linux-kernel - Re: [PATCH 01/31] bitops: add parity functions

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Date:	Sun, 27 Mar 2016 21:38:23 +0800
From:	"zhaoxiu.zeng" <zhaoxiu.zeng@...il.com>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Martin Kepplinger <martink@...teo.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Yury Norov <yury.norov@...il.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 01/31] bitops: add parity functions

On 2016/3/27 20:44, Sam Ravnborg wrote:
> Hi Zeng.
> 
> Looking through the arch specific implementations of __arch_parity().
> Some architectures uses #defines, other uses inline static functions.
> 
> Any particular reason that you select one approach over the other
> in the different cases?
> 
> ia64:
> +#define __arch_parity32(x) ((unsigned int) __arch_parity64((x) & 0xfffffffful))
> +#define __arch_parity16(x) ((unsigned int) __arch_parity64((x) & 0xfffful))
> +#define __arch_parity8(x)  ((unsigned int) __arch_parity64((x) & 0xfful))
> +#define __arch_parity4(x)  ((unsigned int) __arch_parity64((x) & 0xful))
> 
> tile:
> +static inline unsigned int __arch_parity32(unsigned int w)
> +{
> +	return __builtin_popcount(w) & 1;
> +}
> +
> +static inline unsigned int __arch_parity16(unsigned int w)
> +{
> +	return __arch_parity32(w & 0xffff);
> +}
> +
> +static inline unsigned int __arch_parity8(unsigned int w)
> +{
> +	return __arch_parity32(w & 0xff);
> +}
> +
> +static inline unsigned int __arch_parity4(unsigned int w)
> +{
> +	return __arch_parity32(w & 0xf);
> +}
> 

No particular reason, just like the architecture's __arch_hweightN.

> Just two examples.
> 
> Adding the parity helpers seems like veny nice simplifications.
> 
> A few comments to some of those I looked at.
> (I am not subscribed to lkml, so you get it as comments here)
> 

I think the conversion is simple and readable.

> [PATCH 21/31] mtd: use parity16 in ssfdc.c
> The original code semes to check that the parity equals the
> value of first bit in the address.
> This seems lost after the conversion.
> 

The original get_parity return 1 if the number is even, so
if block_address is valid, "block_address & 0x7ff" must be odd.

> [PATCH 20/31] scsi: use parity32 in isci/phy.c
> +	if (parity32(phy_cap.all))
>  		phy_cap.parity = 1;
> Could be written like this - simpler IMO:
> 	phy_cap.parity = parity32(phy_cap.all);
> 
> 
> 	Sam
> 
Yes. Thanks!