linux-kernel - Re: [RFC v2 2/2] base: dma-mapping: Postpone page_to_pfn() on mmap()

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Date:   Mon, 9 Apr 2018 09:25:47 +0200
From:   jacopo mondi <jacopo@...ndi.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        ysato@...rs.sourceforge.jp, dalias@...c.org,
        laurent.pinchart@...asonboard.com, geert@...ux-m68k.org,
        linux-renesas-soc@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org
Subject: Re: [RFC v2 2/2] base: dma-mapping: Postpone page_to_pfn() on mmap()

Hi Robin, Laurent,
    a long time passed, sorry about this.

On Wed, Nov 15, 2017 at 01:38:23PM +0000, Robin Murphy wrote:
> On 14/11/17 17:08, Jacopo Mondi wrote:
> >On SH4 architecture, with SPARSEMEM memory model, translating page to
> >pfn hangs the CPU. Post-pone translation to pfn after
> >dma_mmap_from_dev_coherent() function call as it succeeds and make page
> >translation not necessary.
> >
> >This patch was suggested by Laurent Pinchart and he's working to submit
> >a proper fix mainline. Not sending for inclusion at the moment.
>
> Y'know, I think this patch does have some merit by itself - until we know
> that cpu_addr *doesn't* represent some device-private memory which is not
> guaranteed to be backed by a struct page, calling virt_to_page() on it is
> arguably semantically incorrect, even if it might happen to be benign in
> most cases.

I still need to carry this patch in my trees to have a working dma memory mapping
on SH4 platforms. My understanding from your comment is that there may
be a way forward for this patch, do you still think the same? Have you
got any suggestion on how to improve this eventually for inclusion?

Thanks
   j

>
> Robin.
>
> >Suggested-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> >Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> >---
> >  drivers/base/dma-mapping.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
> >index e584edd..73d64d3 100644
> >--- a/drivers/base/dma-mapping.c
> >+++ b/drivers/base/dma-mapping.c
> >@@ -227,8 +227,8 @@ 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;
> >+	unsigned long pfn;
> >
> >  	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> >
> >@@ -236,6 +236,7 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma,
> >  		return ret;
> >
> >  	if (off < count && user_count <= (count - off)) {
> >+		pfn = page_to_pfn(virt_to_page(cpu_addr));
> >  		ret = remap_pfn_range(vma, vma->vm_start,
> >  				      pfn + off,
> >  				      user_count << PAGE_SHIFT,
> >--
> >2.7.4
> >
> >_______________________________________________
> >iommu mailing list
> >iommu@...ts.linux-foundation.org
> >https://lists.linuxfoundation.org/mailman/listinfo/iommu
> >

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