RE: [v2 4/4] ARM: dma-mapping: IOMMU allocates pages from atomic_pool with GFP_ATOMIC

Marek Szyprowski
2012-08-23T07:33:28Z

Hi Hiroshi,

On Thursday, August 23, 2012 8:10 AM Hiroshi Doyu wrote:

> Makes use of the same atomic pool from DMA, and skips kernel page
> mapping which can involve sleep'able operations at allocating a kernel
> page table.
>
> Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
> ---
> arch/arm/mm/dma-mapping.c | 30 +++++++++++++++++++++++++-----
> 1 files changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 7ab016b..433312a 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1063,7 +1063,6 @@ static struct page **__iommu_alloc_buffer(struct device *dev, size_t
> size,
> struct page **pages;
> int count = size >> PAGE_SHIFT;
> int array_size = count * sizeof(struct page *);
> - int err;
>
> if ((array_size <= PAGE_SIZE) || (gfp & GFP_ATOMIC))
> pages = kzalloc(array_size, gfp);
> @@ -1072,9 +1071,20 @@ static struct page **__iommu_alloc_buffer(struct device *dev, size_t
> size,
> if (!pages)
> return NULL;
>
> - err = __alloc_fill_pages(&pages, count, gfp);
> - if (err)
> - goto error
> + if (gfp & GFP_ATOMIC) {
> + struct page *page;
> + int i;
> + void *addr = __alloc_from_pool(size, &page);
> + if (!addr)
> + goto error;
> +
> + for (i = 0; i < count; i++)
> + pages[i] = page + i;
> + } else {
> + int err = __alloc_fill_pages(&pages, count, gfp);
> + if (err)
> + goto error;
> + }
>
> return pages;
>
> @@ -1091,9 +1101,15 @@ static int __iommu_free_buffer(struct device *dev, struct page **pages,
> size_t s
> int count = size >> PAGE_SHIFT;
> int array_size = count * sizeof(struct page *);
> int i;
> +
> + if (__free_from_pool(page_address(pages[0]), size))
> + goto out;
> +
> for (i = 0; i < count; i++)
> if (pages[i])
> __free_pages(pages[i], 0);
> +
> +out:
> if ((array_size <= PAGE_SIZE) ||
> __in_atomic_pool(page_address(pages[0]), size))
> kfree(pages);
> @@ -1221,6 +1237,9 @@ static void *arm_iommu_alloc_attrs(struct device *dev, size_t size,
> if (*handle == DMA_ERROR_CODE)
> goto err_buffer;
>
> + if (gfp & GFP_ATOMIC)
> + return page_address(pages[0]);
> +
> if (dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
> return pages;
>

I've read the whole code and it looks that it starts to become a little spaghetti - there are too
many places altered by those atomic changes and it is hard to follow the logic at the first sight.

IMHO it will be better to add a following function:

static void *__iommu_alloc_atomic(struct device *dev, size_t size,
dma_addr_t *handle)
{
struct page *page, **pages;
int count = size >> PAGE_SHIFT;
int array_size = count * sizeof(struct page *);
void *addr;
int i;

pages = kzalloc(array_size, gfp);
if (!pages)
return NULL;

addr = __alloc_from_pool(size, &page);
if (!addr)
goto err_pool;

for (i = 0; i < count; i++)
pages[i] = page + i;

*handle = __iommu_create_mapping(dev, pages, size);
if (*handle == DMA_ERROR_CODE)
goto err_mapping;

return addr;

err_mapping:
__free_from_pool(addr, size);
err_pool:
kfree(pages);
return NULL;
}

and then call it at the beginning of the arm_iommu_alloc_attrs():

if (gfp & GFP_ATOMIC)
return __iommu_alloc_atomic(dev, size, handle);

You should also add support for the allocation from atomic_pool to __iommu_get_pages() function
to get dma_mmap() and dma_get_sgtable() working correctly.

> @@ -1279,7 +1298,8 @@ void arm_iommu_free_attrs(struct device *dev, size_t size, void
> *cpu_addr,
> return;
> }
>
> - if (!dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs)) {
> + if (!dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs) ||
> + !__in_atomic_pool(cpu_addr, size)) {
> unmap_kernel_range((unsigned long)cpu_addr, size);
> vunmap(cpu_addr);
> }

Are you sure this one works? __iommu_get_pages() won't find pages array, because atomic
allocations don't get their separate vm_struct area, so this check will be never reached. Maybe
a __iommu_free_atomic() function would also make it safer and easier to understand.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center