Messages in this thread |  | | From | "Hennerich, Michael" <> | Subject | RE: [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug | Date | Tue, 14 Jan 2020 16:19:28 +0000 |
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> -----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);
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