[PATCH v2 2/2] i2c: Add driver for gpio based busses with shared SCL

[Markus Stockhausen <markus.stockhausen@gmx.de>]

Some lower end hardware (especially Realtek based switches) are
designed with multiple I2C busses that share a single clock
(SCL) line. E.g. the D-Link DGS-1250-28X realizes 4 I2C SFP busses
with 5 gpios.

Provide a i2c-gpio-shared driver that handles such hardware designs.
It manages up to 32 buses that share a single SCL line. All data
transfers (including recovery) are synchronized with a central mutex.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/i2c/busses/Kconfig           |  10 ++
 drivers/i2c/busses/Makefile          |   1 +
 drivers/i2c/busses/i2c-gpio-shared.c | 257 +++++++++++++++++++++++++++
 3 files changed, 268 insertions(+)
 create mode 100644 drivers/i2c/busses/i2c-gpio-shared.c

diff --git a/drivers/i2c/busses/Kconfig b/drivers/i2c/busses/Kconfig
index 8c935f867a37..fa05db759b15 100644
--- a/drivers/i2c/busses/Kconfig
+++ b/drivers/i2c/busses/Kconfig
@@ -691,6 +691,16 @@ config I2C_GPIO
 	  This is a very simple bitbanging I2C driver utilizing the
 	  arch-neutral GPIO API to control the SCL and SDA lines.
 
