On Aug 22, 2015, at 15:53, Afzal Mohammed [off-list ref] wrote:

Hi,

On Fri, Aug 21, 2015 at 11:01:41AM +0300, Tomi Valkeinen wrote:

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Possibly the patches are still good for x86 also, but that needs to be
proven.

not exactly, because x86_64 don’t have hardware instruction to do rbit OP,
i compile by test :

For old drivers i386 may be more relevant than x86_64.

It seems asm bit reversal is supported in Kernel on arm & arm64 only,
not sure whether any other arch even provide asm bit reversal
instruction.

i only submit the bit reverse patch for arm / arm64 arch,
i am not sure if there are some other arch also have hard ware bit reverse 
instructions, need arch maintainers to submit if their arch also have these hard
ware instructions . :)

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These kind of optimizations should have some real world measurements,

Not for this case, but once measured on ARM, iirc, a 32-bit asm bit
reversal as compared to doing it in C was taking 1 cycle as opposed to
~225 cycles!, of course writing optimized C could have made it fare
better, but still would reach no-way near asm bit reversal.