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

[Chen-Yu Tsai]

On Fri, Jan 5, 2018 at 3:28 AM, Jernej Škrabec <jernej.skrabec at siol.net> wrote:
> 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 at 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 at 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.

Sounds like we can't use u8 for storing the factors. At least the
intermediate values we use to calculate the rates.

>
> 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 think this is acceptable. M is normally the pre-divider, so this
actually fits how the hardware works, including possible rounding
errors.

ChenYu

> 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
>
>
>
>
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