1=========================
  2Linux I2C fault injection
  3=========================
  4
  5The GPIO based I2C bus master driver can be configured to provide fault
  6injection capabilities. It is then meant to be connected to another I2C bus
  7which is driven by the I2C bus master driver under test. The GPIO fault
  8injection driver can create special states on the bus which the other I2C bus
  9master driver should handle gracefully.
 10
 11Once the Kconfig option I2C_GPIO_FAULT_INJECTOR is enabled, there will be an
 12'i2c-fault-injector' subdirectory in the Kernel debugfs filesystem, usually
 13mounted at /sys/kernel/debug. There will be a separate subdirectory per GPIO
 14driven I2C bus. Each subdirectory will contain files to trigger the fault
 15injection. They will be described now along with their intended use-cases.
 16
 17Wire states
 18===========
 19
 20"scl"
 21-----
 22
 23By reading this file, you get the current state of SCL. By writing, you can
 24change its state to either force it low or to release it again. So, by using
 25"echo 0 > scl" you force SCL low and thus, no communication will be possible
 26because the bus master under test will not be able to clock. It should detect
 27the condition of SCL being unresponsive and report an error to the upper
 28layers.
 29
 30"sda"
 31-----
 32
 33By reading this file, you get the current state of SDA. By writing, you can
 34change its state to either force it low or to release it again. So, by using
 35"echo 0 > sda" you force SDA low and thus, data cannot be transmitted. The bus
 36master under test should detect this condition and trigger a bus recovery (see
 37I2C specification version 4, section 3.1.16) using the helpers of the Linux I2C
 38core (see 'struct bus_recovery_info'). However, the bus recovery will not
 39succeed because SDA is still pinned low until you manually release it again
 40with "echo 1 > sda". A test with an automatic release can be done with the
 41"incomplete transfers" class of fault injectors.
 42
 43Incomplete transfers
 44====================
 45
 46The following fault injectors create situations where SDA will be held low by a
 47device. Bus recovery should be able to fix these situations. But please note:
 48there are I2C client devices which detect a stuck SDA on their side and release
 49it on their own after a few milliseconds. Also, there might be an external
 50device deglitching and monitoring the I2C bus. It could also detect a stuck SDA
 51and will init a bus recovery on its own. If you want to implement bus recovery
 52in a bus master driver, make sure you checked your hardware setup for such
 53devices before. And always verify with a scope or logic analyzer!
 54
 55"incomplete_address_phase"
 56--------------------------
 57
 58This file is write only and you need to write the address of an existing I2C
 59client device to it. Then, a read transfer to this device will be started, but
 60it will stop at the ACK phase after the address of the client has been
 61transmitted. Because the device will ACK its presence, this results in SDA
 62being pulled low by the device while SCL is high. So, similar to the "sda" file
 63above, the bus master under test should detect this condition and try a bus
 64recovery. This time, however, it should succeed and the device should release
 65SDA after toggling SCL.
 66
 67"incomplete_write_byte"
 68-----------------------
 69
 70Similar to above, this file is write only and you need to write the address of
 71an existing I2C client device to it.
 72
 73The injector will again stop at one ACK phase, so the device will keep SDA low
 74because it acknowledges data. However, there are two differences compared to
 75'incomplete_address_phase':
 76
 77a) the message sent out will be a write message
 78b) after the address byte, a 0x00 byte will be transferred. Then, stop at ACK.
 79
 80This is a highly delicate state, the device is set up to write any data to
 81register 0x00 (if it has registers) when further clock pulses happen on SCL.
 82This is why bus recovery (up to 9 clock pulses) must either check SDA or send
 83additional STOP conditions to ensure the bus has been released. Otherwise
 84random data will be written to a device!
 85
 86Lost arbitration
 87================
 88
 89Here, we want to simulate the condition where the master under test loses the
 90bus arbitration against another master in a multi-master setup.
 91
 92"lose_arbitration"
 93------------------
 94
 95This file is write only and you need to write the duration of the arbitration
 96interference (in µs, maximum is 100ms). The calling process will then sleep
 97and wait for the next bus clock. The process is interruptible, though.
 98
 99Arbitration lost is achieved by waiting for SCL going down by the master under
100test and then pulling SDA low for some time. So, the I2C address sent out
101should be corrupted and that should be detected properly. That means that the
102address sent out should have a lot of '1' bits to be able to detect corruption.
103There doesn't need to be a device at this address because arbitration lost
104should be detected beforehand. Also note, that SCL going down is monitored
105using interrupts, so the interrupt latency might cause the first bits to be not
106corrupted. A good starting point for using this fault injector on an otherwise
107idle bus is::
108
109  # echo 200 > lose_arbitration &
110  # i2cget -y <bus_to_test> 0x3f
111
112Panic during transfer
113=====================
114
115This fault injector will create a Kernel panic once the master under test
116started a transfer. This usually means that the state machine of the bus master
117driver will be ungracefully interrupted and the bus may end up in an unusual
118state. Use this to check if your shutdown/reboot/boot code can handle this
119scenario.
120
121"inject_panic"
122--------------
123
124This file is write only and you need to write the delay between the detected
125start of a transmission and the induced Kernel panic (in µs, maximum is 100ms).
126The calling process will then sleep and wait for the next bus clock. The
127process is interruptible, though.
128
129Start of a transfer is detected by waiting for SCL going down by the master
130under test.  A good starting point for using this fault injector is::
131
132  # echo 0 > inject_panic &
133  # i2cget -y <bus_to_test> <some_address>
134
135Note that there doesn't need to be a device listening to the address you are
136using. Results may vary depending on that, though.