linux-kernel - Re: [PATCH v4 2/3] common: DMA-mapping: add DMA_ATTR

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Date:	Fri, 8 Jan 2016 15:05:23 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Russell King <linux@....linux.org.uk>,
	Robin Murphy <robin.murphy@....com>,
	Tomasz Figa <tfiga@...omium.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Pawel Osciak <pawel@...iak.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/3] common: DMA-mapping: add DMA_ATTR_NOHUGEPAGE attribute

Hi,

On Fri, Jan 8, 2016 at 5:42 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Thu, Jan 07, 2016 at 04:36:44PM -0800, Douglas Anderson wrote:
>> This patch adds the DMA_ATTR_NOHUGEPAGE attribute to the DMA-mapping
>> subsystem.
>>
>> This attribute can be used as a hint to the DMA-mapping subsystem that
>> it's likely not worth it to try to allocation large pages behind the
>> scenes.  Large pages are likely to make an IOMMU TLB work more
>> efficiently but may not be worth it.  See the Documentation contained in
>> this patch for more details about this attribute and when to use it.
>>
>> Note that the name of the hint (DMA_ATTR_NOHUGEPAGE) is based on the
>> name MADV_NOHUGEPAGE, which has the same meaning.  If we have expected
>> users, we could also add MADV_HUGEPAGE which has the opposite meaning of
>> this hint.
>
> A user of this features seems to be missing in the series.  Please don't
> add any clutter with unclear usage to the kernel unless there is a real
> need which can be deonstrated in form of patches and numbers.

In my cover letter I tried to address this.  See
<http://comments.gmane.org/gmane.linux.kernel/2121364>.  I said:

> Notably missing from this series is the fourth patch that adds teeth to
> the second and third.  You can find that out of tree at
> <https://chromium-review.googlesource.com/#/c/320498/>.  Unfortunately
> the rk3288_vpu, which is what I'm working on, is out of tree.

...but today I realized that I also needed to do work to get Exynos's
MFC codec using this patch and MFC _is_ upstream.  ...so my next
series will include the MFC patch.  Hopefully that will help address
your concerns.