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.