linux-kernel - Re: [PATCH v3 3/3] swiotlb: Honour dma_alloc_coherent() alignment in swiotlb

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Message-ID: <20240219134141.381dea51@meshulam.tesarici.cz>
Date: Mon, 19 Feb 2024 13:41:41 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
 iommu@...ts.linux.dev, Christoph Hellwig <hch@....de>, Marek Szyprowski
 <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, Petr
 Tesarik <petr.tesarik1@...wei-partners.com>, Dexuan Cui
 <decui@...rosoft.com>
Subject: Re: [PATCH v3 3/3] swiotlb: Honour dma_alloc_coherent() alignment
 in swiotlb_alloc()

On Mon,  5 Feb 2024 19:01:27 +0000
Will Deacon <will@...nel.org> wrote:

> core-api/dma-api-howto.rst states the following properties of
> dma_alloc_coherent():
> 
>   | The CPU virtual address and the DMA address are both guaranteed to
>   | be aligned to the smallest PAGE_SIZE order which is greater than or
>   | equal to the requested size.
> 
> However, swiotlb_alloc() passes zero for the 'alloc_align_mask'
> parameter of swiotlb_find_slots() and so this property is not upheld.
> Instead, allocations larger than a page are aligned to PAGE_SIZE,
> 
> Calculate the mask corresponding to the page order suitable for holding
> the allocation and pass that to swiotlb_find_slots().
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Fixes: e81e99bacc9f ("swiotlb: Support aligned swiotlb buffers")
> Signed-off-by: Will Deacon <will@...nel.org>

Reviewed-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>

Petr T

> ---
>  kernel/dma/swiotlb.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index adbb3143238b..283eea33dd22 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1633,12 +1633,14 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>  	struct io_tlb_pool *pool;
>  	phys_addr_t tlb_addr;
> +	unsigned int align;
>  	int index;
>  
>  	if (!mem)
>  		return NULL;
>  
> -	index = swiotlb_find_slots(dev, 0, size, 0, &pool);
> +	align = (1 << (get_order(size) + PAGE_SHIFT)) - 1;
> +	index = swiotlb_find_slots(dev, 0, size, align, &pool);
>  	if (index == -1)
>  		return NULL;
>