Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Linux I2C fault injection 2 ========================= 3 4 The GPIO based I2C bus master driver can be configured to provide fault 5 injection capabilities. It is then meant to be connected to another I2C bus 6 which is driven by the I2C bus master driver under test. The GPIO fault 7 injection driver can create special states on the bus which the other I2C bus 8 master driver should handle gracefully. 9 10 Once the Kconfig option I2C_GPIO_FAULT_INJECTOR is enabled, there will be an 11 'i2c-fault-injector' subdirectory in the Kernel debugfs filesystem, usually 12 mounted at /sys/kernel/debug. There will be a separate subdirectory per GPIO 13 driven I2C bus. Each subdirectory will contain files to trigger the fault 14 injection. They will be described now along with their intended use-cases. 15 16 "scl" 17 ----- 18 19 By reading this file, you get the current state of SCL. By writing, you can 20 change its state to either force it low or to release it again. So, by using 21 "echo 0 > scl" you force SCL low and thus, no communication will be possible 22 because the bus master under test will not be able to clock. It should detect 23 the condition of SCL being unresponsive and report an error to the upper 24 layers. 25 26 "sda" 27 ----- 28 29 By reading this file, you get the current state of SDA. By writing, you can 30 change its state to either force it low or to release it again. So, by using 31 "echo 0 > sda" you force SDA low and thus, data cannot be transmitted. The bus 32 master under test should detect this condition and trigger a bus recovery (see 33 I2C specification version 4, section 3.1.16) using the helpers of the Linux I2C 34 core (see 'struct bus_recovery_info'). However, the bus recovery will not 35 succeed because SDA is still pinned low until you manually release it again 36 with "echo 1 > sda". A test with an automatic release can be done with the 37 'incomplete_transfer' file. 38 39 "incomplete_transfer" 40 --------------------- 41 42 This file is write only and you need to write the address of an existing I2C 43 client device to it. Then, a transfer to this device will be started, but it 44 will stop at the ACK phase after the address of the client has been 45 transmitted. Because the device will ACK its presence, this results in SDA 46 being pulled low by the device while SCL is high. So, similar to the "sda" file 47 above, the bus master under test should detect this condition and try a bus 48 recovery. This time, however, it should succeed and the device should release 49 SDA after toggling SCL. Please note: there are I2C client devices which detect 50 a stuck SDA on their side and release it on their own after a few milliseconds. 51 Also, there are external devices deglitching and monitoring the I2C bus. They 52 can also detect a stuck SDA and will init a bus recovery on their own. If you 53 want to implement bus recovery in a bus master driver, make sure you checked 54 your hardware setup carefully before.