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>

1 files changed, 54 insertions, 0 deletions

diff --git a/Documentation/i2c/gpio-fault-injection b/Documentation/i2c/gpio-fault-injection
new file mode 100644
index 000000000000..e0c4f775e239
--- /dev/null
+++ b/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.