+config I2C_GPIO_SHARED
+	tristate "multiple GPIO-based bitbanging I2C with shared SCL"
+	depends on GPIOLIB || COMPILE_TEST
+	select I2C_ALGOBIT
+	help
+	  This is an alternative of the I2C GPIO driver for devices with only
+	  few GPIO pins where multiple busses with dedicated SDA lines share
+	  a single SCL line. It can handle up to 32 buses, where N shared I2C
+	  buses are implemented with N+1 GPIO.
+
 config I2C_GPIO_FAULT_INJECTOR
 	bool "GPIO-based fault injector"
 	depends on I2C_GPIO
diff --git a/drivers/i2c/busses/Makefile b/drivers/i2c/busses/Makefile
index 547123ab351f..724b09e613cb 100644
--- a/drivers/i2c/busses/Makefile
+++ b/drivers/i2c/busses/Makefile
@@ -66,6 +66,7 @@ obj-$(CONFIG_I2C_EG20T)		+= i2c-eg20t.o
 obj-$(CONFIG_I2C_EMEV2)		+= i2c-emev2.o
 obj-$(CONFIG_I2C_EXYNOS5)	+= i2c-exynos5.o
 obj-$(CONFIG_I2C_GPIO)		+= i2c-gpio.o
+obj-$(CONFIG_I2C_GPIO_SHARED)	+= i2c-gpio-shared.o
 obj-$(CONFIG_I2C_HIGHLANDER)	+= i2c-highlander.o
 obj-$(CONFIG_I2C_HISI)		+= i2c-hisi.o
 obj-$(CONFIG_I2C_HIX5HD2)	+= i2c-hix5hd2.o
diff --git a/drivers/i2c/busses/i2c-gpio-shared.c b/drivers/i2c/busses/i2c-gpio-shared.c
new file mode 100644
index 000000000000..4c2e4011fcbf
--- /dev/null
+++ b/drivers/i2c/busses/i2c-gpio-shared.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Bitbanging driver for up to 32 I2C buses that share a single SCL pin */
+
+#include <linux/gpio/consumer.h>
+#include <linux/i2c-algo-bit.h>
+#include <linux/i2c.h>
+#include <linux/mod_devicetable.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+
+#define MAX_BUSES 32
+
+struct gpio_shared_ctx;
+
+struct gpio_shared_bus {
+	struct gpio_desc *sda;
+	struct i2c_adapter adap;
+	struct i2c_algo_bit_data bit_data;
+	struct i2c_bus_recovery_info recovery_info;
+	struct gpio_shared_ctx *ctx;
+};
+
+struct gpio_shared_ctx {
+	struct gpio_desc *scl;
+	struct mutex lock;
+	struct gpio_shared_bus bus[];
+};
+
+static struct gpio_shared_bus *adap_to_bus(struct i2c_adapter *adap)
+{
+	return container_of(adap, struct gpio_shared_bus, adap);
+}
+
+static struct gpio_shared_ctx *adap_to_ctx(struct i2c_adapter *adap)
+{
+	return adap_to_bus(adap)->ctx;
+}
+
+static void gpio_shared_setsda(void *data, int state)
+{
+	struct gpio_shared_bus *bus = data;
+
+	gpiod_set_value_cansleep(bus->sda, state);
+}
+
+static void gpio_shared_setscl(void *data, int state)
+{
+	struct gpio_shared_bus *bus = data;
+
+	gpiod_set_value_cansleep(bus->ctx->scl, state);
+}
+
+static int gpio_shared_getsda(void *data)
+{
+	struct gpio_shared_bus *bus = data;
+
+	return gpiod_get_value_cansleep(bus->sda);
+}
+
+static int gpio_shared_getscl(void *data)
+{
+	struct gpio_shared_bus *bus = data;
+
+	return gpiod_get_value_cansleep(bus->ctx->scl);
+}
+
+static int gpio_shared_pre_xfer(struct i2c_adapter *adap)
+{
+	return mutex_lock_interruptible(&adap_to_ctx(adap)->lock);
+}
+
+static void gpio_shared_recovery_set_scl(struct i2c_adapter *adap, int val)
+{
+	gpiod_set_value_cansleep(adap_to_ctx(adap)->scl, val);
+}
+
+static int gpio_shared_recovery_get_scl(struct i2c_adapter *adap)
+{
+	return gpiod_get_value_cansleep(adap_to_ctx(adap)->scl);
+}
+
+static void gpio_shared_recovery_set_sda(struct i2c_adapter *adap, int val)
+{
+	gpiod_set_value_cansleep(adap_to_bus(adap)->sda, val);
+}
+
+static int gpio_shared_recovery_get_sda(struct i2c_adapter *adap)
+{
+	return gpiod_get_value_cansleep(adap_to_bus(adap)->sda);
+}
+
+static void gpio_shared_prepare_recovery(struct i2c_adapter *adap)
+{
+	mutex_lock(&adap_to_ctx(adap)->lock);
+}
+
+static void gpio_shared_unlock(struct i2c_adapter *adap)
+{
+	mutex_unlock(&adap_to_ctx(adap)->lock);
+}
+
+static void gpio_shared_del_adapter(void *data)
+{
+	i2c_del_adapter(data);
+}
+
+static int gpio_shared_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	int bus_count, sda_count, ret;
+	struct gpio_shared_ctx *ctx;
+	u32 used_buses = 0;
+
+	bus_count = device_get_child_node_count(dev);
+	if (!bus_count)
+		return dev_err_probe(dev, -EINVAL, "no buses defined\n");
+
+	if (bus_count > MAX_BUSES)
+		return dev_err_probe(dev, -EINVAL, "maximum of %d buses allowed\n", MAX_BUSES);
+
+	sda_count = gpiod_count(dev, "sda");
+	if (sda_count < 0)
+		return dev_err_probe(dev, sda_count, "sda-gpios missing\n");
+
+	if (sda_count != bus_count)
+		return dev_err_probe(dev, -EINVAL,
+				     "sda-gpios count (%d) must match child node count (%d)\n",
+				     sda_count, bus_count);
+
