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

From: Jiri Slaby
Date: Mon Feb 28 2011 - 10:45:56 EST

On 02/28/2011 04:14 PM, Laurent Pinchart wrote:
> 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 ?

Aha. They actually need not but they would need do the mapping
themselves which they currently do not.

IOW the vbuf-dma-contig allocator is used unconditionally in the few
drivers I checked.

BUT Even if they need only one page and use vbuf-dma-contig, which I
don't see a reason not to, it will cause problems too.

suse labs
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