[PATCH] lib/crc32: slice by 4 is more efficient than the default slice by 8 on Powerpc 8xx.

[Joakim Tjernlund]

I found the same on MPC8321 long time ago(when 64 bits change went in), 
the 32 bits were much faster. I guess the "smaller"
CPUs cannot handle the cache trashing these big tables impose, I didn't 
look into the details though.
So I think this is a good change for 8xx.

Acked-by: Joakim Tjernlund <joakim.tjernlund at transmode.se>

Christophe Leroy <christophe.leroy at c-s.fr> wrote on 2013/11/18 08:04:23:

> From: Christophe Leroy <christophe.leroy at c-s.fr>
> To: Vitaly Bordug <vitb at kernel.crashing.org>, Marcelo Tosatti 
<marcelo at kvack.org>, Joakim Tjernlund <joakim.tjernlund at transmode.se>, Bob 
Pearson <rpearson at systemfabricworks.com>, 
> Cc: linux-kernel at vger.kernel.org, linuxppc-dev at lists.ozlabs.org
> Date: 2013/11/19 13:05
> Subject: [PATCH] lib/crc32: slice by 4 is more efficient than the 
default slice by 8 on Powerpc 8xx.
> 
> On PPC_8xx, CRC32_SLICEBY4 is more efficient (almost twice) than 
CRC32_SLICEBY8,
> as shown below:
> 
> With CRC32_SLICEBY8:
> [    1.109204] crc32: CRC_LE_BITS = 64, CRC_BE BITS = 64
> [    1.114401] crc32: self tests passed, processed 225944 bytes in 
15118910 nsec
> [    1.130655] crc32c: CRC_LE_BITS = 64
> [    1.134235] crc32c: self tests passed, processed 225944 bytes in 
4479879 nsec
> 
> With CRC32_SLICEBY4:
> [    1.097129] crc32: CRC_LE_BITS = 32, CRC_BE BITS = 32
> [    1.101878] crc32: self tests passed, processed 225944 bytes in 
8616242 nsec
> [    1.116298] crc32c: CRC_LE_BITS = 32
> [    1.119607] crc32c: self tests passed, processed 225944 bytes in 
3289576 nsec
> 
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> 
> Index: a/lib/Kconfig
> ===================================================================
> --- a/lib/Kconfig   (révision 5325)
> +++ b/lib/Kconfig   (copie de travail)
> @@ -102,6 +102,7 @@
>  choice
>     prompt "CRC32 implementation"
>     depends on CRC32
> +   default CRC32_SLICEBY4 if PPC_8xx
>     default CRC32_SLICEBY8
>     help
>       This option allows a kernel builder to override the default choice