dm: sandbox: i2c: Add a new 'emulation parent' uclass

Sandbox i2c works using emulation drivers which are currently children of
the i2c device:

	rtc_0: rtc@43 {
		reg = <0x43>;
		compatible = "sandbox-rtc";
		emul {
			compatible = "sandbox,i2c-rtc";
		};
	};

In this case the emulation device is attached to i2c bus on address 0x43
and provides the Real-Time-Clock (RTC) functionality.

However this is not ideal, since every device on an I2C bus has a child
device. This is only really the case for sandbox, but we want to avoid
special-case code for sandbox.

A better approach seems to be to add a separate node on the bus, an
'emulation parent'. This can be given a bogus address (such as 0xff) and
hides all the emulators away. Then we can use a phandle to point from the
device to the correct emualtor, and only on sandbox. The code to find an
emulator does not interfere with normal i2c operation.

Add a new UCLASS_I2C_EMUL_PARENT uclass which allows finding an emulator
given a bus, and finding a bus given an emulator. This will be used in a
follow-on patch.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/i2c/i2c-emul-uclass.c

@@ -6,8 +6,85 @@
 #include <common.h>
 #include <dm.h>
 #include <i2c.h>
+#include <dm/device-internal.h>
+#include <dm/uclass-internal.h>
+
+/*
+ * i2c emulation works using an 'emul' node at the bus level. Each device in
+ * that node is in the UCLASS_I2C_EMUL uclass, and emulates one i2c device. A
+ * pointer to the device it emulates is in the 'dev' property of the emul device
+ * uclass platdata (struct i2c_emul_platdata), put there by i2c_emul_find().
+ * When sandbox wants an emulator for a device, it calls i2c_emul_find() which
+ * searches for the emulator with the correct address. To find the device for an
+ * emulator, call i2c_emul_get_device().
+ *
+ * The 'emul' node is in the UCLASS_I2C_EMUL_PARENT uclass. We use a separate
+ * uclass so avoid having strange devices on the I2C bus.
+ */
+
+/**
+ * struct i2c_emul_uc_platdata - information about the emulator for this device
+ *
+ * This is used by devices in UCLASS_I2C_EMUL to record information about the
+ * device being emulated. It is accessible with dev_get_uclass_platdata()
+ *
+ * @dev: Device being emulated
+ */
+struct i2c_emul_uc_platdata {
+	struct udevice *dev;
+};
+
+struct udevice *i2c_emul_get_device(struct udevice *emul)
+{
+	struct i2c_emul_uc_platdata *uc_plat = dev_get_uclass_platdata(emul);
+
+	return uc_plat->dev;
+}
+
+int i2c_emul_find(struct udevice *dev, struct udevice **emulp)
+{
+	struct i2c_emul_uc_platdata *uc_plat;
+	struct udevice *emul;
+	int ret;
+
+	ret = uclass_find_device_by_phandle(UCLASS_I2C_EMUL, dev,
+					    "sandbox,emul", &emul);
+	if (ret) {
+		log_err("No emulators for device '%s'\n", dev->name);
+		return ret;
+	}
+	uc_plat = dev_get_uclass_platdata(emul);
+	uc_plat->dev = dev;
+	*emulp = emul;
+
+	return device_probe(emul);
+}
 
 UCLASS_DRIVER(i2c_emul) = {
 	.id		= UCLASS_I2C_EMUL,
 	.name		= "i2c_emul",
+	.per_device_platdata_auto_alloc_size =
+		 sizeof(struct i2c_emul_uc_platdata),
+};
+
+/*
+ * This uclass is a child of the i2c bus. Its platdata is not defined here so
+ * is defined by its parent, UCLASS_I2C, which uses struct dm_i2c_chip. See
+ * per_child_platdata_auto_alloc_size in UCLASS_DRIVER(i2c).
+ */
+UCLASS_DRIVER(i2c_emul_parent) = {
+	.id		= UCLASS_I2C_EMUL_PARENT,
+	.name		= "i2c_emul_parent",
+	.post_bind	= dm_scan_fdt_dev,
+};
+
+static const struct udevice_id i2c_emul_parent_ids[] = {
+	{ .compatible = "sandbox,i2c-emul-parent" },
+	{ }
+};
+
+U_BOOT_DRIVER(i2c_emul_parent_drv) = {
+	.name		= "i2c_emul_parent_drv",
+	.id		= UCLASS_I2C_EMUL_PARENT,
+	.of_match	= i2c_emul_parent_ids,
 };

include/dm/uclass-id.h

@@ -21,6 +21,7 @@ enum uclass_id {
 	UCLASS_TEST_DUMMY,
 	UCLASS_SPI_EMUL,	/* sandbox SPI device emulator */
 	UCLASS_I2C_EMUL,	/* sandbox I2C device emulator */
+	UCLASS_I2C_EMUL_PARENT,	/* parent for I2C device emulators */
 	UCLASS_PCI_EMUL,	/* sandbox PCI device emulator */
 	UCLASS_USB_EMUL,	/* sandbox USB bus device emulator */
 	UCLASS_AXI_EMUL,	/* sandbox AXI bus device emulator */

include/i2c.h

@@ -536,6 +536,27 @@ int i2c_chip_ofdata_to_platdata(struct udevice *dev, struct dm_i2c_chip *chip);
  */
 void i2c_dump_msgs(struct i2c_msg *msg, int nmsgs);
 
+/**
+ * i2c_emul_find() - Find an emulator for an i2c sandbox device
+ *
+ * This looks at the device's 'emul' phandle
+ *
+ * @dev: Device to find an emulator for
+ * @emulp: Returns the associated emulator, if found *
+ * @return 0 if OK, -ENOENT or -ENODEV if not found
+ */
+int i2c_emul_find(struct udevice *dev, struct udevice **emulp);
+
+/**
+ * i2c_emul_get_device() - Find the device being emulated
+ *
+ * Given an emulator this returns the associated device
+ *
+ * @emul: Emulator for the device
+ * @return device that @emul is emulating
+ */
+struct udevice *i2c_emul_get_device(struct udevice *emul);
+
 #ifndef CONFIG_DM_I2C
 
 /*