Re: [PATCH v6 2/5] common: DMA-mapping: add DMA_ATTR_ALLOC_SINGLE_PAGES attribute

[Robin Murphy <robin.murphy@arm.com>]

On 11/01/16 17:30, Douglas Anderson wrote:
> This patch adds the DMA_ATTR_ALLOC_SINGLE_PAGES 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 allocate 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_ALLOC_SINGLE_PAGES) is loosely
> based on the name MADV_NOHUGEPAGE.  Just as there is MADV_NOHUGEPAGE
> vs. MADV_HUGEPAGE we could also add an "opposite" attribute to
> DMA_ATTR_ALLOC_SINGLE_PAGES.  Without having the "opposite" attribute
> the lack of DMA_ATTR_ALLOC_SINGLE_PAGES means "use your best judgement
> about whether to use small pages or large pages".

I did ponder the discussion on v4 over the weekend and arrived at 
DMA_ATTR_LOW_ORDER_ALLOC and a shorter, more generalised description, 
but ALLOC_SINGLE_PAGES seems just as good to me.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Thanks,
Robin.

> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
> Changes in v6:
> - renamed DMA_ATTR_NO_HUGE_PAGE to DMA_ATTR_ALLOC_SINGLE_PAGES
>
> Changes in v5:
> - renamed DMA_ATTR_NOHUGEPAGE to DMA_ATTR_NO_HUGE_PAGE
> - s/ping ping/ping pong/
>
> Changes in v4:
> - renamed DMA_ATTR_SEQUENTIAL to DMA_ATTR_NOHUGEPAGE
> - added Marek's ack
>
> Changes in v3:
> - add DMA_ATTR_SEQUENTIAL attribute new for v3
>
> Changes in v2: None
>
>   Documentation/DMA-attributes.txt | 23 +++++++++++++++++++++++
>   include/linux/dma-attrs.h        |  1 +
>   2 files changed, 24 insertions(+)
>
> diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
> index 18dc52c4f2a0..69b7b65ab516 100644
> --- a/Documentation/DMA-attributes.txt
> +++ b/Documentation/DMA-attributes.txt
> @@ -100,3 +100,26 @@ allocated by dma_alloc_attrs() function from individual pages if it can
>   be mapped as contiguous chunk into device dma address space. By
>   specifying this attribute the allocated buffer is forced to be contiguous
>   also in physical memory.
> +
> +DMA_ATTR_ALLOC_SINGLE_PAGES
> +------------------------
> +
> +This is a hint to the DMA-mapping subsystem that it's probably not worth
> +the time to try to allocate memory to in a way that gives better TLB
> +efficiency (AKA it's not worth trying to build the mapping out of larger
> +pages).  You might want to specify this if:
> +- You know that the accesses to this memory won't thrash the TLB.
> +  You might know that the accesses are likely to be sequential or
> +  that they aren't sequential but it's unlikely you'll ping-pong
> +  between many addresses that are likely to be in different physical
> +  pages.
> +- You know that the penalty of TLB misses while accessing the
> +  memory will be small enough to be inconsequential.  If you are
> +  doing a heavy operation like decryption or decompression this
> +  might be the case.
> +- You know that the DMA mapping is fairly transitory.  If you expect
> +  the mapping to have a short lifetime then it may be worth it to
> +  optimize allocation (avoid coming up with large pages) instead of
> +  getting the slight performance win of larger pages.
> +Setting this hint doesn't guarantee that you won't get huge pages, but it
> +means that we won't try quite as hard to get them.
> diff --git a/include/linux/dma-attrs.h b/include/linux/dma-attrs.h
> index 99c0be00b47c..5246239a4953 100644
> --- a/include/linux/dma-attrs.h
> +++ b/include/linux/dma-attrs.h
> @@ -18,6 +18,7 @@ enum dma_attr {
>   	DMA_ATTR_NO_KERNEL_MAPPING,
>   	DMA_ATTR_SKIP_CPU_SYNC,
>   	DMA_ATTR_FORCE_CONTIGUOUS,
> +	DMA_ATTR_ALLOC_SINGLE_PAGES,
>   	DMA_ATTR_MAX,
>   };
>
>