Re: [RFC 3/5] dt-bindings: i3c: Document core bindings

Rob Herring
2017-08-09T23:43:08Z

On Mon, Jul 31, 2017 at 06:24:48PM +0200, Boris Brezillon wrote:
> A new I3C subsystem has been added and a generic description has been
> created to represent the I3C bus and the devices connected on it.
>
> Document this generic representation.
>
> Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> ---
> Documentation/devicetree/bindings/i3c/i3c.txt | 90 +++++++++++++++++++++++++++
> 1 file changed, 90 insertions(+)
> create mode 100644 Documentation/devicetree/bindings/i3c/i3c.txt
>
> diff --git a/Documentation/devicetree/bindings/i3c/i3c.txt b/Documentation/devicetree/bindings/i3c/i3c.txt
> new file mode 100644
> index 000000000000..49261dec7b01
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/i3c/i3c.txt
> @@ -0,0 +1,90 @@
> +Generic device tree bindings for I3C busses
> +===========================================
> +
> +This document describes generic bindings that should be used to describe I3C
> +busses in a device tree.
> +
> +Required properties
> +-------------------
> +
> +- #address-cells - should be <1>. Read more about addresses below.
> +- #size-cells - should be <0>.
> +- compatible - name of I3C bus controller following generic names
> + recommended practice.
> +
> +For other required properties e.g. to describe register sets,
> +clocks, etc. check the binding documentation of the specific driver.
> +
> +Optional properties
> +-------------------
> +
> +These properties may not be supported by all I3C master drivers. Each I3C
> +master bindings should specify which of them are supported.
> +
> +- i3c-scl-frequency: frequency (in Hz) of the SCL signal used for I3C
> + transfers. When undefined the core set it to 12.5MHz.
> +
> +- i2c-scl-frequency: frequency (in Hz) of the SCL signal used for I2C
> + transfers. When undefined, the core looks at LVR values
> + of I2C devices described in the device tree to determine
> + the maximum I2C frequency.
> +
> +I2C devices
> +===========
> +
> +Each I2C device connected to the bus should be described in a subnode with
> +the following properties:
> +
> +All properties described in Documentation/devicetree/bindings/i2c/i2c.txt are
> +valid here.
> +
> +New required properties:
> +------------------------
> +- i3c-lvr: 32 bits integer property (only the lowest 8 bits are meaningful)

What does lvr mean?

> + describing device capabilities as described in the I3C
> + specification.
> +
> + bit[31:8]: unused
> + bit[7:5]: I2C device index. Possible values

index? Seems more like flags

> + * 0: I2C device has a 50 ns spike filter
> + * 1: I2C device does not have a 50 ns spike filter but supports high
> + frequency on SCL
> + * 2: I2C device does not have a 50 ns spike filter and is not
> + tolerant to high frequencies
> + * 3-7: reserved
> +
> + bit[4]: tell whether the device operates in FM or FM+ mode
> + * 0: FM+ mode
> + * 1: FM mode
> +
> + bit[3:0]: device type
> + * 0-15: reserved

That's useful...

> +
> +I3C devices
> +===========
> +
> +I3C are not described in the device tree yet. We could decide to represent them
> +at some point to assign a specific dynamic address to a device or to force an
> +I3C device to act as an I2C device if it has a static address.

I think we need to define this sooner rather than later if there's not a
standard connector. That's the only thing that would enforce any sort of
standard. Of course, that didn't help with SDIO.

> +
> +Example:
> +
> + i3c-master@0d040000 {

The node name should go into the DT spec. I tend to think "i3c" would be
sufficient and aligned with i2c.

> + compatible = "cdns,i3c-master";
> + clocks = <&coreclock>, <&i3csysclock>;
> + clock-names = "pclk", "sysclk";
> + interrupts = <3 0>;
> + reg = <0x0d040000 0x1000>;
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + status = "okay";
> + i2c-scl-frequency = <100000>;
> +
> + nunchuk: nunchuk@52 {
> + compatible = "nintendo,nunchuk";
> + reg = <0x52>;
> + i3c-lvr = <0x10>;
> + };
> + };
> +
> --
> 2.7.4
>