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

[Laurent Pinchart]

Hi Jiri,

On Monday 28 February 2011 16:07:43 Jiri Slaby wrote:
> 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.

What I mean is that videobuf-dma-contig is meant to be used by drivers that 
require physically contiguous memory. If the system has an IOMMU, why would 
drivers need that ?

> 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@xxxxxxx>
> >> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxx>
> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >> ---
> >> 
> >>  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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