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

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Date:	Sun, 10 Jan 2016 01:24:16 +0900
From:	Tomasz Figa <tfiga@...omium.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Doug Anderson <dianders@...omium.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Robin Murphy <robin.murphy@....com>,
	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

On Sat, Jan 9, 2016 at 4:55 PM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, Jan 08, 2016 at 03:31:29PM -0800, Doug Anderson wrote:
>> Ah, that makes so much more sense now!  :)  So you were suggesting
>> something like DMA_ATTR_SMALL_PAGES_OK.  Then you if we wanted all
>> possible states you'd have 0 vs. DMA_ATTR_SMALL_PAGES_OK vs.
>> DMA_ATTR_HUGE_PAGE?  That would avoid the double-negative but does
>> have the downside that it's less obvious that DMA_ATTR_SMALL_PAGES_OK
>> is the opposite of DMA_ATTR_HUGE_PAGE.
>
> or DMA_ATTR_4K_PAGES if that's what you want.

Or maybe just DMA_ATTR_USE_SMALL_PAGES?

> We have at least 4k, 8k,
> 16k and 64k page support in the kernel, not sure if 32k and 256k ever
> made it mainline.  What does your hardware actually require?

The hardware in question doesn't require any specific page size - the
IOMMU can map anything in granularity of 4KiB, which I believe
corresponds to the small page size on its platform (4KiB, ARM). Also
basically this attribute would translate into alloc_pages() with zero
order, so I guess that means small pages. (or we could perhaps make it
DMA_ATTR_ALLOC_SINGLE_PAGES...)

>
> This needs to be documented properly and hpefully also reflected in the
> name of the fag.  Otherwise we'll end up with a giant desaster like
> GFP_DMA that means something slightly different on every architecture.

Agreed.

Best regards,
Tomasz