RE: [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug

[Hennerich, Michael]

> -----Original Message-----
> From: Jonathan Cameron <jic23@xxxxxxxxxx>
> Sent: Samstag, 11. Januar 2020 12:09
> To: Bia, Beniamin <Beniamin.Bia@xxxxxxxxxx>
> Cc: lars@xxxxxxxxxx; Hennerich, Michael <Michael.Hennerich@xxxxxxxxxx>;
> pmeerw@xxxxxxxxxx; linux-iio@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; biabeniamin@xxxxxxxxxxx; knaack.h@xxxxxx
> 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@xxxxxxxxxx> wrote:
> 
> > From: Michael Hennerich <michael.hennerich@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Beniamin Bia <beniamin.bia@xxxxxxxxxx>
> > ---
> >  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);