memremap: don't modify flags

These patches implement a MEMREMAP_WC flag for memremap(), which can be
used to obtain writecombine mappings.  This is then used for setting up
dma_coherent_mem regions which use the DMA_MEMORY_MAP flag.

The motivation is to fix an alignment fault on arm64, and the suggestion
to implement MEMREMAP_WC for this case was made at [1].  That particular
issue is handled in patch 4, which makes sure that the appropriate
memset function is used when zeroing allocations mapped as IO memory.

This patch (of 4):

Don't modify the flags input argument to memremap(). MEMREMAP_WB is
already a special case so we can check for it directly instead of
clearing flag bits in each mapper.
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/memremap.c

@@ -64,6 +64,9 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
 				       IORESOURCE_SYSTEM_RAM, IORES_DESC_NONE);
 	void *addr = NULL;
 
+	if (!flags)
+		return NULL;
+
 	if (is_ram == REGION_MIXED) {
 		WARN_ONCE(1, "memremap attempted on mixed range %pa size: %#lx\n",
 				&offset, (unsigned long) size);
@@ -72,7 +75,6 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
 
 	/* Try all mapping types requested until one returns non-NULL */
 	if (flags & MEMREMAP_WB) {
-		flags &= ~MEMREMAP_WB;
 		/*
 		 * MEMREMAP_WB is special in that it can be satisifed
 		 * from the direct map.  Some archs depend on the
@@ -86,21 +88,19 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
 	}
 
 	/*
-	 * If we don't have a mapping yet and more request flags are
-	 * pending then we will be attempting to establish a new virtual
+	 * If we don't have a mapping yet and other request flags are
+	 * present then we will be attempting to establish a new virtual
 	 * address mapping.  Enforce that this mapping is not aliasing
 	 * System RAM.
 	 */
-	if (!addr && is_ram == REGION_INTERSECTS && flags) {
+	if (!addr && is_ram == REGION_INTERSECTS && flags != MEMREMAP_WB) {
 		WARN_ONCE(1, "memremap attempted on ram %pa size: %#lx\n",
 				&offset, (unsigned long) size);
 		return NULL;
 	}
 
-	if (!addr && (flags & MEMREMAP_WT)) {
-		flags &= ~MEMREMAP_WT;
+	if (!addr && (flags & MEMREMAP_WT))
 		addr = ioremap_wt(offset, size);
-	}
 
 	return addr;
 }