Re: [PATCH 3/8] lib/lzo: enable 64-bit CTZ on Arm

From: "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
To: Dave Rodgman <dave.rodgman@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>
Cc: "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
	"davem@davemloft.net" <davem@davemloft.net>,
	Matt Sealey <Matt.Sealey@arm.com>,
	"nitingupta910@gmail.com" <nitingupta910@gmail.com>,
	"minchan@kernel.org" <minchan@kernel.org>,
	"sergey.senozhatsky.work@gmail.com" 
	<sergey.senozhatsky.work@gmail.com>,
	"sonnyrao@google.com" <sonnyrao@google.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	nd <nd@arm.com>, "sfr@canb.auug.org.au" <sfr@canb.auug.org.au>
Subject: Re: [PATCH 3/8] lib/lzo: enable 64-bit CTZ on Arm
Date: Thu, 6 Dec 2018 16:51:55 +0100	[thread overview]
Message-ID: <5C09459B.5090201@oberhumer.com> (raw)
In-Reply-To: <20181130142600.13782-4-dave.rodgman@arm.com>

I think that LZO_USE_CTZ64 should only be defined for ARM64 and
not for 32-bit ARM.

On 2018-11-30 15:26, Dave Rodgman wrote:
> From: Matt Sealey <matt.sealey@arm.com>
> 
> ARMv6 Thumb state introduced an RBIT instruction which, combined with CLZ
> as present in ARMv5, introduces an extremely fast path for counting
> trailing zeroes.
> 
> Enable the use of the GCC builtin for this on ARMv6+ with
> CONFIG_THUMB2_KERNEL to ensure we get the 'new' instruction usage.
> 
> We do not bother enabling LZO_USE_CTZ64 support for ARMv5 as the builtin
> code path does the same thing as the LZO_USE_CTZ32 code, only with more
> register pressure.
> 
> Link: http://lkml.kernel.org/r/20181127161913.23863-4-dave.rodgman@arm.com
> Signed-off-by: Matt Sealey <matt.sealey@arm.com>
> Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: Markus F.X.J. Oberhumer <markus@oberhumer.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Nitin Gupta <nitingupta910@gmail.com>
> Cc: Richard Purdie <rpurdie@openedhand.com>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> Cc: Sonny Rao <sonnyrao@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
> ---
>  lib/lzo/lzodefs.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/lzo/lzodefs.h b/lib/lzo/lzodefs.h
> index e1b3cf6459a9..c0193f726db0 100644
> --- a/lib/lzo/lzodefs.h
> +++ b/lib/lzo/lzodefs.h
> @@ -33,9 +33,14 @@
>  #define LZO_USE_CTZ32	1
>  #elif defined(CONFIG_X86) || defined(CONFIG_PPC)
>  #define LZO_USE_CTZ32	1
> -#elif defined(CONFIG_ARM) && (__LINUX_ARM_ARCH__ >= 5)
> +#elif defined(CONFIG_ARM)
> +#if (__LINUX_ARM_ARCH__ >= 5)
>  #define LZO_USE_CTZ32	1
>  #endif
> +#if (__LINUX_ARM_ARCH__ >= 6) && defined(CONFIG_THUMB2_KERNEL)
> +#define LZO_USE_CTZ64	1
> +#endif
> +#endif
>  
>  #define M1_MAX_OFFSET	0x0400
>  #define M2_MAX_OFFSET	0x0800
> 

-- 
Markus Oberhumer, <markus@oberhumer.com>, http://www.oberhumer.com/