Kernel GPIO Logic analyzer

This document briefly describes how to run the software based in-kernel logic analyzer. It is currently under discussion and not upstream at this time of writing. A branch can be found here: https://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git/log/?h=renesas/topic/gpio-logic-analyzer

''Note that this is still a last resort analyzer which can be affected by latencies and non-determinant code paths. However, for e.g. remote development, it may be useful to get a first view and aid further debugging.''

= Setup =

Tell the kernel which GPIOs are used as probes. For a DT based system, e.g. a Renesas Salvator-XS board with a R-Car M3-N SoC:

i2c-analyzer { compatible = "gpio-logic-analyzer"; probe-gpios = <&gpio6 21 GPIO_OPEN_DRAIN>, <&gpio6 4 GPIO_OPEN_DRAIN>; probe-names = "SCL", "SDA"; };

The binding documentation is in the misc folder of the Kernel binding documentation.

= Usage =

The logic analyzer is configurable via files in debugfs. However, it is strongly recommended to not use them directly, but to to use the gpio-logic-analyzer script in the tools/debugging directory. Besides checking parameters more extensively, it will isolate a CPU core for you, so you will have least disturbance while measuring.

The script has a help option explaining the parameters. For the above DT snipplet which analyzes an I2C bus at 400KHz, the following settings are used: The isolated CPU shall be CPU1 because it is a big core in a big.LITTLE setup. Because CPU1 is the default, we don't need a parameter. The bus speed is 400kHz. So, the sampling theorem says we need to sample at least at 800kHz. However, falling of both, SDA and SCL, in a start condition is faster, so we need a higher sampling frequency, e.g. -s 1500000 for 1.5MHz. Also, we don't want to sample right away but wait for a start condition on an idle bus. So, we need to set a trigger to a falling edge on SDA, i.e. -t "2F". Last is the duration, let us assume 15ms here which results in the parameter -d 15000. So, altogether:

gpio-logic-analyzer -s 1500000 -t "2F" -d 15000

Note that the process will return you back to the prompt but a sub-process is still sampling in the background. Unless this finished, you will not find a result file in the current or specified directory. Please also note that currently this sub-process is not killable! For the above example, we will need to trigger I2C communication:

i2cdetect -y -r

Result is a .sr file to be consumed with PulseView from the free Sigrok project. It is a zip file which also contains the binary sample data which may be consumed by others. The filename is the logic analyzer instance name plus a since-epoch timestamp.

Here is an picture of pulseview showing output of the above example: