linux-kernel - Re: [PATCH v2] base: dma-mapping: Postpone cpu addr translation on mmap()

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Date:   Fri, 13 Apr 2018 18:43:29 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        laurent.pinchart@...asonboard.com, hch@...radead.org
Cc:     ysato@...rs.sourceforge.jp, dalias@...c.org,
        iommu@...ts.linux-foundation.org, linux-sh@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] base: dma-mapping: Postpone cpu addr translation on
 mmap()

On 13/04/18 18:25, Jacopo Mondi wrote:
> Postpone calling virt_to_page() translation on memory locations not
> guaranteed to be backed by a struct page. Try first to map memory from
> device's coherent memory pool, then perform translation if that fails.
> 
> On some architectures, specifically SH when configured with SPARSEMEM
> memory model, assuming a struct page is always assigned to a memory
> address lead to unexpected hangs during the virtual to page address
> translation. This patch fixes that specific issue but applies in the
> general case too.

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

> Suggested-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> 
> ---
> 
> It has now been clarified this patch does not resolve the issue, but only
> mitigate it on platforms where dma_mmap_from_dev_coherent() succeeds and
> delay page_to_pfn() faulty conversion.
> 
> A suggested proper solution would be not relying on dma_common_mmap() but
> require all platforms to implement an mmap methods known to work, as noted
> by Christoph in v1 review.

Note that that "proper solution" should still involve having 
dma_common_mmap() since we certainly don't want an explosion of code 
duplication. It just means that architectures that do use it should be 
defining their dma_map_ops with an explicit ".mmap = dma_common_mmap" 
instead of relying on dma_mmap_attrs() calling it by default. Thus the 
more architectures this implementation *is* definitely safe for, the 
better :)

Robin.

> v1 -> v2:
> - Save the 'pfn' temp variable performing the page_to_pfn() conversion in the
>    remap_pfn_range() function call as suggested by Christoph.
> 
> ---
>   drivers/base/dma-mapping.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
> index 3b11835..d82566d 100644
> --- a/drivers/base/dma-mapping.c
> +++ b/drivers/base/dma-mapping.c
> @@ -226,7 +226,6 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
>   #ifndef CONFIG_ARCH_NO_COHERENT_DMA_MMAP
>   	unsigned long user_count = vma_pages(vma);
>   	unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> -	unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
>   	unsigned long off = vma->vm_pgoff;
> 
>   	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> @@ -234,12 +233,11 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
>   	if (dma_mmap_from_dev_coherent(dev, vma, cpu_addr, size, &ret))
>   		return ret;
> 
> -	if (off < count && user_count <= (count - off)) {
> +	if (off < count && user_count <= (count - off))
>   		ret = remap_pfn_range(vma, vma->vm_start,
> -				      pfn + off,
> +				      page_to_pfn(virt_to_page(cpu_addr)) + off,
>   				      user_count << PAGE_SHIFT,
>   				      vma->vm_page_prot);
> -	}
>   #endif	/* !CONFIG_ARCH_NO_COHERENT_DMA_MMAP */
> 
>   	return ret;
> --
> 2.7.4
>