linux-kernel - Re: [PATCH] clk: tegra: Return the exact clock rate from clk_round

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Message-ID: <20180925084419.GI7636@tbergstrom-lnx.Nvidia.com>
Date:   Tue, 25 Sep 2018 11:44:19 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     r yang <decatf@...il.com>
CC:     Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: tegra: Return the exact clock rate from
 clk_round_rate

On Mon, Sep 24, 2018 at 03:18:04PM -0400, r yang wrote:
> On Mon, Sep 24, 2018 at 11:08:03AM +0300, Peter De Schrijver wrote:
> > On Fri, Sep 21, 2018 at 06:01:49PM -0400, ryang wrote:
> > > The current behavior is that clk_round_rate would return the same clock
> > > rate passed to it for valid PLL configurations. This change will return
> > > the exact rate the PLL will provide in accordance with clk API.
> > > 
> > > Signed-off-by: ryang <decatf@...il.com>
> > > ---
> > >  drivers/clk/tegra/clk-pll.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/clk/tegra/clk-pll.c b/drivers/clk/tegra/clk-pll.c
> > > index 17a058c3bbc1..36014a6ec42e 100644
> > > --- a/drivers/clk/tegra/clk-pll.c
> > > +++ b/drivers/clk/tegra/clk-pll.c
> > > @@ -595,7 +595,12 @@ static int _calc_rate(struct clk_hw *hw, struct tegra_clk_pll_freq_table *cfg,
> > >  		return -EINVAL;
> > >  	}
> > >  
> > > -	cfg->output_rate >>= p_div;
> > > +	if (cfg->m == 0) {
> > > +		cfg->output_rate = 0;
> > 
> > I think a WARN_ON() is appropriate here. the input divider should never be 0.
> > 
> > Peter.
> > 
> 
> Should it return -EINVAL (or some error) too? _calc_rate is also in the
> clk_set_rate code path. I think we want to avoid programming the
> register to 0 input divider all together?
> 

Yes. writing 0 to the input divider is usually not allowed. In some cases it's
equivalent to writing 1, but better not count on that.

Peter.

> > > +	} else {
> > > +		cfg->output_rate = cfg->n * DIV_ROUND_UP(parent_rate, cfg->m);
> > > +		cfg->output_rate >>= p_div;
> > > +	}
> > >  
> > >  	if (pll->params->pdiv_tohw) {
> > >  		ret = _p_div_to_hw(hw, 1 << p_div);
> > > -- 
> > > 2.17.1
> > >