ARM: 6499/1: Thumb-2: Correct data alignment for CONFIG_THUMB2_KERNEL in bootp/init.S

Directives such as .long and .word do not magically cause the
assembler location counter to become aligned in gas.  As a result,
using these directives in code sections can result in misaligned
data words when building a Thumb-2 kernel (CONFIG_THUMB2_KERNEL).

This is a Bad Thing, since the ABI permits the compiler to assume
that fundamental types of word size or above are word- aligned when
accessing them from C.  If the data is not really word-aligned,
this can cause impaired performance and stray alignment faults in
some circumstances.

In general, the following rules should be applied when using data
word declaration directives inside code sections:

    * .quad and .double:
         .align 3

    * .long, .word, .single, .float:
         .align (or .align 2)

    * .short:
        No explicit alignment required, since Thumb-2
        instructions are always 2 or 4 bytes in size.
        immediately after an instruction.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/boot/bootp/init.S b/arch/arm/boot/bootp/init.S
index 8b0de41c3dcbbe646877a9f31635bb5e48b8b4bd..78b508075161fd4967bdbb3821eab162aa9dd5bc 100644 (file)
--- a/arch/arm/boot/bootp/init.S
+++ b/arch/arm/boot/bootp/init.S
@@ -73,6 +73,8 @@ move:         ldmia   r4!, {r7 - r10}         @ move 32-bytes at a time
 
                .size   _start, . - _start
 
+               .align
+
                .type   data,#object
 data:          .word   initrd_start            @ source initrd address
                .word   initrd_phys             @ destination initrd address