Input: pinephone-keyboard - support the proxied I2C bus

The PinePhone keyboard case contains a battery managed by an integrated
power bank IC. The power bank IC communicates over I2C, and the keyboard
MCU firmware provides an interface to read and write its registers.
Let's use this interface to implement a SMBus adapter, so we can reuse
the driver for the power bank IC.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220618165747.55709-4-samuel@sholland.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/pinephone-keyboard.c

@@ -3,6 +3,7 @@
 // Copyright (C) 2021-2022 Samuel Holland <samuel@sholland.org>
 
 #include <linux/crc8.h>
+#include <linux/delay.h>
 #include <linux/err.h>
 #include <linux/i2c.h>
 #include <linux/input.h>
@@ -10,6 +11,7 @@
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/mod_devicetable.h>
+#include <linux/of.h>
 #include <linux/regulator/consumer.h>
 #include <linux/types.h>
 
@@ -28,6 +30,11 @@
 #define PPKB_SCAN_DATA			0x08
 #define PPKB_SYS_CONFIG			0x20
 #define PPKB_SYS_CONFIG_DISABLE_SCAN		BIT(0)
+#define PPKB_SYS_SMBUS_COMMAND		0x21
+#define PPKB_SYS_SMBUS_DATA		0x22
+#define PPKB_SYS_COMMAND		0x23
+#define PPKB_SYS_COMMAND_SMBUS_READ		0x91
+#define PPKB_SYS_COMMAND_SMBUS_WRITE		0xa1
 
 #define PPKB_ROWS			6
 #define PPKB_COLS			12
@@ -136,6 +143,7 @@ static const struct matrix_keymap_data ppkb_keymap_data = {
 };
 
 struct pinephone_keyboard {
+	struct i2c_adapter adapter;
 	struct input_dev *input;
 	u8 buf[2][PPKB_BUF_LEN];
 	u8 crc_table[CRC8_TABLE_SIZE];
@@ -144,6 +152,57 @@ struct pinephone_keyboard {
 	bool fn_pressed;
 };
 
+static int ppkb_adap_smbus_xfer(struct i2c_adapter *adap, u16 addr,
+				unsigned short flags, char read_write,
+				u8 command, int size,
+				union i2c_smbus_data *data)
+{
+	struct i2c_client *client = adap->algo_data;
+	u8 buf[3];
+	int ret;
+
+	buf[0] = command;
+	buf[1] = data->byte;
+	buf[2] = read_write == I2C_SMBUS_READ ? PPKB_SYS_COMMAND_SMBUS_READ
+					      : PPKB_SYS_COMMAND_SMBUS_WRITE;
+
+	ret = i2c_smbus_write_i2c_block_data(client, PPKB_SYS_SMBUS_COMMAND,
+					     sizeof(buf), buf);
+	if (ret)
+		return ret;
+
+	/* Read back the command status until it passes or fails. */
+	do {
+		usleep_range(300, 500);
+		ret = i2c_smbus_read_byte_data(client, PPKB_SYS_COMMAND);
+	} while (ret == buf[2]);
+	if (ret < 0)
+		return ret;
+	/* Commands return 0x00 on success and 0xff on failure. */
+	if (ret)
+		return -EIO;
+
+	if (read_write == I2C_SMBUS_READ) {
+		ret = i2c_smbus_read_byte_data(client, PPKB_SYS_SMBUS_DATA);
+		if (ret < 0)
+			return ret;
+
+		data->byte = ret;
+	}
+
+	return 0;
+}
+
+static u32 ppkg_adap_functionality(struct i2c_adapter *adap)
+{
+	return I2C_FUNC_SMBUS_BYTE_DATA;
+}
+
+static const struct i2c_algorithm ppkb_adap_algo = {
+	.smbus_xfer		= ppkb_adap_smbus_xfer,
+	.functionality		= ppkg_adap_functionality,
+};
+
 static void ppkb_update(struct i2c_client *client)
 {
 	struct pinephone_keyboard *ppkb = i2c_get_clientdata(client);
@@ -271,6 +330,7 @@ static int ppkb_probe(struct i2c_client *client)
 	struct pinephone_keyboard *ppkb;
 	struct regulator *vbat_supply;
 	u8 info[PPKB_MATRIX_SIZE + 1];
+	struct device_node *i2c_bus;
 	int ret;
 	int error;
 
@@ -330,6 +390,22 @@ static int ppkb_probe(struct i2c_client *client)
 
 	i2c_set_clientdata(client, ppkb);
 
+	i2c_bus = of_get_child_by_name(dev->of_node, "i2c");
+	if (i2c_bus) {
+		ppkb->adapter.owner = THIS_MODULE;
+		ppkb->adapter.algo = &ppkb_adap_algo;
+		ppkb->adapter.algo_data = client;
+		ppkb->adapter.dev.parent = dev;
+		ppkb->adapter.dev.of_node = i2c_bus;
+		strscpy(ppkb->adapter.name, DRV_NAME, sizeof(ppkb->adapter.name));
+
+		error = devm_i2c_add_adapter(dev, &ppkb->adapter);
+		if (error) {
+			dev_err(dev, "Failed to add I2C adapter: %d\n", error);
+			return error;
+		}
+	}
+
 	crc8_populate_msb(ppkb->crc_table, PPKB_CRC8_POLYNOMIAL);
 
 	ppkb->input = devm_input_allocate_device(dev);