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

[Subhajit Ghosh]

On 22/1/24 17:20, Matti Vaittinen wrote:
> On 1/19/24 13:56, Subhajit Ghosh wrote:
> > 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.
>
> Thanks for testing Subhajit! Just to ensure we have no miscommunication - did you test just this division fix, or the rounding fix here:
> https://lore.kernel.org/lkml/37d3aa193e69577353d314e94463a08d488ddd8d.1701780964.git.mazziesaccount@xxxxxxxxx/
You are most welcome. I did not check the above rounding fix pointed out by the link. I will be happy to check it and let you know by the end of this month.
I checked this division fix.

> > 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!
>
> Well, I can't blame you :) Being in a new work at new city sounds like you have a lot on your plate right now. Give it half a year and things will stabilize though :) Oh, and falsely assuming that "when XXX, I will have the time to do YYY" - been there done that :)
>
> Good luck on the new work and city!
Thank you Matti.
> Yours,
>      -- Matti
Regards,
Subhajit Ghosh