[PATCH 1/9] clk: clk-divider: add CLK_DIVIDER_ZERO_GATE clk support

[A.s. Dong]

> -----Original Message-----
> From: Stephen Boyd [mailto:sboyd at codeaurora.org]
> Sent: Saturday, July 01, 2017 8:56 AM
> To: Dong Aisheng
> Cc: A.s. Dong; linux-clk at vger.kernel.org; linux-kernel at vger.kernel.org;
> linux-arm-kernel at lists.infradead.org; mturquette at baylibre.com;
> shawnguo at kernel.org; Anson Huang; Jacky Bai
> Subject: Re: [PATCH 1/9] clk: clk-divider: add CLK_DIVIDER_ZERO_GATE clk
> support
> 
> On 06/20, Dong Aisheng wrote:
> > Hi Stephen,
> >
> > On Mon, Jun 19, 2017 at 06:45:12PM -0700, Stephen Boyd wrote:
> > > On 05/15, Dong Aisheng wrote:
> > > > ---
> > > >  drivers/clk/clk-divider.c    | 2 ++
> > > >  include/linux/clk-provider.h | 4 ++++
> > > >  2 files changed, 6 insertions(+)
> > > >
> > > > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > > > index 96386ff..f78ba7a 100644
> > > > --- a/drivers/clk/clk-divider.c
> > > > +++ b/drivers/clk/clk-divider.c
> > > > @@ -125,6 +125,8 @@ unsigned long divider_recalc_rate(struct
> > > > clk_hw *hw, unsigned long parent_rate,
> > > >
> > > >  	div = _get_div(table, val, flags, divider->width);
> > > >  	if (!div) {
> > > > +		if (flags & CLK_DIVIDER_ZERO_GATE)
> > > > +			return 0;
> > > >  		WARN(!(flags & CLK_DIVIDER_ALLOW_ZERO),
> > >
> > > Why not use the CLK_DIVIDER_ALLOW_ZERO flag? A clk being off doesn't
> > > mean the rate is 0. The divider is just disabled, so we would
> > > consider the rate as whatever the parent is, which is what this code
> > > does before this patch. Similarly, we don't do anything about gate
> > > clocks and return a rate of 0 when they're disabled.
> > >
> >
> > The semantic of CLK_DIVIDER_ALLOW_ZERO seems a bit different.
> >
> > See below definition:
> > * CLK_DIVIDER_ALLOW_ZERO - Allow zero divisors.  For dividers which
> have
> > *      CLK_DIVIDER_ONE_BASED set, it is possible to end up with a zero
> divisor.
> > *      Some hardware implementations gracefully handle this case and
> allow a
> > *      zero divisor by not modifying their input clock
> > *      (divide by one / bypass).
> >
> > zero divisor is simply as divide by one or bypass which is supported
> > by hardware.
> >
> > But it's not true for this hardware.
> >
> > If we consider the rate as whatever the parent is if divider is zero,
> > we may got an issue like below:
> > e.g.
> > Assuming spll_bus_clk divider is 0x0 and it may be enabled by users
> > directly without setting a rate first.
> >
> > Then the clock tree looks like:
> > ...
> > spll_pfd0                    1            1   500210526          0 0
> >   spll_pfd_sel              1            1   500210526          0 0
> >     spll_sel               1            1   500210526          0 0
> >       spll_bus_clk           1            1   500210526          0 0
> >
> > But the spll_bus_clk clock rate actually is wrong and it's even not
> > enabled, not like CLK_DIVIDER_ALLOW_ZERO which zero divider means
> simply bypass.
> >
> > So for this case, we probably can't simply assume zero divider rate as
> > its parent, it is actually set to 0 in hw, although it's something
> > like gate, but a bit different from gate as the normal gate does not
> > affect divider where you can keep the rate.
> >
> > How would you suggest for this?
> >
> 
> It seems that set_rate() and enable/disable are conflated here.
> From what you describe, it sounds like the clk is considered off when the
> divider value is zero, and it's on when the divider value is non-zero.
> 
> I'd suggest you make it so this clk supports enable/disable and set_rate
> with the same register. Something like, set rate when the clk is disabled
> will cache the rate request and only when the clk is enabled will the
> driver actually program the hardware to have the requested divider value.
> Similarly, when the clk is disabled we'll write a 0 there, but when the
> clk is enabled we'll restore whatever rate (divider) was chosen last.
> 
> It does mean that recalc rate will be sort of odd, because when the clk
> is off it will return 0, and when the clk is on it will return the right
> rate. So to make things work, we'll need to return the cached rate in
> recalc rate when the clk is off and read the hardware when the clk is on.
> Probably an if register ==
> 0 then lookup in cache, otherwise do normal division.
> 

Well, good suggestions to me.
It makes the implementation of this clock type more comprehensive.

It seems we still need keep CLK_DIVIDER_ZERO_GATE to distinguish with others.

The change for recacle_rate may looks like:
diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index f78ba7a..7364ac4 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -125,8 +125,9 @@ unsigned long divider_recalc_rate(struct clk_hw *hw, unsigned long parent_rate,
 
        div = _get_div(table, val, flags, divider->width);
        if (!div) {
+               if ((flags & CLK_DIVIDER_ZERO_GATE) && clk_hw_is_enabled(hw))
+                       return divider->cached_rate;
+
                WARN(!(flags & CLK_DIVIDER_ALLOW_ZERO),
                        "%s: Zero divisor and CLK_DIVIDER_ALLOW_ZERO not set\n",
                        clk_hw_get_name(hw));

And for the initial disabled state (div = 0), the calc_rate will still return
0 rate as there's still no set_rate happened.

If any issue, please let me know.

Regards
Dong Aisheng

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