csky: Fixup dma_alloc_coherent with PAGE_SO attribute

This bug is from commit: 2b070ccd (fixup abiv2 mmap(... O_SYNC)
failed). In that patch we remove the _PAGE_SO for memory noncache
mapping and this will cause problem when drivers use dma descriptors
to control the transcations without dma_w/rmb().

After referencing other archs' implementation, pgprot_writecombine is
introduced for mmap(... O_SYNC).
Signed-off-by: Guo Ren <ren_guo@c-sky.com>

arch/csky/include/asm/pgtable.h

@@ -258,6 +258,16 @@ static inline pgprot_t pgprot_noncached(pgprot_t _prot)
 {
 	unsigned long prot = pgprot_val(_prot);
 
+	prot = (prot & ~_CACHE_MASK) | _CACHE_UNCACHED | _PAGE_SO;
+
+	return __pgprot(prot);
+}
+
+#define pgprot_writecombine pgprot_writecombine
+static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
+{
+	unsigned long prot = pgprot_val(_prot);
+
 	prot = (prot & ~_CACHE_MASK) | _CACHE_UNCACHED;
 
 	return __pgprot(prot);

arch/csky/mm/ioremap.c

@@ -29,8 +29,7 @@ void __iomem *ioremap(phys_addr_t addr, size_t size)
 
 	vaddr = (unsigned long)area->addr;
 
-	prot = __pgprot(_PAGE_PRESENT | __READABLE | __WRITEABLE |
-			_PAGE_GLOBAL | _CACHE_UNCACHED | _PAGE_SO);
+	prot = pgprot_noncached(PAGE_KERNEL);
 
 	if (ioremap_page_range(vaddr, vaddr + size, addr, prot)) {
 		free_vm_area(area);
@@ -51,10 +50,9 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 			      unsigned long size, pgprot_t vma_prot)
 {
 	if (!pfn_valid(pfn)) {
-		vma_prot.pgprot |= _PAGE_SO;
 		return pgprot_noncached(vma_prot);
 	} else if (file->f_flags & O_SYNC) {
-		return pgprot_noncached(vma_prot);
+		return pgprot_writecombine(vma_prot);
 	}
 
 	return vma_prot;