uncachable access to physical pages(ARM, xScale)

[Nickolay <nickolay@protei.ru>]

Hello Guys!

I want to change pgprot(unset L_PTE_CACHEABLE and L_PTE_BUFFERABLE) for
several pages, allocated by alloc_pages, in kernel VM, because this memory
used for sharing data between kernelspace and userland(via .mmap).
For userland VM, i done that successul next way:
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
remap_pfn_range(...);

but for kernelspace VM i has some trouble.

I was trying directly unset L_PTE_CACHEABLE | L_PTE_BUFFERABLE flags from
pte(see below), but it doesn't working, and i has problem with cache 
cogerency
between userland and kernelspace:

pgd_t *pgd;
pmd_t *pmd;
pte_t *pte;

/* find pte */
addr = page_address(my_page);
pgd = pgd_offset_k(addr & PAGE_MASK);
pmd = pmd_offset(pgd, addr & PAGE_MASK);
pte = pte_offset_kernel(pmd, addr & PAGE_MASK);

/* ok, we get pte, now unset L_PTE_CACHEABLE and
    L_PTE_BUFFERABLE flags
*/
pte_val(*pte) &= ~(L_PTE_CACHEABLE|L_PTE_BUFFERABLE);

The problem descriprion simple, kernel at some time doesn't see
data, which userland writes to shared buffer, without inserting
flush_cache_all after each operation with shared buffer from kernel.

thanks,

Nickolay.