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 16:07:43 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
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

On 02/28/2011 11:53 AM, Laurent Pinchart wrote:
> 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.

Which the DMA allocation shall be.

>  * Note: Drivers should NOT use these. 

This is weird.

> They are the wrong
>  * translation for translating DMA addresses.  Use the driver
>  * DMA support - see dma-mapping.h.

Yes, ACK, and vice versa. DMA addresses cannot be used as physical ones.

>  */
> 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 ?

Sorry, what? When IOMMU is used, dma_alloc_* functions may return "tags"
as a DMA address, not a physical address. So using these DMA "addresses"
directly (e.g. in remap_pfn_range) is a bug.

Maybe there is a better way to convert a kernel address of the DMA
mapping into a physical frame, but I'm not aware of any...

>> 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,
-- 
js
suse labs
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