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.