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
96intereference (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.
137