linux-kernel - Re: [PATCH v2 -resend#1 1/1] V4L: videobuf, don't use dma addr as physical

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Date:	Mon, 28 Feb 2011 11:53:30 +0100
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	mchehab@...radead.org, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, jirislaby@...il.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 -resend#1 1/1] V4L: videobuf, don't use dma addr as physical

Hi Jiri,

On Monday 28 February 2011 10:37:02 Jiri Slaby wrote:
> mem->dma_handle is a dma address obtained by dma_alloc_coherent which
> needn't be a physical address in presence of IOMMU. So ensure we are
> remapping (remap_pfn_range) the right page in __videobuf_mmap_mapper
> by using virt_to_phys(mem->vaddr) and not mem->dma_handle.

Quoting arch/arm/include/asm/memory.h,

/*
 * These are *only* valid on the kernel direct mapped RAM memory.
 * Note: Drivers should NOT use these.  They are the wrong
 * translation for translating DMA addresses.  Use the driver
 * DMA support - see dma-mapping.h.
 */
static inline unsigned long virt_to_phys(const volatile void *x)
{
        return __virt_to_phys((unsigned long)(x));
}

Why would you use physically contiguous memory if you have an IOMMU anyway ?

> While at it, use PFN_DOWN instead of explicit shift.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Mauro Carvalho Chehab <mchehab@...radead.org>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  drivers/media/video/videobuf-dma-contig.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/media/video/videobuf-dma-contig.c
> b/drivers/media/video/videobuf-dma-contig.c index c969111..19d3e4a 100644
> --- a/drivers/media/video/videobuf-dma-contig.c
> +++ b/drivers/media/video/videobuf-dma-contig.c
> @@ -300,7 +300,7 @@ static int __videobuf_mmap_mapper(struct videobuf_queue
> *q,
> 
>  	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  	retval = remap_pfn_range(vma, vma->vm_start,
> -				 mem->dma_handle >> PAGE_SHIFT,
> +				 PFN_DOWN(virt_to_phys(mem->vaddr))
>  				 size, vma->vm_page_prot);
>  	if (retval) {
>  		dev_err(q->dev, "mmap: remap failed with error %d. ", retval);

-- 
Regards,

Laurent Pinchart
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