Re: [PATCH next] crypto: qat - replace avg_array() with a better function

[David Laight]

On Tue, 24 Feb 2026 18:11:37 +0000
Giovanni Cabiddu <giovanni.cabiddu@xxxxxxxxx> wrote:

> On Fri, Feb 06, 2026 at 09:09:40PM +0000, david.laight.linux@xxxxxxxxx wrote:
> > From: David Laight <david.laight.linux@xxxxxxxxx>
> > 
> > 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.  
> Thanks David, this is a great optimization.
> 
> I also reviewed the algorithm and confirmed it is functionally equivalent
> to the previous version. I tested it on a platform with QAT and it
> behaves as expected.
> 
> Some minor comments below.
> 
> > This will be massively faster as it does two divisions rather than
> > one for each element.  
> NIT: probably not `massively faster` as the maximum value for len in the
> current implementation is 4.

It is still a lot faster - but probably not significant to system performance.

Actually if the max for len is 65536 you can do better (esp. for 32bit).
Instead of splitting 32/32 split 48/16, then sum_lo needs only be u32.

giving:

{
	u64 sum_hi = 0;
	u32 sum_lo = 0;
	size_t i;

	for (i = 0; i < len; i++) {
		sum_hi += array[i] >> 16;
		sum_lo += array[i] & 0xffff;
	}

	sum_lo += do_div(sum_hi, len) << 16;

	return (sum_hi << 16) + sum_lo / len;
}

OTOH aren't those values performance counts of some kind?
Adding four of them together isn't going to wrap.

> 
> > 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@xxxxxxxxx>
> > ---
> >  .../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.)  
> Should this be 2^31 or 1 << 31?
> Alternatively: `Provided len fits in u32`?

Not sure why I wrote 2 << 31 :-)

The condition is that sum_lo must not overflow.
The worst case is all the low bits being 1.
If len is 2^32 then sum_lo is then (2^32 - 1) * 2^32.
The remainder from the sum_hi divide is shifted and added in,
giving (2^32 - 1) * (2^32 + 1) which is what my maths teacher called a
'cow and goat' - (cow + goat) * (cow - goat) = cow squared - goat squared,
so then maximum for sum_lo is 2^64 - 1 which fits.
Which means it should have been 'len <= 2^32'.

	David
 

> 
> >   *
> >   * 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)  
> Shall size_t len be u32 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,  
> 
> Thanks,
>