ARM: DMA coherent allocator: align remapped addresses

The DMA coherent remap area is used to provide an uncached mapping
of memory for coherency with DMA engines.  Currently, we look for
any free hole which our allocation will fit in with page alignment.

However, this can lead to fragmentation of the area, and allows small
allocations to cross L1 entry boundaries.  This is undesirable as we
want to move towards allocating sections of memory.

Align allocations according to the size, limiting the alignment between
the page and section sizes.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 14 insertions, 1 deletions

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 9e7742f0a102..c704eed63c5d 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -183,6 +183,8 @@ static void *
 __dma_alloc_remap(struct page *page, size_t size, gfp_t gfp, pgprot_t prot)
 {
         struct arm_vmregion *c;
+        size_t align;
+        int bit;
 
         if (!consistent_pte[0]) {
                 printk(KERN_ERR "%s: not initialised\n", __func__);
@@ -191,9 +193,20 @@ __dma_alloc_remap(struct page *page, size_t size, gfp_t gfp, pgprot_t prot)
         }
 
         /*
+         * Align the virtual region allocation - maximum alignment is
+         * a section size, minimum is a page size.  This helps reduce
+         * fragmentation of the DMA space, and also prevents allocations
+         * smaller than a section from crossing a section boundary.
+         */
+        bit = fls(size - 1) + 1;
+        if (bit > SECTION_SHIFT)
+                bit = SECTION_SHIFT;
+        align = 1 << bit;
+
+        /*
          * Allocate a virtual address in the consistent mapping region.
          */
-        c = arm_vmregion_alloc(&consistent_head, size,
+        c = arm_vmregion_alloc(&consistent_head, align, size,
                             gfp & ~(__GFP_DMA | __GFP_HIGHMEM));
         if (c) {
                 pte_t *pte;