emul: Add support for non-bus emulators

Adds a stub API for a non bus emulators. The stub is used to keep the
rest of the emulation consistent.

Signed-off-by: Yuval Peress <peress@google.com>

Author: yperess

include/zephyr/drivers/emul.h

@@ -36,6 +36,7 @@ enum emul_bus_type {
 	EMUL_BUS_TYPE_I2C,
 	EMUL_BUS_TYPE_ESPI,
 	EMUL_BUS_TYPE_SPI,
+	EMUL_BUS_TYPE_NONE,
 };
 
 /**
@@ -62,6 +63,14 @@ struct emul_list_for_bus {
  */
 typedef int (*emul_init_t)(const struct emul *emul, const struct device *parent);
 
+/**
+ * Emulator API stub when an emulator is not actually placed on a bus.
+ */
+struct no_bus_emul {
+	void *api;
+	uint16_t addr;
+};
+
 /** An emulator instance - represents the *target* emulated device/peripheral that is
  * interacted with through an emulated bus. Instances of emulated bus nodes (e.g. i2c_emul)
  * and emulators (i.e. struct emul) are exactly 1..1
@@ -82,6 +91,7 @@ struct emul {
 		struct i2c_emul *i2c;
 		struct espi_emul *espi;
 		struct spi_emul *spi;
+		struct no_bus_emul *none;
 	} bus;
 	/** Address of the API structure exposed by the emulator instance */
 	const void *backend_api;
@@ -101,10 +111,10 @@ struct emul {
 #define Z_EMUL_REG_BUS_IDENTIFIER(_dev_node_id) (_CONCAT(_CONCAT(__emulreg_, _dev_node_id), _bus))
 
 /* Conditionally places text based on what bus _dev_node_id is on. */
-#define Z_EMUL_BUS(_dev_node_id, _i2c, _espi, _spi)                                                \
+#define Z_EMUL_BUS(_dev_node_id, _i2c, _espi, _spi, _none)                                         \
 	COND_CODE_1(DT_ON_BUS(_dev_node_id, i2c), (_i2c),                                          \
 		    (COND_CODE_1(DT_ON_BUS(_dev_node_id, espi), (_espi),                           \
-				 (COND_CODE_1(DT_ON_BUS(_dev_node_id, spi), (_spi), (-EINVAL))))))
+				 (COND_CODE_1(DT_ON_BUS(_dev_node_id, spi), (_spi), (_none))))))
 /**
  * @brief Define a new emulator
  *
@@ -120,10 +130,10 @@ struct emul {
  * @param _backend_api emulator-specific backend api
  */
 #define EMUL_DT_DEFINE(node_id, init_fn, data_ptr, cfg_ptr, bus_api, _backend_api)                 \
-	static struct Z_EMUL_BUS(node_id, i2c_emul, espi_emul, spi_emul)                           \
+	static struct Z_EMUL_BUS(node_id, i2c_emul, espi_emul, spi_emul, no_bus_emul)              \
 		Z_EMUL_REG_BUS_IDENTIFIER(node_id) = {                                             \
 			.api = bus_api,                                                            \
-			.Z_EMUL_BUS(node_id, addr, chipsel, chipsel) = DT_REG_ADDR(node_id),       \
+			.Z_EMUL_BUS(node_id, addr, chipsel, chipsel, addr) = DT_REG_ADDR(node_id), \
 	};                                                                                         \
 	const STRUCT_SECTION_ITERABLE(emul, EMUL_DT_NAME_GET(node_id))                             \
 	__used = {                                                                                 \
@@ -132,8 +142,8 @@ struct emul {
 		.cfg = (cfg_ptr),                                                                  \
 		.data = (data_ptr),                                                                \
 		.bus_type = Z_EMUL_BUS(node_id, EMUL_BUS_TYPE_I2C, EMUL_BUS_TYPE_ESPI,             \
-				       EMUL_BUS_TYPE_SPI),                                         \
-		.bus = {.Z_EMUL_BUS(node_id, i2c, espi, spi) =                                     \
+				       EMUL_BUS_TYPE_SPI, EMUL_BUS_TYPE_NONE),                     \
+		.bus = {.Z_EMUL_BUS(node_id, i2c, espi, spi, none) =                               \
 				&(Z_EMUL_REG_BUS_IDENTIFIER(node_id))},                            \
 		.backend_api = (_backend_api),                                                     \
 	};

subsys/emul/emul.c

@@ -55,6 +55,8 @@ int emul_init_for_bus(const struct device *dev)
 		case EMUL_BUS_TYPE_SPI:
 			emul->bus.spi->target = emul;
 			break;
+		case EMUL_BUS_TYPE_NONE:
+			break;
 		}
 		int rc = emul->init(emul, dev);