Kernel GPIO Logic analyzer

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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/gpio-logic-analyzer-v2

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, you need to use the following bindings. Because these bindings are only for debugging, there is no official yaml file:

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

Note that you must provide a name for every GPIO specified. Currently a maximum of 8 probes are supported. 32 are likely possible but are not implemented yet.

Usage

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

The script has a help option explaining the parameters. For the above DT snippet which analyzes an I2C bus at 400KHz on a Renesas Salvator-XS board, 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 edges of both signals in an I2C start condition happen 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 while SCL stays high, i.e. -t 1H+2F. Last is the duration, let us assume 15ms here which results in the parameter -d 15000. So, altogether:

   gpio-sloppy-logic-analyzer -s 1500000 -t 1H+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 has 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 then need to trigger I2C communication:

   i2cdetect -y -r <your bus number>

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

Result

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

Kernel la demo.png