Tests:I2C-fault-injection

This document describes how to test the I2C fault injection feature. The Renesas Salvator-X (R-Car M3-W) and Lager (R-Car H2) boards are used. =Setup=

Kernel Version and Configuration
Support for this feature is expected to land upstream in v4.15. It is currently available in a topic branch:

git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git renesas/topic/i2c-fault-injection

The following kernel options need to be set in addition to the defconfig: CONFIG_I2C_GPIO=y CONFIG_I2C_GPIO_FAULT_INJECTOR=y

Salvator-X/r8a7796 (R-Car M3-W SoC)
On the EXIO_B connector, you need to connect the pins:

57 <-> 73 (for SCL) 54 <-> 74 (for SDA)

This will hook two GPIO to the I2C2 bus. The above branch will update the DTS to enable the GPIO as a bit-banged I2C bus.

Lager/r8a7790 (R-Car H2 SoC)
You need to connect the pins:

EXIO_C 78 <-> EXIO_A 58 (for SCL) EXIO_C 80 <-> EXIO_A 60 (for SDA)

This will connect the I2C1 and the I2C2 bus. We will use the I2C demultiplexer at runtime to switch I2C1 to GPIO:
 * 1) echo 2 > /sys/devices/platform/i2c-11/current_master

Software Environment
Only a busybox with standard commands like cd, cat, echo is needed.

=Testing= On both boards, you should go to the i2c fault injector directory and see the following files: incomplete_transfer scl sda
 * 1) cd /sys/kernel/debug/i2c-fault-injector/i2c-8/
 * 2) ls -1

Please refer to the documentation file in Documentation/i2c/gpio-fault-injection about the meaning of the files.

Incomplete transfer
An incomplete transfer is used to make a device keep SDA low.

Salvator-X/r8a7796 (R-Car M3-W SoC)
Use these commands:
 * 1) cd /sys/kernel/debug/i2c-fault-injector/i2c-8/
 * 2) echo 0x10 > incomplete_transfer

With a scope, you can see the following happening:

You can see that SDA is kept low. After 45ms, SCL is toggled again, so the device releases SDA again. This is done by the external LTC4313 chip which monitors the bus and tries to reactivate it. So, the bus stalled condition was met which made the LTC4313 reactivate the bus.

Lager/r8a7790 (R-Car H2 SoC)
Use these commands:
 * 1) echo 2 > /sys/devices/platform/i2c-11/current_master
 * 2) cd /sys/kernel/debug/i2c-fault-injector/i2c-8/
 * 3) echo 0x5a > incomplete_transfer

With a scope, you can see the following happening:

You can see that SDA is kept low. After 30ms, SDA raises without SCL being toggled. This is because the DA9xxx chips detect the timeout on SCL and release SDA on their own. So, the bus stalled condition was met which made the DA9xxx reactivate the bus.

Checking another device:
 * 1) echo 0x12 > incomplete_transfer

With a scope, you can see the following happening:

You can see that SDA is kept low forever. The audio codec does not have any timeout detection and there is no external chip to reactivate the bus. It is now the I2C bus masters turn to detect this condition and to recover the bus by toggling SCL.

Pin SDA low
Use these commands on the Salvator-X:
 * 1) cd /sys/kernel/debug/i2c-fault-injector/i2c-8/
 * 2) echo 0 > sda

With a scope, you can see the following happening:

Now, SDA is pinned low by us until we release it. You can see the LTC4313 trying to recover the bus, but to no avail. The bus can only be released with
 * 1) echo 1 > sda

On the Lager board, nothing special will happen because there is no LTC4313. SDA just stays low until it gets manually released again.

Pin SCL low
Use these commands on the Salvator-X or Lager:
 * 1) cd /sys/kernel/debug/i2c-fault-injector/i2c-8/
 * 2) echo 0 > scl

With a scope, you can see the following happening:

Now, SCL is pinned low by us until we release it. No communication is possible anymore. The bus can only be released with:
 * 1) echo 1 > scl

= Conclusion = It could be demonstrated that the "incomplete_transfer" mechanism creates a stalled bus. It could be observed that some devices detect this condition and try to recover the bus. Cases which create a completely stalled bus could also be created (e.g. by the audio codec on Lager). This can serve as a test case for the bus recovery feature of the Linux I2C core. Also, the features of pinning SCL and SDA permanently low could be demonstrated. This can be used to make I2C drivers handle this abnormal situation gracefully.