diff options
Diffstat (limited to 'Documentation/unaligned-memory-access.txt')
| -rw-r--r-- | Documentation/unaligned-memory-access.txt | 28 |
1 files changed, 19 insertions, 9 deletions
diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt index f866c72291bf..a445da098bc6 100644 --- 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, |