linux-kernel - Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc

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Date:   Mon, 27 May 2019 03:55:29 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     hch@....de, robin.murphy@....com, m.szyprowski@...sung.com,
        vdumpa@...dia.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, chris@...kel.net, jcmvbkbc@...il.com,
        joro@...tes.org, dwmw2@...radead.org, tony@...mide.com,
        akpm@...ux-foundation.org, sfr@...b.auug.org.au,
        treding@...dia.com, keescook@...omium.org, iamjoonsoo.kim@....com,
        wsa+renesas@...g-engineering.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, iommu@...ts.linux-foundation.org,
        dann.frazier@...onical.com
Subject: Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for
 single pages

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