i2c: gpio: add fault injector

Add fault injection capabilities to the i2c-gpio driver. When connected
to another I2C bus, it can create unusual states which the other I2C bus
master driver needs to handle. Only for debugging!

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Documentation/i2c/gpio-fault-injection

@@ -0,0 +1,54 @@
+Linux I2C fault injection
+=========================
+
+The GPIO based I2C bus master driver can be configured to provide fault
+injection capabilities. It is then meant to be connected to another I2C bus
+which is driven by the I2C bus master driver under test. The GPIO fault
+injection driver can create special states on the bus which the other I2C bus
+master driver should handle gracefully.
+
+Once the Kconfig option I2C_GPIO_FAULT_INJECTOR is enabled, there will be an
+'i2c-fault-injector' subdirectory in the Kernel debugfs filesystem, usually
+mounted at /sys/kernel/debug. There will be a separate subdirectory per GPIO
+driven I2C bus. Each subdirectory will contain files to trigger the fault
+injection. They will be described now along with their intended use-cases.
+
+"scl"
+-----
+
+By reading this file, you get the current state of SCL. By writing, you can
+change its state to either force it low or to release it again. So, by using
+"echo 0 > scl" you force SCL low and thus, no communication will be possible
+because the bus master under test will not be able to clock. It should detect
+the condition of SCL being unresponsive and report an error to the upper
+layers.
+
+"sda"
+-----
+
+By reading this file, you get the current state of SDA. By writing, you can
+change its state to either force it low or to release it again. So, by using
+"echo 0 > sda" you force SDA low and thus, data cannot be transmitted. The bus
+master under test should detect this condition and trigger a bus recovery (see
+I2C specification version 4, section 3.1.16) using the helpers of the Linux I2C
+core (see 'struct bus_recovery_info'). However, the bus recovery will not
+succeed because SDA is still pinned low until you manually release it again
+with "echo 1 > sda". A test with an automatic release can be done with the
+'incomplete_transfer' file.
+
+"incomplete_transfer"
+---------------------
+
+This file is write only and you need to write the address of an existing I2C
+client device to it. Then, a transfer to this device will be started, but it
+will stop at the ACK phase after the address of the client has been
+transmitted. Because the device will ACK its presence, this results in SDA
+being pulled low by the device while SCL is high. So, similar to the "sda" file
+above, the bus master under test should detect this condition and try a bus
+recovery. This time, however, it should succeed and the device should release
+SDA after toggling SCL. Please note: there are I2C client devices which detect
+a stuck SDA on their side and release it on their own after a few milliseconds.
+Also, there are external devices deglitching and monitoring the I2C bus. They
+can also detect a stuck SDA and will init a bus recovery on their own. If you
+want to implement bus recovery in a bus master driver, make sure you checked
+your hardware setup carefully before.

drivers/i2c/busses/Kconfig

@@ -603,6 +603,14 @@ 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_FAULT_INJECTOR
+	bool "GPIO-based fault injector"
+	depends on I2C_GPIO
+	help
+	  This adds some functionality to the i2c-gpio driver which can inject
+	  faults to an I2C bus, so another bus master can be stress-tested.
+	  This is for debugging. If unsure, say 'no'.
+
 config I2C_HIGHLANDER
 	tristate "Highlander FPGA SMBus interface"
 	depends on SH_HIGHLANDER

drivers/i2c/busses/i2c-gpio.c

@@ -7,6 +7,8 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+#include <linux/debugfs.h>
+#include <linux/delay.h>
 #include <linux/i2c.h>
 #include <linux/i2c-algo-bit.h>
 #include <linux/i2c-gpio.h>
@@ -23,6 +25,9 @@ struct i2c_gpio_private_data {
 	struct i2c_adapter adap;
 	struct i2c_algo_bit_data bit_data;
 	struct i2c_gpio_platform_data pdata;
+#ifdef CONFIG_I2C_GPIO_FAULT_INJECTOR
+	struct dentry *debug_dir;
+#endif
 };
 
 /*
@@ -64,6 +69,108 @@ static int i2c_gpio_getscl(void *data)
 	return gpiod_get_value(priv->scl);
 }
 
