Re: [PATCH 01/11] clk: sunxi-ng: Don't set k if width is 0 for nkmp plls

["Jernej Škrabec" <jernej.skrabec@siol.net>]

Hi,

Dne četrtek, 04. januar 2018 ob 15:45:18 CET je Chen-Yu Tsai napisal(a):
> On Sun, Dec 31, 2017 at 5:01 AM, Jernej Skrabec <jernej.skrabec@siol.net> 
wrote:
> > For example, A83T have nmp plls which are modelled as nkmp plls. Since k
> > is not specified, it has offset 0, shift 0 and lowest value 1. This
> > means that LSB bit is always set to 1, which may change clock rate.
> > 
> > Fix that by applying k factor only if k width is greater than 0.
> > 
> > Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
> > ---
> > 
> >  drivers/clk/sunxi-ng/ccu_nkmp.c | 21 +++++++++++++--------
> >  1 file changed, 13 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/clk/sunxi-ng/ccu_nkmp.c
> > b/drivers/clk/sunxi-ng/ccu_nkmp.c index e58c95787f94..709f528af2b3 100644
> > --- a/drivers/clk/sunxi-ng/ccu_nkmp.c
> > +++ b/drivers/clk/sunxi-ng/ccu_nkmp.c
> > @@ -81,7 +81,7 @@ static unsigned long ccu_nkmp_recalc_rate(struct clk_hw
> > *hw,> 
> >                                         unsigned long parent_rate)
> >  
> >  {
> >  
> >         struct ccu_nkmp *nkmp = hw_to_ccu_nkmp(hw);
> > 
> > -       unsigned long n, m, k, p;
> > +       unsigned long n, m, k = 1, p;
> > 
> >         u32 reg;
> >         
> >         reg = readl(nkmp->common.base + nkmp->common.reg);
> > 
> > @@ -92,11 +92,13 @@ static unsigned long ccu_nkmp_recalc_rate(struct
> > clk_hw *hw,> 
> >         if (!n)
> >         
> >                 n++;
> > 
> > -       k = reg >> nkmp->k.shift;
> > -       k &= (1 << nkmp->k.width) - 1;
> > -       k += nkmp->k.offset;
> > -       if (!k)
> > -               k++;
> > +       if (nkmp->k.width) {
> > +               k = reg >> nkmp->k.shift;
> > +               k &= (1 << nkmp->k.width) - 1;
> > +               k += nkmp->k.offset;
> > +               if (!k)
> > +                       k++;
> > +       }
> 
> The conditional shouldn't be necessary. With nkmp->k.width = 0,
> you'd simply get k & 0, which is 0, which then gets bumped up to 1,
> unless k.offset > 1, which would be a bug.
> 
> >         m = reg >> nkmp->m.shift;
> >         m &= (1 << nkmp->m.width) - 1;
> > 
> > @@ -153,12 +155,15 @@ static int ccu_nkmp_set_rate(struct clk_hw *hw,
> > unsigned long rate,> 
> >         reg = readl(nkmp->common.base + nkmp->common.reg);
> >         reg &= ~GENMASK(nkmp->n.width + nkmp->n.shift - 1, nkmp->n.shift);
> > 
> > -       reg &= ~GENMASK(nkmp->k.width + nkmp->k.shift - 1, nkmp->k.shift);
> > +       if (nkmp->k.width)
> > +               reg &= ~GENMASK(nkmp->k.width + nkmp->k.shift - 1,
> > +                               nkmp->k.shift);
> > 
> >         reg &= ~GENMASK(nkmp->m.width + nkmp->m.shift - 1, nkmp->m.shift);
> >         reg &= ~GENMASK(nkmp->p.width + nkmp->p.shift - 1, nkmp->p.shift);
> >         
> >         reg |= (_nkmp.n - nkmp->n.offset) << nkmp->n.shift;
> > 
> > -       reg |= (_nkmp.k - nkmp->k.offset) << nkmp->k.shift;
> > +       if (nkmp->k.width)
> > +               reg |= (_nkmp.k - nkmp->k.offset) << nkmp->k.shift;
> 
> I think a better way would be
> 
>         reg |= ((_nkmp.k - nkmp->k.offset) << nkmp->k.shift) &
>                GENMASK(nkmp->k.width + nkmp->k.shift - 1, nkmp->k.shift);
> 
> And do this for all the factors, not just k. This pattern is what
> regmap_update_bits does, which seems much safer. I wonder what
> GENMASK() with a negative value would do though...

You're right, GENMASK(-1, 0) equals 0 (calculated by hand, not tested). This 
seems much more elegant solution. 

Semi-related question: All nmp PLLs have much wider N range than real nkmp 
PLLs. This causes integer overflow when using nkmp formula from datasheet. 
Usually, N is 1-256 for nmp PLLs, which means that for very high N factors, it 
overflows. This also causes issue that M factor is never higher than 1.

I was wondering, if patch would be acceptable which would change this formula:

RATE = (24MHz * N * K) / (M * P)

to this:

RATE ((24MHz / M) * N * K) / P

I checked all M factors and are all in 1-4 or 1-2 range, which means it 
wouldn't have any impact for real nkmp PLLs when parent is 24 MHz clock which 
is probably always.

What do you think?

I discovered that when I tried to set A83T PLL_VIDEO to 346.5 MHz which is 
possible only when above formula is changed.

Best regards,
Jernej

> 
> ChenYu
> 
> >         reg |= (_nkmp.m - nkmp->m.offset) << nkmp->m.shift;
> >         reg |= ilog2(_nkmp.p) << nkmp->p.shift;
> > 
> > --
> > 2.15.1