Date:   Tue, 24 Jan 2017 12:05:20 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Stephen Boyd <stephen.boyd@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v3 1/3] of: Support parsing phandle argument lists through
 a nexus node

On Tue, Jan 24, 2017 at 12:36 AM, Frank Rowand <frowand.list@...il.com> wrote:
> Hi Stephen,
>
> Sorry I did not get to v1 and v2 in a timely manner.
>
>
> On 01/23/17 12:48, Stephen Boyd wrote:
>> Platforms like 96boards have a standardized connector/expansion
>> slot that exposes signals like GPIOs to expansion boards in an
>> SoC agnostic way. We'd like the DT overlays for the expansion
>> boards to be written once without knowledge of the SoC on the
>> other side of the connector. This avoids the unscalable
>> combinatorial explosion of a different DT overlay for each
>> expansion board and SoC pair.
>>
>> We need a way to describe the GPIOs routed through the connector
>> in an SoC agnostic way. Let's introduce nexus property parsing
>> into the OF core to do this. This is largely based on the
>> interrupt nexus support we already have. This allows us to remap
>> a phandle list in a consumer node (e.g. reset-gpios) through a
>> connector in a generic way (e.g. via gpio-map). Do this in a
>> generic routine so that we can remap any sort of variable length
>> phandle list.
>>
>> Taking GPIOs as an example, the connector would be a GPIO nexus,
>> supporting the remapping of a GPIO specifier space to multiple
>> GPIO providers on the SoC. DT would look as shown below, where
>> 'soc_gpio1' and 'soc_gpio2' are inside the SoC, 'connector' is an
>> expansion port where boards can be plugged in, and
>> 'expansion_device' is a device on the expansion board.
>>
>>       soc {
>>               soc_gpio1: gpio-controller1 {
>>                       #gpio-cells = <2>;
>>               };
>>
>>               soc_gpio2: gpio-controller2 {
>>                       #gpio-cells = <2>;
>>               };
>>       };
>>
>>       connector: connector {
>>               #gpio-cells = <2>;
>>               gpio-map = <0 0 &soc_gpio1 1 0>,
>>                          <1 0 &soc_gpio2 4 0>,
>>                          <2 0 &soc_gpio1 3 0>,
>>                          <3 0 &soc_gpio2 2 0>;
>>               gpio-map-mask = <0xf 0x0>;
>>               gpio-map-pass-thru = <0x0 0x1>
>>       };
>>
>>       expansion_device {
>>               reset-gpios = <&connector 2 GPIO_ACTIVE_LOW>;
>>       };
>
> The how to architect connectors and plugs threads fell asleep before
> coming to a resolution.  We need to revive that discussion.
>
> One of the concepts of the plug and connector architecture is that
> a main board may contain multiple connectors of the same type (or
> different types, but the same type is sufficient for this discussion).
>
> The node describing the card that plugs into one of the connectors
> does not know the phandle of the connector it is going to be
> connected to.  Some other mechanism is provided to allow a card
> to be plugged into any of the available connectors.  If there are
> two identical cards plugged into two connectors, then both cards
> have the same exact device tree node.  But some mechanism will
> exist to resolve (or "link") the two card nodes to the different
> connector nodes.
>
> As a result of this, in the above example the reset-gpios property
> in the node 'expansion_device' can not contain '&connector'.  The
> concept of &connector belongs to the entire expansion_device node,
> not to individual properties within the node.

I think this is easily solved with a connector having 2 halves and
that we need to search parents for *-map properties. Inheriting from
parents is a common pattern in DT though perhaps not walking the
parents of a phandle. So we'd have something like this:

base-connector-1 {
  gpio-map = ...
  connector {
    child {
      some-gpios = <&connector 1>;
    };
  };
};

base-connector-2 {
  gpio-map = ...
  connector {
    child {
      some-gpios = <&connector 1>;
    };
  };
};

Now, how we resolve that /connector from an overlay targets
/base-connector-1 and /base-connector-2 is an orthogonal issue and one
that's going to be connector specific (at least for probe-able
connectors).

Rob

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