linux-kernel - Re: [PATCH] clk: fractional-divider: prevent division by zero

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Date:	Mon, 19 Jan 2015 13:56:50 +0200
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: fractional-divider: prevent division by zero

On Fri, Jan 16, 2015 at 02:29:13PM -0800, Stephen Boyd wrote:
> On 01/16/2015 04:37 AM, Heikki Krogerus wrote:
> > Preventing division by zero condition by making sure that
> > the initial n and m values are not 0.
> >
> > Signed-off-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > ---
> >  drivers/clk/clk-fractional-divider.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
> > index 82a59d0..dc91da7 100644
> > --- a/drivers/clk/clk-fractional-divider.c
> > +++ b/drivers/clk/clk-fractional-divider.c
> > @@ -104,6 +104,7 @@ struct clk *clk_register_fractional_divider(struct device *dev,
> >  	struct clk_fractional_divider *fd;
> >  	struct clk_init_data init;
> >  	struct clk *clk;
> > +	u32 val;
> >  
> >  	fd = kzalloc(sizeof(*fd), GFP_KERNEL);
> >  	if (!fd) {
> > @@ -126,6 +127,14 @@ struct clk *clk_register_fractional_divider(struct device *dev,
> >  	fd->lock = lock;
> >  	fd->hw.init = &init;
> >  
> > +	/* Prevent division by zero */
> > +	val = clk_readl(fd->reg);
> > +	if (!(val & fd->nmask))
> > +		val |= 1 << fd->nshift;
> > +	if (!(val & fd->mmask))
> > +		val |= 1 << fd->mshift;
> > +	clk_writel(val, fd->reg);
> > +
> >  	clk = clk_register(dev, &fd->hw);
> >  	if (IS_ERR(clk))
> >  		kfree(fd);
> 
> Sorry I don't understand this at all. Does your hardware support a value
> of 0 in the register? Doesn't that mean "bypass" so that the rate of the
> parent bypasses the divider? If so, why aren't we fixing the
> recalc_rate() logic to check for a special case of 0?

OK. Makes sense.

-- 
heikki
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