linux-kernel - Re: [RFC PATCH 1/2] bitops: add generic parity calculation for u8

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Message-ID: <0f4bb1d6-19f1-4999-b09c-e514c18364b2@roeck-us.net>
Date: Sat, 14 Dec 2024 07:10:35 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hwmon@...r.kernel.org, Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [RFC PATCH 1/2] bitops: add generic parity calculation for u8

On Sat, Dec 14, 2024 at 09:58:31AM +0100, Wolfram Sang 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>

> ---
>  include/linux/bitops.h | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index ba35bbf07798..4ed430934ffc 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);
>  }
>  
> +/**
> + * get_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
> + *
> + * Note: This function informs you about the current parity. Example to bail
> + * out when parity is odd:
> + *
> + *	if (get_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 (get_parity8(val) == 0)
> + *		val |= BIT(7);
> + */
> +static inline int get_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
> -- 
> 2.45.2
> 
>