spi: Add support for Realtek RTL838x/RTL839x SoC SPI

v4:
- Added SoC series-specific compatible matches.

v3:
- Added cpu_relax() to busy loop.
- Dropped .remove callback from driver struct.
- Use (variations of) realtek-rtl as prefix.
- Dropped Kconfig entry, and use MACH_REALTEK_RTL setting to build the driver, since there's no point booting without the SPI-connected flash.

v2:
- Rewrote from spi-nor driver to regular spi driver, implementing only set_cs() and transfer_one(). (Thanks Chuanhong Guo!)

Bert Vermeulen (2):
dt-bindings: spi: Realtek RTL838x/RTL839x SPI controller
spi: realtek-rtl: Add support for Realtek RTL838x/RTL839x SoC SPI
controllers

 .../bindings/spi/realtek,rtl-spi.yaml         |  38 ++++
 MAINTAINERS                                   |   6 +
 drivers/spi/Makefile                          |   1 +
 drivers/spi/spi-realtek-rtl.c                 | 206 ++++++++++++++++++
 4 files changed, 251 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/realtek,rtl-spi.yaml
 create mode 100644 drivers/spi/spi-realtek-rtl.c

2.25.1

---
 .../bindings/spi/realtek,rtl-spi.yaml         | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/realtek,rtl-spi.yaml

new file mode 100644
index 000000000000..c600e85b819b
@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/realtek,rtl-spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Realtek RTL838x/RTL839x SPI controller
+
+maintainers:
+  - Bert Vermeulen <[email protected]>
+  - Birger Koblitz <[email protected]>
+
+allOf:
+  - $ref: "spi-controller.yaml#"
+
+properties:
+  compatible:
+    oneOf:
+      - const: realtek,rtl838x-spi
+      - const: realtek,rtl839x-spi
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    spi: spi@1200 {
+        compatible = "realtek,rtl838x-spi";
+        reg = <0x1200 0x100>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+    };

2.25.1

This driver likely also supports earlier (RTL8196) and later (RTL93xx) SoCs.

The SPI hardware in these SoCs is specifically intended for connecting NOR bootflash chips, and only used for that in dozens of examined devices. However boiled down to basics, it's really just a half-duplex SPI controller.

The hardware appears to have a vestigial second chip-select control, but it hasn't been seen in the wild and is thus not supported.

---
 MAINTAINERS                   |   6 +
 drivers/spi/Makefile          |   1 +
 drivers/spi/spi-realtek-rtl.c | 206 ++++++++++++++++++++++++++++++++++
 3 files changed, 213 insertions(+)
 create mode 100644 drivers/spi/spi-realtek-rtl.c

index ad0e34bf8453..c6cb954e5ecf 100644
@@ -15048,6 +15048,12 @@ F:	Documentation/devicetree/bindings/net/dsa/realtek-smi.txt
 F:	drivers/net/dsa/realtek-smi*
 F:	drivers/net/dsa/rtl83*
 
+REALTEK RTL83XX SPI DRIVER
+M:	Bert Vermeulen <[email protected]>
+M:	Birger Koblitz <[email protected]>
+S:	Maintained
+F:	drivers/spi/spi-realtek-rtl.c
+
 REALTEK WIRELESS DRIVER (rtlwifi family)
 M:	Ping-Ke Shih <[email protected]>
 L:	[email protected]

index 6fea5821662e..182adef17013 100644
@@ -94,6 +94,7 @@ obj-$(CONFIG_SPI_QCOM_QSPI)		+= spi-qcom-qspi.o
 obj-$(CONFIG_SPI_QUP)			+= spi-qup.o
 obj-$(CONFIG_SPI_ROCKCHIP)		+= spi-rockchip.o
 obj-$(CONFIG_SPI_RB4XX)			+= spi-rb4xx.o
+obj-$(CONFIG_MACH_REALTEK_RTL)		+= spi-realtek-rtl.o
 obj-$(CONFIG_SPI_RPCIF)			+= spi-rpc-if.o
 obj-$(CONFIG_SPI_RSPI)			+= spi-rspi.o
 obj-$(CONFIG_SPI_S3C24XX)		+= spi-s3c24xx-hw.o

2.25.1