Date:	Thu, 3 Apr 2014 15:32:48 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Tomasz Figa <tomasz.figa@...il.com>
Cc:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Russell King <linux@....linux.org.uk>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Kumar Gala <galak@...eaurora.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Tomasz Figa <t.figa@...sung.com>,
	Mark Brown <broonie@...nel.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2 01/11] base: power: Add generic OF-based power domain look-up

On 3 April 2014 14:30, Tomasz Figa <tomasz.figa@...il.com> wrote:
> Hi Ulf,
>
>
> On 03.04.2014 14:16, Ulf Hansson wrote:
>>
>> On 3 March 2014 17:02, Tomasz Figa <tomasz.figa@...il.com> wrote:
>>>
>>> This patch introduces generic code to perform power domain look-up using
>>> device tree and automatically bind devices to their power domains.
>>> Generic device tree binding is introduced to specify power domains of
>>> devices in their device tree nodes.
>>>
>>> Backwards compatibility with legacy Samsung-specific power domain
>>> bindings is provided, but for now the new code is not compiled when
>>> CONFIG_ARCH_EXYNOS is selected to avoid collision with legacy code. This
>>> will change as soon as Exynos power domain code gets converted to use
>>> the generic framework in further patch.
>>>
>>> Signed-off-by: Tomasz Figa <tomasz.figa@...il.com>
>>> ---
>>>   .../devicetree/bindings/power/power_domain.txt     |  51 ++++
>>>   drivers/base/power/domain.c                        | 298
>>> +++++++++++++++++++++
>>>   include/linux/pm_domain.h                          |  46 ++++
>>>   kernel/power/Kconfig                               |   4 +
>>>   4 files changed, 399 insertions(+)
>>>   create mode 100644
>>> Documentation/devicetree/bindings/power/power_domain.txt
>>>
>>> diff --git a/Documentation/devicetree/bindings/power/power_domain.txt
>>> b/Documentation/devicetree/bindings/power/power_domain.txt
>>> new file mode 100644
>>> index 0000000..93be5d9
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/power/power_domain.txt
>>> @@ -0,0 +1,51 @@
>>> +* Generic power domains
>>> +
>>> +System on chip designs are often divided into multiple power domains
>>> that
>>> +can be used for power gating of selected IP blocks for power saving by
>>> +reduced leakage current.
>>> +
>>> +This device tree binding can be used to bind power domain consumer
>>> devices
>>> +with their power domains provided by power domain providers. A power
>>> domain
>>> +provider can be represented by any node in the device tree and can
>>> provide
>>> +one or more power domains. A consumer node can refer to the provider by
>>> +a phandle and a set of phandle arguments (so called power domain
>>> specifier)
>>> +of length specified by #power-domain-cells property in the power domain
>>> +provider node.
>>> +
>>> +==Power domain providers==
>>> +
>>> +Required properties:
>>> + - #power-domain-cells : Number of cells in a power domain specifier;
>>> +   Typically 0 for nodes representing a single power domain and 1 for
>>> nodes
>>> +   providing multiple power domains (e.g. power controllers), but can be
>>> +   any value as specified by device tree binding documentation of
>>> particular
>>> +   provider.
>>
>>
>> I am trying to understand if using a value > 1, ever would make sense.
>> Wouldn't that mean each consumer (device) would actually be a part of
>> more than one power domain? That won't work, right!?
>
>
> Not exactly. Each phandle + #power-domain-cells cells are used just for
> single power domain.
>
> The cells here are used merely as the identifier used by power domain driver
> to translate a power domain specifier from DT to a kernel pointer. It's up
> to driver bindings to select the number of cells to properly identify a
> power domain.
>
> As an example (from different world, but showing the same mechanism), let's
> see a common pattern of GPIO banks on some SoC:
>
> GPA0
> GPA1
> GPB0
> GPB1
> GPC0
> GPC1
>
> One might assign a single-cell ID to each bank ending with a namespace of
> integers from 0 to 5 that would be used as follows:
>
> #define GPA0 0
> #define GPA1 1
> #define GPB0 2
> #define GPB1 3
> #define GPC0 4
> #define GPC1 5
>
> reset-gpios = <&gpio GPA0 4>;
>
> However one might also consider assigning one cell to bank set (e.g. GPA)
> and one cell to identify the bank inside of a set, like on the following
> example:
>
> #define GPA 0
> #define GPB 1
> #define GPC 2
>
> reset-gpios = <&gpio GPA 0 4>;
>
> Good bindings should allow arbitrary identification schemes to let a driver
> developer use the one that suits the hardware he's working on.
>
>
>>
>> Additionally, there are no corresponding parsing method (like
>> of_genpd_xlate_onecell() ) that support more than one cell.
>
>
> There are two generic xlate helpers provided for the most common cases that
> are likely to be reused by most drivers. For more complex cases it's up to
> the driver to implement its own mapping function. It can be promoted to a
> generic one later if such need shows up.
>
> Best regards,
> Tomasz

Tomasz, thanks for the clarification! I still have more to learn about DT. :-)

Not sure if some additional comments would make this more clear though
- or if it's juts my untrained eye that had a few problems
understanding.

Kind regards
Ulf Hansson
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