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

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 14 Jan 2020 16:19:28 +0000
"Hennerich, Michael" <Michael.Hennerich@analog.com> wrote:

> > -----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.

Not in mainline it doesn't.   I took a look at the ADI git hub and I see it does
there.  I think we are missing a precursor patch.

Probably this one from blame...

https://github.com/analogdevicesinc/linux/commit/c8e6b341abf749f78e00326cb92b365d90d9de1f

Jonathan

> 
> -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);  
>