linux-kernel - Re: [PATCH next] crypto: qat - replace avg

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Message-ID: <20260207105130.4af50ce1@pumpkin>
Date: Sat, 7 Feb 2026 10:51:30 +0000
From: David Laight <david.laight.linux@...il.com>
To: Giovanni Cabiddu <giovanni.cabiddu@...el.com>, Herbert Xu
 <herbert@...dor.apana.org.au>, "David S. Miller" <davem@...emloft.net>,
 Suman Kumar Chakraborty <suman.kumar.chakraborty@...el.com>, Vijay Sundar
 Selvamani <vijay.sundar.selvamani@...el.com>, George Abraham P
 <george.abraham.p@...el.com>, qat-linux@...el.com,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH next] crypto: qat - replace avg_array() with a better
 function

On Fri,  6 Feb 2026 21:09:40 +0000
david.laight.linux@...il.com wrote:

Cc the people discussing unqual_scalar_typeof() for arm64 LTO READ_ONCE().

> From: David Laight <david.laight.linux@...il.com>
> 
> avg_array() is defined as a 'type independant' #define.
> However the algorithm is only valid for unsigned types and the
> implementation is only valid for u64.
> All the callers pass temporary kmalloc() allocated arrays of u64.
> 
> Replace with a function that takes a pointer to a u64 array.
> 
> Change the implementation to sum the low and high 32bits of each
> value separately and then compute the average.
> This will be massively faster as it does two divisions rather than
> one for each element.
> 
> Also removes some very pointless __unqual_scalar_typeof().
> They could be 'auto _x = 0 ? x + 0 : 0;' even if the types weren't fixed.
> 
> Only compile tested.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
>  .../intel/qat/qat_common/adf_tl_debugfs.c     | 38 ++++++++-----------
>  1 file changed, 15 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/crypto/intel/qat/qat_common/adf_tl_debugfs.c b/drivers/crypto/intel/qat/qat_common/adf_tl_debugfs.c
> index b81f70576683..a084437a2631 100644
> --- a/drivers/crypto/intel/qat/qat_common/adf_tl_debugfs.c
> +++ b/drivers/crypto/intel/qat/qat_common/adf_tl_debugfs.c
> @@ -77,32 +77,24 @@ static int tl_collect_values_u64(struct adf_telemetry *telemetry,
>   * @len: Number of elements.
>   *
>   * This algorithm computes average of an array without running into overflow.
> + * (Provided len is less than 2 << 31.)
>   *
>   * Return: average of values.
>   */
> -#define avg_array(array, len) (				\
> -{							\
> -	typeof(&(array)[0]) _array = (array);		\
> -	__unqual_scalar_typeof(_array[0]) _x = 0;	\
> -	__unqual_scalar_typeof(_array[0]) _y = 0;	\
> -	__unqual_scalar_typeof(_array[0]) _a, _b;	\
> -	typeof(len) _len = (len);			\
> -	size_t _i;					\
> -							\
> -	for (_i = 0; _i < _len; _i++) {			\
> -		_a = _array[_i];			\
> -		_b = do_div(_a, _len);			\
> -		_x += _a;				\
> -		if (_y >= _len - _b) {			\
> -			_x++;				\
> -			_y -= _len - _b;		\
> -		} else {				\
> -			_y += _b;			\
> -		}					\
> -	}						\
> -	do_div(_y, _len);				\
> -	(_x + _y);					\
> -})
> +static u64 avg_array(const u64 *array, size_t len)
> +{
> +	u64 sum_hi = 0, sum_lo = 0;
> +	size_t i;
> +
> +	for (i = 0; i < len; i++) {
> +		sum_hi += array[i] >> 32;
> +		sum_lo += (u32)array[i];
> +	}
> +
> +	sum_lo += (u64)do_div(sum_hi, len) << 32;
> +
> +	return (sum_hi << 32) + div_u64(sum_lo, len);
> +}
>  
>  /* Calculation function for simple counter. */
>  static int tl_calc_count(struct adf_telemetry *telemetry,