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:

quoted

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.

quoted

+	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

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