+#ifdef CONFIG_I2C_GPIO_FAULT_INJECTOR
+static struct dentry *i2c_gpio_debug_dir;
+
+#define setsda(bd, val)	((bd)->setsda((bd)->data, val))
+#define setscl(bd, val)	((bd)->setscl((bd)->data, val))
+#define getsda(bd)	((bd)->getsda((bd)->data))
+#define getscl(bd)	((bd)->getscl((bd)->data))
+
+#define WIRE_ATTRIBUTE(wire) \
+static int fops_##wire##_get(void *data, u64 *val)	\
+{							\
+	struct i2c_gpio_private_data *priv = data;	\
+							\
+	i2c_lock_adapter(&priv->adap);			\
+	*val = get##wire(&priv->bit_data);		\
+	i2c_unlock_adapter(&priv->adap);		\
+	return 0;					\
+}							\
+static int fops_##wire##_set(void *data, u64 val)	\
+{							\
+	struct i2c_gpio_private_data *priv = data;	\
+							\
+	i2c_lock_adapter(&priv->adap);			\
+	set##wire(&priv->bit_data, val);		\
+	i2c_unlock_adapter(&priv->adap);		\
+	return 0;					\
+}							\
+DEFINE_DEBUGFS_ATTRIBUTE(fops_##wire, fops_##wire##_get, fops_##wire##_set, "%llu\n")
+
+WIRE_ATTRIBUTE(scl);
+WIRE_ATTRIBUTE(sda);
+
+static int fops_incomplete_transfer_set(void *data, u64 addr)
+{
+	struct i2c_gpio_private_data *priv = data;
+	struct i2c_algo_bit_data *bit_data = &priv->bit_data;
+	int i, pattern;
+
+	if (addr > 0x7f)
+		return -EINVAL;
+
+	/* ADDR (7 bit) + RD (1 bit) + SDA hi (1 bit) */
+	pattern = (addr << 2) | 3;
+
+	i2c_lock_adapter(&priv->adap);
+
+	/* START condition */
+	setsda(bit_data, 0);
+	udelay(bit_data->udelay);
+
+	/* Send ADDR+RD, request ACK, don't send STOP */
+	for (i = 8; i >= 0; i--) {
+		setscl(bit_data, 0);
+		udelay(bit_data->udelay / 2);
+		setsda(bit_data, (pattern >> i) & 1);
+		udelay((bit_data->udelay + 1) / 2);
+		setscl(bit_data, 1);
+		udelay(bit_data->udelay);
+	}
+
+	i2c_unlock_adapter(&priv->adap);
+
+	return 0;
+}
+DEFINE_DEBUGFS_ATTRIBUTE(fops_incomplete_transfer, NULL, fops_incomplete_transfer_set, "%llu\n");
+
+static void i2c_gpio_fault_injector_init(struct platform_device *pdev)
+{
+	struct i2c_gpio_private_data *priv = platform_get_drvdata(pdev);
+
+	/*
+	 * If there will be a debugfs-dir per i2c adapter somewhen, put the
+	 * 'fault-injector' dir there. Until then, we have a global dir with
+	 * all adapters as subdirs.
+	 */
+	if (!i2c_gpio_debug_dir) {
+		i2c_gpio_debug_dir = debugfs_create_dir("i2c-fault-injector", NULL);
+		if (!i2c_gpio_debug_dir)
+			return;
+	}
+
+	priv->debug_dir = debugfs_create_dir(pdev->name, i2c_gpio_debug_dir);
+	if (!priv->debug_dir)
+		return;
+
+	debugfs_create_file_unsafe("scl", 0600, priv->debug_dir, priv, &fops_scl);
+	debugfs_create_file_unsafe("sda", 0600, priv->debug_dir, priv, &fops_sda);
+	debugfs_create_file_unsafe("incomplete_transfer", 0200, priv->debug_dir,
+				   priv, &fops_incomplete_transfer);
+}
+
+static void i2c_gpio_fault_injector_exit(struct platform_device *pdev)
+{
+	struct i2c_gpio_private_data *priv = platform_get_drvdata(pdev);
+
+	debugfs_remove_recursive(priv->debug_dir);
+}
+#else
+static inline void i2c_gpio_fault_injector_init(struct platform_device *pdev) {}
+static inline void i2c_gpio_fault_injector_exit(struct platform_device *pdev) {}
+#endif /* CONFIG_I2C_GPIO_FAULT_INJECTOR*/
+
 static void of_i2c_gpio_get_props(struct device_node *np,
 				  struct i2c_gpio_platform_data *pdata)
 {
@@ -228,6 +335,8 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 		 pdata->scl_is_output_only
 		 ? ", no clock stretching" : "");
 
+	i2c_gpio_fault_injector_init(pdev);
+
 	return 0;
 }
 
@@ -236,6 +345,8 @@ static int i2c_gpio_remove(struct platform_device *pdev)
 	struct i2c_gpio_private_data *priv;
 	struct i2c_adapter *adap;
 
+	i2c_gpio_fault_injector_exit(pdev);
+
 	priv = platform_get_drvdata(pdev);
 	adap = &priv->adap;