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

[Jerome Brunet <jbrunet@baylibre.com>]

On Sun, 2017-04-02 at 23:34 +0200, Jerome Brunet wrote:
> 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@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.

So I checked, when N2 is less than the documented minimum (4), the clock get
into some weird state but is not grounded.
For example, on gxbb with N2 the rate of the mpll is around 4Mhz.

We have to return an error code here.
I've reworked the patch ... i'll send it very soon

> 
> > 
> > > > 
> > > > Fixes: 1c50da4f27 ("clk: meson: add mpll support")
> > > > Signed-off-by: Martin Blumenstingl <martin.blumenstingl@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,