Re: [PATCH 3/4] bus: Add DT bindings for 96Boards low speed connector

From: Rob Herring <robh+dt@kernel.org>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Michal Simek <michal.simek@xilinx.com>,
	dev@lists.96boards.org, Mark Brown <broonie@kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Andy Shevchenko <andy@infradead.org>,
	Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
	Frank Rowand <frowand.list@gmail.com>,
	"moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 3/4] bus: Add DT bindings for 96Boards low speed connector
Date: Thu, 23 Aug 2018 08:45:52 -0500	[thread overview]
Message-ID: <CAL_JsqK2tdGHkeJ0OceLbBiki7VUqKrstDSiB7KRpZ-FExHtQw@mail.gmail.com> (raw)
In-Reply-To: <20180823103332.32047-4-linus.walleij@linaro.org>

On Thu, Aug 23, 2018 at 5:33 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> We need to be able to grab resources on a 96Boards low
> speed connector to populate and use a daughterboard, so
> define some very rudimentary properties simply passing
> phandles for the resources used on the low speed
> connector.

This only works for describing what's on the connector, but does
nothing for describing downstream devices on the connector. We already
have a way to do the former with label properties at least for uart,
i2c and spi. And gpio has it's own method of labeling lines.

> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  .../bus/96boards,low-speed-connector.txt      | 50 +++++++++++++++++++
>  1 file changed, 50 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/bus/96boards,low-speed-connector.txt
>
> diff --git a/Documentation/devicetree/bindings/bus/96boards,low-speed-connector.txt b/Documentation/devicetree/bindings/bus/96boards,low-speed-connector.txt
> new file mode 100644
> index 000000000000..6631b3edd01f
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/bus/96boards,low-speed-connector.txt
> @@ -0,0 +1,50 @@
> +96Boards Low Speed Connectors
> +
> +The 96Boards connectors represent a non-dicoverable Low Speed (LS)
> +and High Speed (HS) external bus on the 96Board development
> +boards. Each 96Board presents both of these connectors.
> +
> +The Low Speed (LS) connector is a 40 pin 2x20 female pin header:
> +- Power
> +- Two buttons: power and reset
> +- 2 x UART one optional with just RX/TX, one required with modem
> +  CTS/RTS
> +- 2 x I2C
> +- 1 x SPI
> +- 1 x I2S (audio)
> +- 12 x GPIO lines named GPIO-A thru GPIO-L
> +
> +Further details on the electronics and signals are available in
> +"96Boards Consumer Edition, Low Cost Hardware Platform Specification"
> +As of writing version 1.0, January 2015.
> +
> +Required properties:
> +
> +- compatible: shall be "96boards,low-speed-connector"
> +- i2c0: phandle to the I2C0 bus
> +- i2c1: phandle to the I2C1 bus
> +- spi: phandle to the SPI bus
> +- gpios: a list of phandles to the GPIOs connected to the
> +  connector. The handles need to be uniformly specified with the
> +  0 flag as the connector is not the end consumer.
> +
> +Example:
> +
> +lscon: connector {
> +       compatible = "96boards,low-speed-connector";
> +       i2c0 = <&i2csw_0>;
> +       i2c1 = <&i2csw_1>;
> +       spi = <&spi0>;

David G has suggested using aliases (in this node, not top-level) for
this purpose of mapping things like spi and i2c, and I think that's a
good use of aliases (versus spi and i2c aliases at the top level which
I don't). The hard part is still how to structure the child devices.

> +       gpios = <&gpio 36 0>, /* GPIO-A */

We defined 'gpio-map' specifically for handling GPIOs thru connectors.
It's in the DT spec (though not a released version yet). Use it here.

Sorry, but the only halfway solution I'm going to accept is one which
can evolve into a full solution. Define the basic structure and add
each interface one by one. Maybe that starts with just:

lscon: connector {
       compatible = "96boards,low-speed-connector";
};

That's not really my recommendation as that's really just kicking the
can down the road.

Rob