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

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Date:	Mon, 13 Jul 2015 16:31:05 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Vaibhav Hiremath <vaibhav.hiremath@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Cc:	devicetree@...r.kernel.org, sameo@...ux.intel.com,
	linux-kernel@...r.kernel.org, robh+dt@...nel.org,
	lee.jones@...aro.org
Subject: Re: [PATCH 3/6] mfd: devicetree: bindings: 88pm800: Add DT property
 for 32KHz output enable

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@...aro.org>
>>> ---
>>>   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 μA
>                     CLK32K2 = 0  => 18 μW

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).

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

>> 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.

Best regards,
Krzysztof

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