linux-kernel - Re: [PATCH RFT v2 1/5] bitops: add generic parity calculation for u8

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Message-ID: <20241229111129.2602d219@dsl-u17-10>
Date: Sun, 29 Dec 2024 11:11:29 +0000
From: David Laight <david.laight.linux@...il.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: linux-i3c@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org, Rasmus Villemoes
 <linux@...musvillemoes.dk>, Guenter Roeck <linux@...ck-us.net>, Geert
 Uytterhoeven <geert+renesas@...der.be>, Yury Norov <yury.norov@...il.com>,
 Kuan-Wei Chiu <visitorckw@...il.com>
Subject: Re: [PATCH RFT v2 1/5] bitops: add generic parity calculation for
 u8

On Sun, 29 Dec 2024 11:12:29 +0100
Wolfram Sang <wsa+renesas@...g-engineering.com> wrote:

> There are multiple open coded implementations for getting the parity of
> a byte in the kernel, even using different approaches. Take the pretty
> efficient version from SPD5118 driver and make it generally available by
> putting it into the bitops header. As long as there is just one parity
> calculation helper, the creation of a distinct 'parity.h' header was
> discarded. Also, the usage of hweight8() for architectures having a
> popcnt instruction is postponed until a use case within hot paths is
> desired. The motivation for this patch is the frequent use of odd parity
> in the I3C specification and to simplify drivers there.
> 
> Changes compared to the original SPD5118 version are the addition of
> kernel documentation, switching the return type from bool to int, and
> renaming the argument of the function.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
> Tested-by: Guenter Roeck <linux@...ck-us.net>
> Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> Acked-by: Yury Norov <yury.norov@...il.com>
> Reviewed-by: Kuan-Wei Chiu <visitorckw@...il.com>
> Tested-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> Changes since v1:
> 
> * renamed from 'get_parity8' to 'parity8'
> * use XOR instead of OR in the kdoc example (Thanks, Rasmus, for both)
> * rebased to 6.13-rc4
> 
>  include/linux/bitops.h | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index ba35bbf07798..c1cb53cf2f0f 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -229,6 +229,37 @@ static inline int get_count_order_long(unsigned long l)
>  	return (int)fls_long(--l);
>  }
>  
> +/**
> + * parity8 - get the parity of an u8 value
> + * @value: the value to be examined
> + *
> + * Determine the parity of the u8 argument.
> + *
> + * Returns:
> + * 0 for even parity, 1 for odd parity

I think I'd return 0x80 for even and 0 for odd.
That just need the 'magic constant' changing and masking with 0x80.
Also rename to parity8_even() - since it returns non-zero for even parity.

> + *
> + * Note: This function informs you about the current parity. Example to bail
> + * out when parity is odd:
> + *
> + *	if (parity8(val) == 1)
> + *		return -EBADMSG;
> + *
> + * If you need to calculate a parity bit, you need to draw the conclusion from
> + * this result yourself. Example to enforce odd parity, parity bit is bit 7:
> + *
> + *	if (parity8(val) == 0)
> + *		val ^= BIT(7);

That then becomes:
	val ^= parity8_even(val);
which is what a lot of the code seems to want to do.
With your definition you could do:
	val ^= (parity8(val) << 7) ^ 0x80;

(and I'm sorry, but IMHO 0x80 is better than BIT(7))

	David

> + */
> +static inline int parity8(u8 val)
> +{
> +	/*
> +	 * One explanation of this algorithm:
> +	 * https://funloop.org/codex/problem/parity/README.html
> +	 */
> +	val ^= val >> 4;
> +	return (0x6996 >> (val & 0xf)) & 1;
> +}
> +
>  /**
>   * __ffs64 - find first set bit in a 64 bit word
>   * @word: The 64 bit word