Merge tag 'for-linus-4.9-rc2-ofs-1' of git://git.kernel.org/pub/scm/linux/kernel...

[cascardo/linux.git] / Documentation / unaligned-memory-access.txt

diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt
index f866c72..a445da0 100644 (file)
--- a/Documentation/unaligned-memory-access.txt
+++ b/Documentation/unaligned-memory-access.txt
@@ -137,24 +137,34 @@ Code that causes unaligned access
 =================================
 
 With the above in mind, let's move onto a real life example of a function
-that can cause an unaligned memory access. The following function adapted
+that can cause an unaligned memory access. The following function taken
 from include/linux/etherdevice.h is an optimized routine to compare two
 ethernet MAC addresses for equality.
 
-unsigned int compare_ether_addr(const u8 *addr1, const u8 *addr2)
+bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
 {
-       const u16 *a = (const u16 *) addr1;
-       const u16 *b = (const u16 *) addr2;
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+       u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) |
+                  ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
+
+       return fold == 0;
+#else
+       const u16 *a = (const u16 *)addr1;
+       const u16 *b = (const u16 *)addr2;
        return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
+#endif
 }
 
-In the above function, the reference to a[0] causes 2 bytes (16 bits) to
-be read from memory starting at address addr1. Think about what would happen
-if addr1 was an odd address such as 0x10003. (Hint: it'd be an unaligned
-access.)
+In the above function, when the hardware has efficient unaligned access
+capability, there is no issue with this code.  But when the hardware isn't
+able to access memory on arbitrary boundaries, the reference to a[0] causes
+2 bytes (16 bits) to be read from memory starting at address addr1.
+
+Think about what would happen if addr1 was an odd address such as 0x10003.
+(Hint: it'd be an unaligned access.)
 
 Despite the potential unaligned access problems with the above function, it
-is included in the kernel anyway but is understood to only work on
+is included in the kernel anyway but is understood to only work normally on
 16-bit-aligned addresses. It is up to the caller to ensure this alignment or
 not use this function at all. This alignment-unsafe function is still useful
 as it is a decent optimization for the cases when you can ensure alignment,