usage of dma_common_mmap

[Malte Vesper]

Hello,

I am trying to use dma_mmap_attrs . However I keep getting the following 
error:

    mmap:6420 map pfn RAM range req uncached-minus for [mem
    0xfd0000000-0xfd0000fff], got write-back


Assuming that dma_common_map is used 
(http://lxr.free-electrons.com/source/drivers/base/dma-mapping.c#L246) I 
tracked the error source to be comming from: remap_pfn_range
After struding the codeflow I think that this macro is the culprit:

12 #define pgprot_noncached(prot)                                          \
  13         ((boot_cpu_data.x86 > 
3)                                        \
  14          ? (__pgprot(pgprot_val(prot) 
|                                 \
  15 cachemode2protval(_PAGE_CACHE_MODE_UC_MINUS)))     \
  16          : (prot))

since the "got write-back" is calculated from the memory address I 
assume I can not change that bit.
Could anyone explain to me, why the page protection depends on 
boot_cpu_data?

Googling turned up further hints 
http://comments.gmane.org/gmane.linux.kernel.mm/125059. Is this a Bug or 
am I missing something?
How do I correctly use dma_mmap_attrs? Currently I try:


mmap function:

    return dma_mmap_attrs(&pcidev->dev, vma, buffer->memoryAddress,
    buffer->dmaAddress, BUF_SIZE, 0);


testfile:

         char* fpath = "/proc/myDevice";
         uint64_t* buffer;
         int filepointer = open(fpath, O_RDWR);

         if(filepointer < 0) {
             printf("Could not open %s.\n", fpath);
             return -1;
         }

         buffer = (uint64_t*) mmap(0, 4096, PROT_READ | PROT_WRITE,
    MAP_PRIVATE, filepointer, 0);

Thanks for any help,
Malte
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