Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc_pages

Will Deacon
2016-03-29T17:02:19Z

On Mon, Mar 28, 2016 at 02:32:11PM +0800, Yong Wu wrote:
> Currently __iommu_dma_alloc_pages assumes that all the IOMMU support
> the granule of PAGE_SIZE. It call alloc_page to try allocating memory
> in the last time. Fortunately the mininum pagesize in all the
> current IOMMU is SZ_4K, so this works well.
>
> But there may be a case in which the mininum granule of IOMMU may be
> larger than PAGE_SIZE, then it will abort as the IOMMU cann't map the
> discontinuous memory within a granule. For example, the pgsize_bitmap
> of the IOMMU only has SZ_16K while the PAGE_SIZE is SZ_4K, then we
> have to prepare SZ_16K continuous memory at least for each a granule
> iommu mapping.

Did you find a driver/platform that actually needs this? I certainly
think it's possible, but we don't necessarily need to fix it if it's
not broken yet! Just adding a comment would be enough.

Either way, a couple of review comments inline.

>
> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
>
> ---
> v2:
> -rebase on v4.6-rc1.
> -add a new patch([1/2] add pgsize_bitmap) here.
>
> drivers/iommu/dma-iommu.c | 22 +++++++++++++---------
> 1 file changed, 13 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 72d6182..75ce71e 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -190,11 +190,13 @@ static void __iommu_dma_free_pages(struct page **pages, int count)
> kvfree(pages);
> }
>
> -static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
> +static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp,
> + unsigned long pgsize_bitmap)
> {
> struct page **pages;
> unsigned int i = 0, array_size = count * sizeof(*pages);
> - unsigned int order = MAX_ORDER;
> + int min_order = get_order(1 << __ffs(pgsize_bitmap));

1UL

> + int order = MAX_ORDER;
>
> if (array_size <= PAGE_SIZE)
> pages = kzalloc(array_size, GFP_KERNEL);
> @@ -213,13 +215,16 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
> /*
> * Higher-order allocations are a convenience rather
> * than a necessity, hence using __GFP_NORETRY until
> - * falling back to single-page allocations.
> + * falling back to min size allocations.
> */
> - for (order = min_t(unsigned int, order, __fls(count));
> - order > 0; order--) {
> - page = alloc_pages(gfp | __GFP_NORETRY, order);
> + for (order = min_t(int, order, __fls(count));
> + order >= min_order; order--) {
> + page = alloc_pages((order == min_order) ? gfp :
> + gfp | __GFP_NORETRY, order);
> if (!page)
> continue;
> + if (!order)
> + break;

Isn't this handled by the loop condition?

Will