From: "Hennerich, Michael" <Michael.Hennerich@analog.com>
To: Jonathan Cameron <jic23@kernel.org>,
"Bia, Beniamin" <Beniamin.Bia@analog.com>
Cc: "lars@metafoo.de" <lars@metafoo.de>,
"pmeerw@pmeerw.net" <pmeerw@pmeerw.net>,
"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"biabeniamin@outlook.com" <biabeniamin@outlook.com>,
"knaack.h@gmx.de" <knaack.h@gmx.de>
Subject: RE: [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug
Date: Tue, 14 Jan 2020 16:19:28 +0000 [thread overview]
Message-ID: <BN6PR03MB25966D74469371C11AFC4E4C8E340@BN6PR03MB2596.namprd03.prod.outlook.com> (raw)
In-Reply-To: <20200111110848.7c45a4f3@archlinux>
> -----Original Message-----
> From: Jonathan Cameron <jic23@kernel.org>
> Sent: Samstag, 11. Januar 2020 12:09
> To: Bia, Beniamin <Beniamin.Bia@analog.com>
> Cc: lars@metafoo.de; Hennerich, Michael <Michael.Hennerich@analog.com>;
> pmeerw@pmeerw.net; linux-iio@vger.kernel.org; linux-
> kernel@vger.kernel.org; biabeniamin@outlook.com; knaack.h@gmx.de
> Subject: Re: [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug
>
> On Tue, 7 Jan 2020 15:15:59 +0200
> Beniamin Bia <beniamin.bia@analog.com> wrote:
>
> > From: Michael Hennerich <michael.hennerich@analog.com>
> >
> > During initialization adf4371_pll_fract_n_get_rate() is called on all
> > output channels to determine if the device was setup. In this case
> > mod2 is zero which can cause a divide by zero exception.
> > Return before that can happen.
> I'm confused by this description vs the code.
>
> As far as I can see fract_n_get_rate is only called on a sysfs read of the
> frequency.
That's not the case. The failure mechanism comes via adf4371_channel_config().
It calls adf4371_pll_fract_n_get_rate() prior adf4371_set_freq() which initializes
st->mod2 via adf4371_pll_fract_n_compute(). This only happens the first time
during probe and setup. So the solution was to return 0 if st->mod2 == 0.
-Michael
>
> mod2 is set when fract_n_compute is called in the relevant set_freq calls.
> This seems to occur on a sysfs set frequency call.
>
> So the issue here is that a sysfs read before a write of the frequency will cause a
> div zero? If so, is there a sane set of initial values we can put in mod2 and
> friends before exposing them via the device register?
>
> If mod2==0 is a valid value and indicates for example that the channel is turned
> off, then the description should make that clear.
>
> Jonathan
>
> >
> > Fixes: 7f699bd149134 ("iio: frequency: adf4371: Add support for
> > ADF4371 PLL")
> > Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
> > Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
> > ---
> > drivers/iio/frequency/adf4371.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/iio/frequency/adf4371.c
> > b/drivers/iio/frequency/adf4371.c index e2a599b912e5..c21462238314
> > 100644
> > --- a/drivers/iio/frequency/adf4371.c
> > +++ b/drivers/iio/frequency/adf4371.c
> > @@ -191,6 +191,9 @@ static unsigned long long
> adf4371_pll_fract_n_get_rate(struct adf4371_state *st,
> > unsigned long long val, tmp;
> > unsigned int ref_div_sel;
> >
> > + if (st->mod2 == 0)
> > + return 0;
> > +
> > val = (((u64)st->integer * ADF4371_MODULUS1) + st->fract1) * st-
> >fpfd;
> > tmp = (u64)st->fract2 * st->fpfd;
> > do_div(tmp, st->mod2);
next prev parent reply other threads:[~2020-01-14 16:19 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-07 13:15 [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug Beniamin Bia
2020-01-11 11:08 ` Jonathan Cameron
2020-01-14 16:19 ` Hennerich, Michael [this message]
2020-01-18 11:29 ` Jonathan Cameron
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