Re: [PATCH] iio: gts-helper: Fix division loop

[Subhajit Ghosh]

On 8/1/24 02:52, Jonathan Cameron wrote:
> On Thu, 4 Jan 2024 11:34:28 +0200
> Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
>
> > The loop based 64bit division may run for a long time when dividend is a
> > lot bigger than the divider. Replace the division loop by the
> > div64_u64() which implementation may be significantly faster.
> >
> > Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
> > Fixes: 38416c28e168 ("iio: light: Add gain-time-scale helpers")
>
> Hmm. Fix or not perf improvement?  I'm going to take the middle ground
> and leave the fixes tag, but not rush this in.
>
> So applied to the togreg branch of iio.git and for now just pushed out
> as testing for 0-day etc to take a look before I rebase that tree after
> rc1.
>
>
>
> > ---
> >
> > I've implemented also a fixup series for supporting rounding of
> > gains/scales:
> > https://lore.kernel.org/lkml/37d3aa193e69577353d314e94463a08d488ddd8d.1701780964.git.mazziesaccount@xxxxxxxxx/
> >
> > That series does also remove the offending loop.
> >
> > We don't currently have any in-tree users of GTS helpers which would
> > need the rounding support so pushing the rounding is not urgent (and I
> > haven't heard of Subjahit whose driver required the rounding). Hence, we
> > may want to only take this loop fix in for now (?) and reconsider
> > rounding when someone need that.
> >
> > Jonathan, what's your take on this?
> Agreed - let us wait for the rounding to have a user, but makes sense
> to tidy this corner up in the meantime.
>
> Thanks,
>
> Jonathan
>
> >   drivers/iio/industrialio-gts-helper.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/iio/industrialio-gts-helper.c b/drivers/iio/industrialio-gts-helper.c
> > index 7653261d2dc2..abcab2d38589 100644
> > --- a/drivers/iio/industrialio-gts-helper.c
> > +++ b/drivers/iio/industrialio-gts-helper.c
> > @@ -34,7 +34,7 @@
> >   static int iio_gts_get_gain(const u64 max, const u64 scale)
> >   {
> >   	u64 full = max;
> > -	int tmp = 1;
> > +	int tmp = 0;
> >   
> >   	if (scale > full || !scale)
> >   		return -EINVAL;
> > @@ -48,8 +48,7 @@ static int iio_gts_get_gain(const u64 max, const u64 scale)
> >   		tmp++;
> >   	}
> >   
> > -	while (full > scale * (u64)tmp)
> > -		tmp++;
> > +	tmp += div64_u64(full, scale);
> >   
> >   	return tmp;
> >   }
> >
> > base-commit: 2cc14f52aeb78ce3f29677c2de1f06c0e91471ab
Hi Matti,

Your fix works beautifully with the latest version of apds9306 driver which I am working on.
All available scale values can be set without any errors. Thank you.

Moving to a new city with a new full time job with the assumption of getting more time
for my list of opensource projects and contributions proved to be utterly wrong!

Regards,
Subhajit Ghosh