linux-kernel - Re: [PATCH] clk: mediatek: Add MT8173 MMPLL change rate support

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Date:	Tue, 7 Jul 2015 17:48:45 +0800
From:	James Liao <jamesjj.liao@...iatek.com>
To:	Heiko Stübner <heiko@...ech.de>
CC:	<devicetree@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	<srv_heupstream@...iatek.com>, Stephen Boyd <sboyd@...eaurora.org>,
	<linux-kernel@...r.kernel.org>, Ricky Liang <jcliang@...omium.org>,
	"Rob Herring" <robh+dt@...nel.org>,
	<linux-mediatek@...ts.infradead.org>,
	"Sascha Hauer" <kernel@...gutronix.de>,
	Matthias Brugger <matthias.bgg@...il.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clk: mediatek: Add MT8173 MMPLL change rate support

Hi Heiko,

On Tue, 2015-07-07 at 11:34 +0200, Heiko Stübner wrote:
> > > > @@ -135,16 +138,26 @@ static void mtk_pll_calc_values(struct mtk_clk_pll
> > > > *pll, u32 *pcw, u32 *postdiv, u32 freq, u32 fin)
> > > > 
> > > >  {
> > > >  
> > > >  	unsigned long fmin = 1000 * MHZ;
> > > > 
> > > > +	const unsigned long *div_rate = pll->data->div_rate;
> > > > 
> > > >  	u64 _pcw;
> > > >  	u32 val;
> > > >  	
> > > >  	if (freq > pll->data->fmax)
> > > >  	
> > > >  		freq = pll->data->fmax;
> > > > 
> > > > -	for (val = 0; val < 4; val++) {
> > > > +	if (div_rate) {
> > > > +		for (val = 1; div_rate[val] != 0; val++) {
> > > > +			if (freq > div_rate[val])
> > > > +				break;
> > > > +		}
> > > > +		val--;
> > > 
> > > if you're changing the table struct, this of course also would need to be
> > > adapted.
> > > 
> > > 
> > > Hmm, what I don't understand is, what does MT8173_PLL_FMAX in the table,
> > > if
> > > you ignore it here all the time?
> > > 
> > > So the table should probably look more like [when using the concept from
> > > above]
> > > 
> > > static const struct mtk_pll_div_table mmpll_div_rate[] = {
> > > 
> > > 	{ .freq = 1000000000, .div = 0 },
> > > 	{ .freq = 702000000, .div = 1 },
> > > 	{ .freq = 253500000, .div = 2 },
> > > 	{ .freq = 126750000, .div = 3 },
> > > 	{ /* sentinel */ },
> > > 
> > > };
> > 
> > The freq-div table describes the maximum frequency of each divider
> > setting. Although the first element doesn't used in current
> > implementation, I think it's better to keep freq-div table's
> > completeness.
> 
> the issue I see is, that its value is currently 0 and the code substracts 1. 
> So if anything would (accidentially) select MT8173_PLL_FMAX, the u32 val would 
> wrap around, as you're subtracting 1 from 0 .

Subtracting 1 from val is safe now because it starts from 1:

  for (val = 1; div_rate[val] != 0; val++) {
    ...
  }
  val--;

I can change this implementation to a more readable one such as:

  for (val = 0; div_rate[val + 1] != 0; val++) {
    if (freq <= div_rate[val] && freq > div_rate[val + 1]) {
      ...

Do you think it is OK?


Best regards,

James

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