Date:	Mon, 14 Dec 2015 16:42:29 -0800
From:	Kevin Hilman <khilman@...nel.org>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	Philipp Zabel <p.zabel@...gutronix.de>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	"Alexandre Courbot" <gnurou@...il.com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Vince Hsu <vinceh@...dia.com>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
	<linux-pm@...r.kernel.org>
Subject: Re: [PATCH V4 12/16] Documentation: DT: bindings: Add power domain info for NVIDIA PMC

Jon Hunter <jonathanh@...dia.com> writes:

> On 09/12/15 12:23, Jon Hunter wrote:
>> 
>> On 08/12/15 19:07, Kevin Hilman wrote:
>>> Jon Hunter <jonathanh@...dia.com> writes:
>>>
>>>> Add power-domain binding documentation for the NVIDIA PMC driver in
>>>> order to support generic power-domains.
>>>>
>>>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
>>>>
>>>> ---
>>>>
>>>> Please note that I have been debating whether I add this
>>>> "nvidia,powergate-clock-disable" property or just leave the clocks
>>>> disabled by default. Some downstream kernels leave the clocks enabled
>>>> for the audio power-domain because the clocks required for powering up
>>>> the power-domain are needed by all modules within the power-domain.
>>>> However are the same time there are other power-domains that may need
>>>> to be on, but not always clocked and so having the ability to specify if
>>>> the clocks should be disabled seems useful. However, I can also remove
>>>> this and just have the appropriate devices turn on the clocks as well.
>>>> ---
>>>>  .../bindings/arm/tegra/nvidia,tegra20-pmc.txt      | 61 ++++++++++++++++++++++
>>>>  1 file changed, 61 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-pmc.txt b/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-pmc.txt
>>>> index 838e1a69ec0a..8e4641db51a9 100644
>>>> --- a/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-pmc.txt
>>>> +++ b/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-pmc.txt
>>>> @@ -1,5 +1,7 @@
>>>>  NVIDIA Tegra Power Management Controller (PMC)
>>>>  
>>>> +== Power Management Controller Node ==
>>>> +
>>>>  The PMC block interacts with an external Power Management Unit. The PMC
>>>>  mostly controls the entry and exit of the system from different sleep
>>>>  modes. It provides power-gating controllers for SoC and CPU power-islands.
>>>> @@ -69,6 +71,10 @@ Optional properties for hardware-triggered thermal reset (inside 'i2c-thermtrip'
>>>>                       Defaults to 0. Valid values are described in section 12.5.2
>>>>                       "Pinmux Support" of the Tegra4 Technical Reference Manual.
>>>>  
>>>> +Optional nodes:
>>>> +- pm-domains : This node contains a hierarchy of PM domain nodes, which should
>>>> +	       match the power-domains on the Tegra SoC.
>>>> +
>>>>  Example:
>>>>  
>>>>  / SoC dts including file
>>>> @@ -114,3 +120,58 @@ pmc@...0f400 {
>>>>  	};
>>>>  	...
>>>>  };
>>>> +
>>>> +
>>>> +== PM Domain Nodes ==
>>>> +
>>>> +Each of the PM domain nodes represents a power-domain on the Tegra SoC
>>>> +that can be power-gated by the PMC and should be named appropriately.
>>>> +
>>>> +Required properties:
>>>> +  - clocks: Must contain an entry for each clock required by the PMC for
>>>> +    controlling a power-gate. See ../clocks/clock-bindings.txt for details.
>>>
>>> We've had this discussiona for a couple of other SoCs, so I need to
>>> ask...
>>>
>>> Presumably these are not device clocks that a runtime PM enabled driver
>>> should be managing for a device, right?  IOW, We want to make sure that the
>>> PM domain isn't managing clocks for drivers that should be doing it.
>>>
>>> I understand there are legitimate reasons for the PM domain to manage
>>> clocks in addition to device drivers (e.g. for synchronous reset), but
>>> just want to be sure it's not a shortcut for having a proper driver.
>> 
>> So some clocks may also be used by devices, but they are needed as part
>> of the power ungating/gating sequence. The general power-up sequence for
>> tegra is ...
>> 
>> 1. Enable the power-domain
>> 2. Enable the clock(s)
>> 3. Remove signal clamps
>> 4. De-assert reset(s)
>> 5. Disable clocks (optional)
>> 
>> You may say we should only handle #1 above for the powering up sequence,
>> but we can't do this. The reason is that there is one bit for each
>> power-domain that controls the signaling clamps and so we need to turn
>> on all the clocks specified in the TRM before we do this. Once we have
>> done this and released the reset(s), we can then disable the clocks
>> again (shown above an optional as it is not mandatory from a design
>> perspective) and then the devices in the power-domain should enable the
>> clocks they need as and when they want them.
>> 
>> Please note that I 100% agree that all clocks required by a device are
>> handled by the device and we do not implement any short-cuts here. The
>> only question I had was if there are clocks that may be bus clocks in
>> the power-domain that are required by all device in the power-domain.
>> However, may be this should be represented as a bus driver and all the
>> devices are children of it?
>
> Although the "nvidia,powergate-disable-clocks" optional property I had
> proposed could be seen as a bit of a short-cut, it is true :-)
> May be I should make the disabling of clocks again mandatory for the
> power-up sequence.

IIUC, that looks like a flag that tells the power-domain to just leave
all the clocks enabled after the domain power up?  Is that right?

To me that looks like possibly useful bringup hack, but a short-cut
that's ripe for abuse and would likely be used so that drivers don't
ever need to do their own clock management.

Kevin
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