usage of dma_common_mmap

[Malte Vesper <malte.vesper@xxxxxxxxxxxxxxxxxxxxxxxxx>]

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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