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SubjectRe: [PATCH 1/7] arm-nommu: remove the partial DMA_ATTR_NON_CONSISTENT support
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On 6/14/19 3:44 PM, Christoph Hellwig wrote:
> The arm-nommu DMA code supports DMA_ATTR_NON_CONSISTENT allocations, but
> does not provide a cache_sync operation. This means any user of it
> will never be able to actually transfer cache ownership and thus cause
> coherency bugs.

By the way, Documentation/DMA-attributes.txt doesn't specify cache_sync() as
requirement for DMA_ATTR_NON_CONSISTENT it only states that it is responsibility
of the driver to have all the correct and necessary sync points.

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> arch/arm/mm/dma-mapping-nommu.c | 24 +++---------------------
> 1 file changed, 3 insertions(+), 21 deletions(-)
>
> diff --git a/arch/arm/mm/dma-mapping-nommu.c b/arch/arm/mm/dma-mapping-nommu.c
> index f304b10e23a4..bc003df45546 100644
> --- a/arch/arm/mm/dma-mapping-nommu.c
> +++ b/arch/arm/mm/dma-mapping-nommu.c
> @@ -39,18 +39,7 @@ static void *arm_nommu_dma_alloc(struct device *dev, size_t size,
> unsigned long attrs)
>
> {
> - void *ret;
> -
> - /*
> - * Try generic allocator first if we are advertised that
> - * consistency is not required.
> - */
> -
> - if (attrs & DMA_ATTR_NON_CONSISTENT)
> - return dma_direct_alloc_pages(dev, size, dma_handle, gfp,
> - attrs);
> -
> - ret = dma_alloc_from_global_coherent(size, dma_handle);
> + void *ret = dma_alloc_from_global_coherent(size, dma_handle);
>
> /*
> * dma_alloc_from_global_coherent() may fail because:
> @@ -70,16 +59,9 @@ static void arm_nommu_dma_free(struct device *dev, size_t size,
> void *cpu_addr, dma_addr_t dma_addr,
> unsigned long attrs)
> {
> - if (attrs & DMA_ATTR_NON_CONSISTENT) {
> - dma_direct_free_pages(dev, size, cpu_addr, dma_addr, attrs);
> - } else {
> - int ret = dma_release_from_global_coherent(get_order(size),
> - cpu_addr);
> -
> - WARN_ON_ONCE(ret == 0);
> - }
> + int ret = dma_release_from_global_coherent(get_order(size), cpu_addr);
>
> - return;
> + WARN_ON_ONCE(ret == 0);
> }
>
> static int arm_nommu_dma_mmap(struct device *dev, struct vm_area_struct *vma,
>

FWIW:

Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>

Cheers
Vladimir

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