linux-kernel - Re: [PATCH] staging: iio: impedance-analyzer: ad5933: use div64_ul() instead of do

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Message-ID: <aXI_HnvDNbX68-lq@smile.fi.intel.com>
Date: Thu, 22 Jan 2026 17:15:42 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Archit Anant <architanant5@...il.com>
Cc: jic23@...nel.org, lars@...afoo.de, Michael.Hennerich@...log.com,
	gregkh@...uxfoundation.org, dlechner@...libre.com,
	nuno.sa@...log.com, andy@...nel.org, linux-iio@...r.kernel.org,
	linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: iio: impedance-analyzer: ad5933: use div64_ul()
 instead of do_div()

On Thu, Jan 22, 2026 at 05:12:42PM +0200, Andy Shevchenko wrote:
> On Thu, Jan 22, 2026 at 08:26:33PM +0530, Archit Anant wrote:

...

> > -	freqreg = (u64)freq * (u64)(1 << 27);
> > -	do_div(freqreg, st->mclk_hz / 4);
> > +	freqreg = div64_ul((u64)freq * (u64)(1 << 27),
> > +			   st->mclk_hz / 4);
> 
> It can be one line to begin with.
> Then drop that ugly castings and explicit big shifts.
> 
> 	freqreg = div64_ul(BIT_ULL(27) * freq, st->mclk_hz / 4);
> 
> Now you can see That 4 is only 2 bits, so this can be written in
> simpler way:
> 
> 	freqreg = div64_ul(BIT_ULL(29) * freq, st->mclk_hz);
> 
> which may give a better precision at the end of the day.

It also might be worth to add a comment on top to explain (with given context
I don't know if there is already one on top of the function, though).

And I think we want AD people to comment on this and maybe explain better
the calculations done (and why the original code drops precision, was it
deliberate?).

-- 
With Best Regards,
Andy Shevchenko