[RFC PATCH] Avoid aliasing mappings in DMA coherent allocator

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Dec 28, 2010 at 09:29:58PM +0000, Janusz Krzysztofik wrote:
> Works fine on OMAP1 based Amstrad Delta, both default 2MB/2MB and shrinked
> to 1MB/1MB configurations. No need to preallocate dma coherent memory for
> camera video buffers on boot.

That's good news.

> However, in 1MB/1MB configuration it failed to allocate 600kB of
> writecombined omapfb vram, required for double-buffering, and reserving
> up to 4MB of machine's total 32MB seems too expensive.

Do you have other users of the writecombined memory?  If so, what size
are they?

As the 600k allocation can only be satisfied at the beginning of the
region, we could do top-down rather than bottom-up allocation (untested):

diff --git a/arch/arm/mm/vmregion.c b/arch/arm/mm/vmregion.c
index 935993e..036fdbf 100644
--- a/arch/arm/mm/vmregion.c
+++ b/arch/arm/mm/vmregion.c
@@ -38,7 +38,7 @@ struct arm_vmregion *
 arm_vmregion_alloc(struct arm_vmregion_head *head, size_t align,
 		   size_t size, gfp_t gfp)
 {
-	unsigned long addr = head->vm_start, end = head->vm_end - size;
+	unsigned long start = head->vm_start, addr = head->vm_end;
 	unsigned long flags;
 	struct arm_vmregion *c, *new;
 
@@ -54,21 +54,20 @@ arm_vmregion_alloc(struct arm_vmregion_head *head, size_t align,
 
 	spin_lock_irqsave(&head->vm_lock, flags);
 
-	list_for_each_entry(c, &head->vm_list, vm_list) {
-		if ((addr + size) < addr)
-			goto nospc;
-		if ((addr + size) <= c->vm_start)
+	addr = rounddown(addr - size, align);
+	list_for_each_entry_reverse(c, &head->vm_list, vm_list) {
+		if (addr >= c->vm_end)
 			goto found;
-		addr = ALIGN(c->vm_end, align);
-		if (addr > end)
+		addr = rounddown(c->vm_start - size, align);
+		if (addr < start)
 			goto nospc;
 	}
 
  found:
 	/*
-	 * Insert this entry _before_ the one we found.
+	 * Insert this entry after the one we found.
 	 */
-	list_add_tail(&new->vm_list, &c->vm_list);
+	list_add(&new->vm_list, &c->vm_list);
 	new->vm_start = addr;
 	new->vm_end = addr + size;
 	new->vm_active = 1;