On Sun, 29 Dec 2024 11:12:29 +0100
Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Reviewed-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> Acked-by: Yury Norov <yury.norov@xxxxxxxxx>
> Reviewed-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
> Tested-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
> ---
> 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
