linux-kernel - Re: [RESEND PATCH v2] iio: gts-helper: Fix division loop

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Message-ID: <20240217162724.767f2ab6@jic23-huawei>
Date: Sat, 17 Feb 2024 16:27:24 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Subhajit Ghosh <subhajit.ghosh@...aklogic.com>
Cc: Matti Vaittinen <mazziesaccount@...il.com>, Matti Vaittinen
 <matti.vaittinen@...rohmeurope.com>, Lars-Peter Clausen <lars@...afoo.de>,
 linux-kernel@...r.kernel.org, David Laight <David.Laight@...lab.com>,
 linux-iio@...r.kernel.org
Subject: Re: [RESEND PATCH v2] iio: gts-helper: Fix division loop

On Sun, 18 Feb 2024 01:09:33 +1030
Subhajit Ghosh <subhajit.ghosh@...aklogic.com> wrote:

> On 17/2/24 00:28, Jonathan Cameron wrote:
> > On Mon, 12 Feb 2024 13:20:09 +0200
> > Matti Vaittinen <mazziesaccount@...il.com> 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@...il.com>
> >> Fixes: 38416c28e168 ("iio: light: Add gain-time-scale helpers")
> >>
> >> ---
> >> This is a resend. Only change is the base which is now the v6.8-rc4 and
> >> not the v6.8-rc1  
> > Given I'm not rushing this in, it is going via my togreg tree, so the
> > rebase wasn't really helpful (thankfully didn't stop it applying).
> > Would have been fine to send a ping response to the first posting of it.
> > 
> > I was leaving some time for David or Subhajit to have time to take
> > another look, but guess they are either happy with this or busy.
> > 
> > Applied to the togreg branch of iio.git and pushed out as testing for
> > all the normal reasons.
> > 
> > Jonathan
> >   
> >>
> >> This change was earlier applied and reverted as it confusingly lacked of
> >> the removal of the overflow check (which is only needed when we do
> >> looping "while (full > scale * (u64)tmp)". As this loop got removed, the
> >> check got also obsolete and leaving it to the code caused some
> >> confusion.
> >>
> >> So, I marked this as a v2, where v1 is the reverted change discussed
> >> here:
> >> https://lore.kernel.org/linux-iio/ZZZ7pJBGkTdFFqiY@dc78bmyyyyyyyyyyyyydt-3.rev.dnainternet.fi/
> >>
> >> Revision history:
> >> v1 => v2:
> >>   - Drop the obsolete overflow check
> >>   - Rebased on top of the v6.8-rc4
> >>
> >> iio: gts: loop fix fix
> >> ---
> >>   drivers/iio/industrialio-gts-helper.c | 15 +--------------
> >>   1 file changed, 1 insertion(+), 14 deletions(-)
> >>
> >> diff --git a/drivers/iio/industrialio-gts-helper.c b/drivers/iio/industrialio-gts-helper.c
> >> index 7653261d2dc2..b51eb6cb766f 100644
> >> --- a/drivers/iio/industrialio-gts-helper.c
> >> +++ b/drivers/iio/industrialio-gts-helper.c
> >> @@ -34,24 +34,11 @@
> >>   static int iio_gts_get_gain(const u64 max, const u64 scale)
> >>   {
> >>   	u64 full = max;
> >> -	int tmp = 1;
> >>   
> >>   	if (scale > full || !scale)
> >>   		return -EINVAL;
> >>   
> >> -	if (U64_MAX - full < scale) {
> >> -		/* Risk of overflow */
> >> -		if (full - scale < scale)
> >> -			return 1;
> >> -
> >> -		full -= scale;
> >> -		tmp++;
> >> -	}
> >> -
> >> -	while (full > scale * (u64)tmp)
> >> -		tmp++;
> >> -
> >> -	return tmp;
> >> +	return div64_u64(full, scale);
> >>   }
> >>   
> >>   /**  
> Hi Matti and Jonathan,
> 
> I somehow missed testing this patch earlier. The above patch works fine with apds9306 v7 driver(which work in progress!).
> There are no errors.
> My test script is simple:
> #!/bin/bash
> D=0
> S=`cat /sys/bus/iio/devices/iio:device${D}/in_illuminance_scale_available`
> 
> for s in $S; do
> 	echo $s
> 	echo $s > /sys/bus/iio/devices/iio:device${D}/in_illuminance_scale
> 	sleep 5
> done
> 
> One question - if I test a patch like this, do I put a "Tested-by" tag or just mention that I have tested it?
Both are useful - so thanks for this email.

Preference for a formal tag though as that goes in the git commit and we have
a convenient record that both says you tested it + that we should make sure
to cc you on related changes as you may well be in a position to test those
as well!

Thanks,

Jonathan

> 
> Regards,
> Subhajit Ghosh
> 
> >>
> >> base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de  
> >   
>