Re: [PATCH 3/6] mfd: devicetree: bindings: 88pm800: Add DT property for 32KHz output enable

[Vaibhav Hiremath]

On Monday 13 July 2015 01:01 PM, Krzysztof Kozlowski wrote:
> On 13.07.2015 16:24, Vaibhav Hiremath wrote:
> > On Saturday 11 July 2015 12:41 PM, Krzysztof Kozlowski wrote:
> > > W dniu 09.07.2015 o 20:47, Vaibhav Hiremath pisze:
> > > > 88PM800 family of device supports output of 32KHz clock (low jitter)
> > > > on CLK32K2/3 pin which can be supplied to other peripherals on the
> > > > board.
> > > >
> > > > This patch adds the devicetree binding to enable this feature.
> > > >
> > > > Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@xxxxxxxxxx>
> > > > ---
> > > >    Documentation/devicetree/bindings/mfd/88pm800.txt | 6 ++++++
> > > >    1 file changed, 6 insertions(+)
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/mfd/88pm800.txt
> > > > b/Documentation/devicetree/bindings/mfd/88pm800.txt
> > > > index dec842f..ae1311c 100644
> > > > --- a/Documentation/devicetree/bindings/mfd/88pm800.txt
> > > > +++ b/Documentation/devicetree/bindings/mfd/88pm800.txt
> > > > @@ -9,6 +9,12 @@ Required parent device properties:
> > > >    - #interrupt-cells     : should be 1.
> > > >                  The cell is the 88pm80x local IRQ number
> > > >
> > > > +Optional properties :
> > > > + - marvell,88pm800-32khz-xolj-out-en    : If set, driver will enable
> > > > low jitter
> > > > +   version of 32Khz clock output on
> > >
> > > I am not sure if I understand it correctly. The hardware always has such
> > > clocks and you only want to enable/disable it in DT? Any reasons why
> > > these should not be enabled always?
> >
> > Small amount of Power savings...
> > Although currently I do not have power numbers to justify this.
> >
> > As per spec, (it only talks about power consumption in power down state)
> >
> > Power-down State => VSYS > 2.8   => 4.5 ÎA
> >                      CLK32K2 = 0  => 18 ÎW
>
> This would be a power saving if it could be enabled/disabled runtime.
> But with DT you will either:
> 1. enable it always so there won't be any power saving,
> 2. disable it always so there won't be such clock.
>
> I can find a use case - when on some boards the clock output is not
> wired to anything so it cannot be used. In other cases (there is some
> potential user) this should be triggered per-use (runtime enabled/disabled).

Exactly,

> Do you have such case? I mean boards where this is not connected at all
> and boards where this is used?

The PXA1928 based board which I have,

CLK32K1 (G3 pad)  = Used by Wireless chip
CLK32K2 (G11 pad) = Not connected to anything, so we need to disable
                    this output

> > > Enabling it in DT does not look like a job for DT. Maybe you there
> > > should be just a clock driver (clock provider)?
> >
> > It's init time (and one time) settings, wouldn't clock-provider
> > be overkill for this?
>
> The clock provider would be a proper way to do it and some PMICs I know
> do this. Examples are max77686 and s5m8767/s2mps11. These are PMICs used
> for Samsung SoCs. They have two or three 32kHz clocks.

This is good to know.
Let me take a look at these drivers.

Thanks,
Vaibhav

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