+	ctx = devm_kzalloc(dev, struct_size(ctx, bus, bus_count), GFP_KERNEL);
+	if (!ctx)
+		return dev_err_probe(dev, -ENOMEM, "memory allocation failed\n");
+
+	ret = devm_mutex_init(dev, &ctx->lock);
+	if (ret)
+		return dev_err_probe(dev, ret, "mutex initialization failed\n");
+
+	platform_set_drvdata(pdev, ctx);
+
+	ctx->scl = devm_gpiod_get(dev, "scl", GPIOD_OUT_HIGH_OPEN_DRAIN);
+	if (IS_ERR(ctx->scl))
+		return dev_err_probe(dev, PTR_ERR(ctx->scl), "shared SCL GPIO not found\n");
+
+	device_for_each_child_node_scoped(dev, child) {
+		struct i2c_algo_bit_data *bit_data;
+		struct i2c_bus_recovery_info *ri;
+		struct gpio_shared_bus *bus;
+		struct i2c_adapter *adap;
+		u32 bus_num, clock_freq;
+
+		/* Use the child's reg value as the index into the sda-gpios array. */
+		ret = fwnode_property_read_u32(child, "reg", &bus_num);
+		if (ret)
+			return dev_err_probe(dev, ret,
+					     "missing reg property in child node\n");
+
+		if (bus_num >= sda_count)
+			return dev_err_probe(dev, -EINVAL,
+					     "reg value %u out of range (max %d)\n",
+					     bus_num, sda_count - 1);
+
+		if (used_buses & BIT(bus_num))
+			return dev_err_probe(dev, -EINVAL,
+					     "duplicate definition of bus %d\n", bus_num);
+
+		bus = &ctx->bus[bus_num];
+		bit_data = &bus->bit_data;
+		ri = &bus->recovery_info;
+		adap = &bus->adap;
+
+		bus->sda = devm_gpiod_get_index(dev, "sda", bus_num,
+						GPIOD_OUT_HIGH_OPEN_DRAIN);
+		if (IS_ERR(bus->sda))
+			return dev_err_probe(dev, PTR_ERR(bus->sda),
+					     "SDA GPIO for bus %u not found\n", bus_num);
+		/*
+		 * Data transfer synchronization between multiple busses is realized by mutex
+		 * locking/unlocking within pre_xfer and post_xfer helpers.
+		 */
+		bus->ctx = ctx;
+		bit_data->data = bus;
+		bit_data->setsda = gpio_shared_setsda;
+		bit_data->setscl = gpio_shared_setscl;
+		bit_data->getsda = gpio_shared_getsda;
+		bit_data->getscl = gpio_shared_getscl;
+		bit_data->pre_xfer = gpio_shared_pre_xfer;
+		bit_data->post_xfer = gpio_shared_unlock;
+		bit_data->timeout = msecs_to_jiffies(100);
+		/*
+		 * clock-frequency specifies the I2C bus frequency. Convert to the half-period
+		 * delay in microseconds that i2c-algo-bit expects. Default to 5 us (~100 kHz)
+		 * if not specified. This is usually lower than the configured frequency,
+		 * especially near the 400 kHz limit.
+		 */
+		if (!fwnode_property_read_u32(child, "clock-frequency", &clock_freq) &&
+		    clock_freq > 0)
+			bit_data->udelay = max(1u, DIV_ROUND_UP(1000000, 2 * clock_freq));
+		else
+			bit_data->udelay = 5;
+		/*
+		 * i2c_recover_bus() is called by the I2C core without going through pre_xfer
+		 * and post_xfer. So the shared SCL mutex is NOT held at that point. Provide
+		 * prepare_recovery/unprepare_recovery to explicitly prevent concurrent SCL
+		 * access from another bus.
+		 */
+		ri->recover_bus = i2c_generic_scl_recovery;
+		ri->set_scl = gpio_shared_recovery_set_scl;
+		ri->get_scl = gpio_shared_recovery_get_scl;
+		ri->set_sda = gpio_shared_recovery_set_sda;
+		ri->get_sda = gpio_shared_recovery_get_sda;
+		ri->prepare_recovery = gpio_shared_prepare_recovery;
+		ri->unprepare_recovery = gpio_shared_unlock;
+
+		adap->dev.parent = dev;
+		adap->owner = THIS_MODULE;
+		adap->algo_data = bit_data;
+		adap->bus_recovery_info = ri;
+		device_set_node(&adap->dev, child);
+		snprintf(adap->name, sizeof(adap->name), "i2c-gpio-shared:%u", bus_num);
+
+		ret = devm_add_action_or_reset(dev, gpio_shared_del_adapter, adap);
+		if (ret)
+			return dev_err_probe(dev, ret,
+					     "bus %u cleanup registration failed\n", bus_num);
+
+		ret = i2c_bit_add_bus(adap);
+		if (ret)
+			return dev_err_probe(dev, ret, "failed to register bus %u\n", bus_num);
+
+		dev_info(dev, "registered I2C bus %u with shared SCL (udelay %d us)\n",
+			 bus_num, bit_data->udelay);
+
+		used_buses |= BIT(bus_num);
+	}
+
+	return 0;
+}
+
+static const struct of_device_id gpio_shared_of_match[] = {
+	{ .compatible = "i2c-gpio-shared" },
+	{}
+};
+MODULE_DEVICE_TABLE(of, gpio_shared_of_match);
+
+static struct platform_driver gpio_shared_driver = {
+	.probe = gpio_shared_probe,
+	.driver = {
+		.name = "i2c-gpio-shared",
+		.of_match_table = gpio_shared_of_match,
+	},
+};
+
+module_platform_driver(gpio_shared_driver);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Markus Stockhausen <markus.stockhausen@gmx.de>");
+MODULE_DESCRIPTION("bitbanging multi I2C driver for shared SCL");
-- 
2.54.0