Re: [PATCH v4 2/3] common: DMA-mapping: add DMA_ATTR_NOHUGEPAGE attribute

From: Tomasz Figa
Date: Sat Jan 09 2016 - 11:24:43 EST

On Sat, Jan 9, 2016 at 4:55 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.


> 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

> 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.


Best regards,