[PATCH 1/2] clk: meson: mpll: fix division by zero in rate_from_params

[Jerome Brunet]

On Sun, 2017-04-02 at 20:43 +0200, Martin Blumenstingl wrote:
> Hi Jerome,
> 
> On Sun, Apr 2, 2017 at 4:49 PM, Jerome Brunet <jbrunet at baylibre.com> wrote:
> > On Sat, 2017-04-01 at 15:02 +0200, Martin Blumenstingl wrote:
> > > According to the public datasheet all register bits in HHI_MPLL_CNTL7,
> > > HHI_MPLL_CNTL8 and HHI_MPLL_CNTL9 default to zero. On all GX SoCs these
> > > seem to be initialized by the bootloader to some default value.
> > > However, on my Meson8 board they are not initialized, leading to a
> > > division by zero in rate_from_params as the math is:
> > > (parent_rate * SDM_DEN) / ((SDM_DEN * 0) + 0)
> > > 
> > > Although the rate_from_params function was only introduced recently the
> > > original bug has been there for much longer. It was only exposed
> > > recently when the MPLL clocks were added to the Meson8b clock driver.
> > 
> > The Documentation of the S905 also state that N2 minimal value is 4 and SDM
> > minimal is 1 ... I should have been more careful ! Thanks for catching this
> > Martin. We definitely need to fix this.
> > 
> > Patch seems ok but I have one question. Is the rate actually zero when the
> > divisor is zero ?
> > I'll be away from my boards until Wednesday but I can check this when I get
> > back
> > if you can't.
> 
> is there a way how I can check this? I have a consumer device so I
> can't measure the frequencies at some pin.

I'll try through the audio pins on wednesday. Don't worry about it. 

> 
> > > 
> > > Fixes: 1c50da4f27 ("clk: meson: add mpll support")
> > > Signed-off-by: Martin Blumenstingl <martin.blumenstingl at googlemail.com>
> > > ---
> > >  drivers/clk/meson/clk-mpll.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/clk/meson/clk-mpll.c b/drivers/clk/meson/clk-mpll.c
> > > index 540dabe5adad..551aa2a5b291 100644
> > > --- a/drivers/clk/meson/clk-mpll.c
> > > +++ b/drivers/clk/meson/clk-mpll.c
> > > @@ -76,7 +76,12 @@ static unsigned long rate_from_params(unsigned long
> > > parent_rate,
> > >                                     unsigned long sdm,
> > >                                     unsigned long n2)
> > >  {
> > > -     return (parent_rate * SDM_DEN) / ((SDM_DEN * n2) + sdm);
> > > +     unsigned long divisor = (SDM_DEN * n2) + sdm;
> > > +
> > > +     if (divisor == 0)
> > > +             return 0;
> > > +     else
> > > +             return (parent_rate * SDM_DEN) / divisor;
> > >  }
> > > 
> > >  static void params_from_rate(unsigned long requested_rate,