SPI
===

The Duckbill SPI is intended to be used in the evaluation kit “PLC bundle for Internet of Things”.
Thus the device tree file contains a section which refers to the Qualcomm Atheros QCA7000 serial-to-powerline bridge.

The settings for the QCA driver are listed in the following table.

+------------------------------+-----------------+-------------------+
| Attribute                    | Source          | SSP2              |
+==============================+=================+===================+
| Type                         | Modprobe config | External          |
+------------------------------+-----------------+-------------------+
| MAC address                  | Device tree     | Value from OTP    |
+------------------------------+-----------------+-------------------+
| Max SPI clock rate           | Device tree     | 8 MHz             |
+------------------------------+-----------------+-------------------+
| Actual SPI clock rate        | Clock tree      | 6 MHz             |
+------------------------------+-----------------+-------------------+
| SPI mode                     | Device tree     | 3                 |
+------------------------------+-----------------+-------------------+
| Expected SPI slave           | Device tree     | QCA7000           |
+------------------------------+-----------------+-------------------+
| Expected command mode        | Device tree     | burst             |
+------------------------------+-----------------+-------------------+

The device tree binding for the QCA driver can be found in the Kernel sources:
Documentation/devicetree/bindings/net/qca-qca7000-spi.txt

Note 1: The MAC address is added dynamically by U-Boot into the device tree.

Note 2: The reference clock for SSP2 is switched from ref_io1 to ref_xtal by an additional kernel patch.
This limits the maximum SPI frequency to 12 MHz. This is due to observed bit errors when ref_io1 is
used as clock source. However, for the use-case with QCA7000, the maximum clock rate is already limited
by the QCA7000 which only supports up to 12 MHz. The reason for the actual 6 MHz is, that the SPI leaves the
Duckbill via the connector and “flying wires”.

Due the limitation of the selected clock and SPI the following SPI clock rates are available:

* 12.0 MHz
* 6.0 MHz
* 4.0 MHz
* 3.0 MHz
* 2.4 MHz
* 2.0 MHz
* 1.72 MHz
* 1.5 MHz
* 1.34 MHz
* 1.2 MHz
* 1.1 MHz
* 1.0 MHz