The data transferred by the system over the i2c bus (and smbus) can be snooped using the kernel’s FTRACE facility. To use this, the following kernel options need to be enabled:

• CONFIG_FTRACE
• CONFIG_ENABLE_DEFAULT_TRACERS

i2c tracing can then be enabled by:

echo 1 >/sys/kernel/debug/tracing/events/i2c/enable

Note that this will, by default, trace the traffic through all i2c adapters on the system. You can be selective about what you trace by something like:

echo adapter_nr==1 >/sys/kernel/debug/tracing/events/i2c/filter

The trace output can be found in prettified text form by:

cat /sys/kernel/debug/tracing/trace

This will look something like:

... i2c_write: i2c-5 #0 a=044 f=0000 l=2 [02-14]
... i2c_read: i2c-5 #1 a=044 f=0001 l=4
... i2c_reply: i2c-5 #1 a=044 f=0001 l=4 [33-00-00-00]
... i2c_result: i2c-5 n=2 ret=2

Where the components are:

• i2c-<adapter-nr>
• #<message-array-index>
• a=<addr>
• f=<flags>
• l=<datalen>
• n=<message-array-size>
• ret=<result>
• [<data-transferred>]