non cached memory for user space DMA

I am trying to write a MAC driver for Arm 11 MPcore in 2.6.23. The DMA'ble memory required is supposed to be non-cached for performance reasons.

A chunk of memory(~ 6MB) is allocated at startup using bigphys area patch. This physical area in the RAM is mmap'd to userspace as follows and thereafter carved into network buffers:

fd = open("/dev/mem", O_RDWR|O_SYNC);
uptr = mmap(0, sz, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_LOCKED,fd, pptr);

Inspite of using the O_SYNC flag the user space mapping is cached and the DMA'd data is not visible. I tried writing up my own mmap and marked the vma non-cached without any results:

size_t size = vma->vm_end - vma->vm_start;
unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
if (remap_pfn_range(vma, vma->vm_start, offset, size, vma->vm_page_prot)) {
return -EAGAIN;
}

I found that the bigphys patch allocated memory from bootmem and bootmem maps the memory as MT_MEMORY. Is there a way I could reduce bootmem allocation at startup (using kernel boot cmd mem=x ?) ?

The idea is to map the rest of the available memory as MT_DEVICE(assuming it removes the BUFFERABLE & CACHEABLE bits while mapping) so that the kernel mapping is also non-cacheable. Hopefully, this will ease the userspace mapping to non-cached as well ?

thanks,
Deb Rupam Banerjee

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