Add a fault injector simulating 'arbitration lost' from multi-master
setups. Read the docs for its usage.

A helper function for future fault injectors using SCL interrupts is
created to achieve this.

Signed-off-by: Wolfram Sang <wsa+rene...@sang-engineering.com>
---
 Documentation/i2c/gpio-fault-injection | 25 ++++++++++++
 drivers/i2c/busses/i2c-gpio.c          | 74 ++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+)

diff --git a/Documentation/i2c/gpio-fault-injection 
b/Documentation/i2c/gpio-fault-injection
index 1a44e3edc0c4..1f1bb96a64bd 100644
--- a/Documentation/i2c/gpio-fault-injection
+++ b/Documentation/i2c/gpio-fault-injection
@@ -83,3 +83,28 @@ This is why bus recovery (up to 9 clock pulses) must either 
check SDA or send
 additional STOP conditions to ensure the bus has been released. Otherwise
 random data will be written to a device!
 
+Lost arbitration
+================
+
+Here, we want to simulate the condition where the master under test loses the
+bus arbitration against another master in a multi-master setup.
+
+"lose_arbitration"
+------------------
+
+This file is write only and you need to write the duration of the arbitration
+intereference (in ┬Ás, maximum is 100ms). The calling process will then sleep
+and wait for the next bus clock. The process is interruptible, though.
+
+Arbitration lost is achieved by waiting for SCL going down by the master under
+test and then pulling SDA low for some time. So, the I2C address sent out
+should be corrupted and that should be detected properly. That means that the
+address sent out should have a lot of '1' bits to be able to detect corruption.
+There doesn't need to be a device at this address because arbitration lost
+should be detected beforehand. Also note, that SCL going down is monitored
+using interrupts, so the interrupt latency might cause the first bits to be not
+corrupted. A good starting point for using this fault injector on an otherwise
+idle bus is:
+
+# echo 200 > lose_arbitration &
+# i2cget -y <bus_to_test> 0x3f
diff --git a/drivers/i2c/busses/i2c-gpio.c b/drivers/i2c/busses/i2c-gpio.c
index 2d532cc042f5..76e43783f50f 100644
--- a/drivers/i2c/busses/i2c-gpio.c
+++ b/drivers/i2c/busses/i2c-gpio.c
@@ -7,12 +7,14 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+#include <linux/completion.h>
 #include <linux/debugfs.h>
 #include <linux/delay.h>
 #include <linux/gpio/consumer.h>
 #include <linux/i2c-algo-bit.h>
 #include <linux/i2c.h>
 #include <linux/init.h>
+#include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_data/i2c-gpio.h>
@@ -27,6 +29,9 @@ struct i2c_gpio_private_data {
        struct i2c_gpio_platform_data pdata;
 #ifdef CONFIG_I2C_GPIO_FAULT_INJECTOR
        struct dentry *debug_dir;
+       /* these must be protected by bus lock */
+       struct completion scl_irq_completion;
+       u64 scl_irq_data;
 #endif
 };
 
@@ -162,6 +167,70 @@ static int fops_incomplete_write_byte_set(void *data, u64 
addr)
 }
 DEFINE_DEBUGFS_ATTRIBUTE(fops_incomplete_write_byte, NULL, 
fops_incomplete_write_byte_set, "%llu\n");
 
+static int i2c_gpio_fi_act_on_scl_irq(struct i2c_gpio_private_data *priv,
+                                      irqreturn_t handler(int, void*))
+{
+       int ret, irq = gpiod_to_irq(priv->scl);
+
+       if (irq < 0)
+               return irq;
+
+       i2c_lock_bus(&priv->adap, I2C_LOCK_ROOT_ADAPTER);
+
+       ret = gpiod_direction_input(priv->scl);
+       if (ret)
+               goto unlock;
+
+       reinit_completion(&priv->scl_irq_completion);
+
+       ret = request_irq(irq, handler, IRQF_TRIGGER_FALLING,
+                         "i2c_gpio_fault_injector_scl_irq", priv);
+       if (ret)
+               goto output;
+
+       wait_for_completion_interruptible(&priv->scl_irq_completion);
+
+       free_irq(irq, priv);
+ output:
+       ret = gpiod_direction_output(priv->scl, 1) ?: ret;
+ unlock:
+       i2c_unlock_bus(&priv->adap, I2C_LOCK_ROOT_ADAPTER);
+
+       return ret;
+}
+
+static irqreturn_t lose_arbitration_irq(int irq, void *dev_id)
+{
+       struct i2c_gpio_private_data *priv = dev_id;
+
+       setsda(&priv->bit_data, 0);
+       udelay(priv->scl_irq_data);
+       setsda(&priv->bit_data, 1);
+
+       complete(&priv->scl_irq_completion);
+
+       return IRQ_HANDLED;
+}
+
+static int fops_lose_arbitration_set(void *data, u64 duration)
+{
+       struct i2c_gpio_private_data *priv = data;
+
+       if (duration > 100 * 1000)
+               return -EINVAL;
+
+       priv->scl_irq_data = duration;
+       /*
+        * Interrupt on falling SCL. This ensures that the master under test has
+        * really started the transfer. Interrupt on falling SDA did only
+        * exercise 'bus busy' detection on some HW but not 'arbitration lost'.
+        * Note that the interrupt latency may cause the first bits to be
+        * transmitted correctly.
+        */
+       return i2c_gpio_fi_act_on_scl_irq(priv, lose_arbitration_irq);
+}
+DEFINE_DEBUGFS_ATTRIBUTE(fops_lose_arbitration, NULL, 
fops_lose_arbitration_set, "%llu\n");
+
 static void i2c_gpio_fault_injector_init(struct platform_device *pdev)
 {
        struct i2c_gpio_private_data *priv = platform_get_drvdata(pdev);
@@ -181,10 +250,15 @@ static void i2c_gpio_fault_injector_init(struct 
platform_device *pdev)
        if (!priv->debug_dir)
                return;
 
+       init_completion(&priv->scl_irq_completion);
+
        debugfs_create_file_unsafe("incomplete_address_phase", 0200, 
priv->debug_dir,
                                   priv, &fops_incomplete_addr_phase);
        debugfs_create_file_unsafe("incomplete_write_byte", 0200, 
priv->debug_dir,
                                   priv, &fops_incomplete_write_byte);
+       if (priv->bit_data.getscl)
+               debugfs_create_file_unsafe("lose_arbitration", 0200, 
priv->debug_dir,
+                                          priv, &fops_lose_arbitration);
        debugfs_create_file_unsafe("scl", 0600, priv->debug_dir, priv, 
&fops_scl);
        debugfs_create_file_unsafe("sda", 0600, priv->debug_dir, priv, 
&fops_sda);
 }
-- 
2.11.0

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