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From: Nicolin Chen <nicoleotsuka@gmail.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: hch@lst.de, robin.murphy@arm.com, m.szyprowski@samsung.com,
	vdumpa@nvidia.com, linux@armlinux.org.uk,
	catalin.marinas@arm.com, will.deacon@arm.com, chris@zankel.net,
	jcmvbkbc@gmail.com, joro@8bytes.org, dwmw2@infradead.org,
	tony@atomide.com, akpm@linux-foundation.org,
	sfr@canb.auug.org.au, treding@nvidia.com, keescook@chromium.org,
	iamjoonsoo.kim@lge.com, wsa+renesas@sang-engineering.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	iommu@lists.linux-foundation.org, dann.frazier@canonical.com
Subject: Re: [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for single pages
Date: Mon, 27 May 2019 03:55:29 -0700	[thread overview]
Message-ID: <20190527105528.GA26916@Asurada-Nvidia.nvidia.com> (raw)
In-Reply-To: <20190524161618.GB23100@iweiny-DESK2.sc.intel.com>

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

  reply	other threads:[~2019-05-27 10:56 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-24  4:06 [PATCH v3 0/2] Optimize dma_*_from_contiguous calls Nicolin Chen
2019-05-24  4:06 ` [PATCH v3 1/2] dma-contiguous: Abstract dma_{alloc,free}_contiguous() Nicolin Chen
2019-05-29 18:35   ` Nathan Chancellor
2019-05-29 22:48     ` Nicolin Chen
2019-07-25 16:06   ` Ezequiel Garcia
2019-07-25 16:50     ` Nicolin Chen
2019-07-25 17:31       ` Dafna Hirschfeld
2019-07-25 23:42         ` Nicolin Chen
2019-05-24  4:06 ` [PATCH v3 2/2] dma-contiguous: Use fallback alloc_pages for single pages Nicolin Chen
2019-05-24 16:16   ` Ira Weiny
2019-05-27 10:55     ` Nicolin Chen [this message]
2019-05-24 20:03 ` [PATCH v3 0/2] Optimize dma_*_from_contiguous calls dann frazier
2019-05-28  6:04 ` Christoph Hellwig
2019-05-29 23:06   ` Nicolin Chen

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