might be quite a
> > lot CMA pages being allocated for single pages.
> > 
> > However, there is also a concern that a device might care where a
> > page comes from -- it might expect the page from CMA area and act
> > differently if the page doesn't.
> 
> How does a device know, after this call, if a CMA area was used?  From the
> patches I figured a device should not care.

A device doesn't know. But that doesn't mean a device won't care
at all. There was a concern from Robin and Christoph, as a corner
case that device might act differently if the memory isn't in its
own CMA region. That's why we let it still use its device specific
CMA area.

> > +	if (dev && dev->cma_area)
> > +		cma = dev->cma_area;
> > +	else if (count > 1)
> > +		cma = dma_contiguous_default_area;
> 
> Doesn't dev_get_dma_area() already do this?

Partially yes. But unwrapping it makes the program flow clear in
my opinion. Actually I should have mentioned that this patch was
suggested by Christoph also.

Otherwise, it would need an override like:
	cma = dev_get_dma_area();
	if (count > 1 && cma == dma_contiguous_default_area)
		cma = NULL;

Which doesn't look that bad though..

Thanks
Nicolin