static void dma_cache_maint_page(struct page *page, unsigned long offset,
	size_t size, enum dma_data_direction dir,
	void (*op)(const void *, size_t, int))
{
	unsigned long pfn;
	size_t left = size;
	pfn = page_to_pfn(page) + offset / PAGE_SIZE;
	offset %= PAGE_SIZE;
	/*
	 * A single sg entry may refer to multiple physically contiguous
	 * pages.  But we still need to process highmem pages individually.
	 * If highmem is not configured then the bulk of this loop gets
	 * optimized out.
	 */
	do {
		size_t len = left;
		void *vaddr;
		page = pfn_to_page(pfn);
		if (PageHighMem(page)) {
			if (len + offset > PAGE_SIZE)
				len = PAGE_SIZE - offset;
			vaddr = kmap_high_get(page);
			if (vaddr) {
				vaddr += offset;
				op(vaddr, len, dir);
				kunmap_high(page);
			} else if (cache_is_vipt()) {
				/* unmapped pages might still be cached */
				vaddr = kmap_atomic(page);
				op(vaddr + offset, len, dir);
				kunmap_atomic(vaddr);
			}
		} else {
			vaddr = page_address(page) + offset;
			op(vaddr, len, dir);
		}
		offset = 0;
		pfn++;
		left -= len;
	} while (left);
}