ARM: PL08x: ensure pl08x_pre_boundary() works for any value of addr

pl08x_pre_boundary() was unsafe with addresses towards the top of
memory space:

	boundary = ((addr >> PL08X_BOUNDARY_SHIFT) + 1)
			<< PL08X_BOUNDARY_SHIFT;

This can overflow a 32-bit number, producing zero.  When it does:

	if (boundary < addr + len)
		return boundary - addr;
	else
		return len;

results in (boundary - addr) returning either a large positive value.
Also if addr + len overflows, this calculation also fails.

We can fix this trivially as the only thing we're actually interested
in is the value of the least significant PL08X_BOUNDARY_SHIFT bits:

	boundary_len = PL08X_BOUNDARY_SIZE -
		(addr & (PL08X_BOUNDARY_SIZE - 1));

gives us the number of bytes before 'addr' becomes a multiple of
PL08X_BOUNDARY_SIZE.  We can then just take the min() of the two
calculated lengths.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/amba-pl08x.c

@@ -547,19 +547,15 @@ static void pl08x_fill_lli_for_desc(struct pl08x_driver_data *pl08x,
 }
 
 /*
- * Return number of bytes to fill to boundary, or len
+ * Return number of bytes to fill to boundary, or len.
+ * This calculation works for any value of addr.
  */
 static inline size_t pl08x_pre_boundary(u32 addr, size_t len)
 {
-	u32 boundary;
+	size_t boundary_len = PL08X_BOUNDARY_SIZE -
+			(addr & (PL08X_BOUNDARY_SIZE - 1));
 
-	boundary = ((addr >> PL08X_BOUNDARY_SHIFT) + 1)
-		<< PL08X_BOUNDARY_SHIFT;
-
-	if (boundary < addr + len)
-		return boundary - addr;
-	else
-		return len;
+	return min(boundary_len, len);
 }
 
 /*