Re: [PATCH v2] iio: frequency: adf4350: replace loop with fls_long()

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Sat, Mar 14, 2026 at 01:20:06PM -0400, Neel Bullywon wrote:
> Address the TODO in adf4350_set_freq() by replacing the iterative
> power-of-2 shift loop with a constant-time bitwise calculation.
> 
> By comparing the highest set bits of the target constant and freq
> using fls_long(), we can calculate the required RF divider selection
> in a single step without relying on expensive 64-bit division.
> 
> This ensures freq is properly shifted to meet or exceed the minimum
> VCO frequency.

...

>  	st->r4_rf_div_sel = 0;

> -	while (freq < ADF4350_MIN_VCO_FREQ) {
> -		freq <<= 1;
> -		st->r4_rf_div_sel++;
> +	if (freq < ADF4350_MIN_VCO_FREQ) {
> +		st->r4_rf_div_sel = fls_long(ADF4350_MIN_VCO_FREQ - 1) - fls_long(freq);
> +		if ((freq << st->r4_rf_div_sel) < ADF4350_MIN_VCO_FREQ)
> +			st->r4_rf_div_sel++;
>  	}
>  
> +	freq <<= st->r4_rf_div_sel;

Yeah, unfortunately it looks like too complicated in comparison with the
original code. Have you checked binary output? My gut feelings that this
gives also a lot of code in addition to.

Little optimisation can be like:

	if (freq < ADF4350_MIN_VCO_FREQ) {
		st->r4_rf_div_sel = fls_long(ADF4350_MIN_VCO_FREQ - 1) - fls_long(freq);
		freq <<= st->r4_rf_div_sel;
		if (freq < ADF4350_MIN_VCO_FREQ) {
			st->r4_rf_div_sel++;
			freq <<= 1;
		}
	}

but it is still too much.

...

Maybe we simply need to replace TODO with a NOTE explaining that if better algo
is found we can replace.

-- 
With Best Regards,
Andy Shevchenko