1.. SPDX-License-Identifier: GPL-2.0
2
3=============================================
4Linux Kernel GPIO based sloppy logic analyzer
5=============================================
6
7:Author: Wolfram Sang
8
9Introduction
10============
11
12This document briefly describes how to run the GPIO based in-kernel sloppy
13logic analyzer running on an isolated CPU.
14
15The sloppy logic analyzer will utilize a few GPIO lines in input mode on a
16system to rapidly sample these digital lines, which will, if the Nyquist
17criteria is met, result in a time series log with approximate waveforms as they
18appeared on these lines. One way to use it is to analyze external traffic
19connected to these GPIO lines with wires (i.e. digital probes), acting as a
20common logic analyzer.
21
22Another feature is to snoop on on-chip peripherals if the I/O cells of these
23peripherals can be used in GPIO input mode at the same time as they are being
24used as inputs or outputs for the peripheral. That means you could e.g. snoop
25I2C traffic without any wiring (if your hardware supports it). In the pin
26control subsystem such pin controllers are called "non-strict": a certain pin
27can be used with a certain peripheral and as a GPIO input line at the same
28time.
29
30Note that this is a last resort analyzer which can be affected by latencies,
31non-deterministic code paths and non-maskable interrupts. It is called 'sloppy'
32for a reason. However, for e.g. remote development, it may be useful to get a
33first view and aid further debugging.
34
35Setup
36=====
37
38Your kernel must have CONFIG_DEBUG_FS and CONFIG_CPUSETS enabled. Ideally, your
39runtime environment does not utilize cpusets otherwise, then isolation of a CPU
40core is easiest. If you do need cpusets, check that helper script for the
41sloppy logic analyzer does not interfere with your other settings.
42
43Tell the kernel which GPIOs are used as probes. For a Device Tree based system,
44you need to use the following bindings. Because these bindings are only for
45debugging, there is no official schema::
46
47    i2c-analyzer {
48            compatible = "gpio-sloppy-logic-analyzer";
49            probe-gpios = <&gpio6 21 GPIO_OPEN_DRAIN>, <&gpio6 4 GPIO_OPEN_DRAIN>;
50            probe-names = "SCL", "SDA";
51    };
52
53Note that you must provide a name for every GPIO specified. Currently a
54maximum of 8 probes are supported. 32 are likely possible but are not
55implemented yet.
56
57Usage
58=====
59
60The logic analyzer is configurable via files in debugfs. However, it is
61strongly recommended to not use them directly, but to use the script
62``tools/gpio/gpio-sloppy-logic-analyzer``. Besides checking parameters more
63extensively, it will isolate the CPU core so you will have the least
64disturbance while measuring.
65
66The script has a help option explaining the parameters. For the above DT
67snippet which analyzes an I2C bus at 400kHz on a Renesas Salvator-XS board, the
68following settings are used: The isolated CPU shall be CPU1 because it is a big
69core in a big.LITTLE setup. Because CPU1 is the default, we don't need a
70parameter. The bus speed is 400kHz. So, the sampling theorem says we need to
71sample at least at 800kHz. However, falling edges of both signals in an I2C
72start condition happen faster, so we need a higher sampling frequency, e.g.
73``-s 1500000`` for 1.5MHz. Also, we don't want to sample right away but wait
74for a start condition on an idle bus. So, we need to set a trigger to a falling
75edge on SDA while SCL stays high, i.e. ``-t 1H+2F``. Last is the duration, let
76us assume 15ms here which results in the parameter ``-d 15000``. So,
77altogether::
78
79    gpio-sloppy-logic-analyzer -s 1500000 -t 1H+2F -d 15000
80
81Note that the process will return you back to the prompt but a sub-process is
82still sampling in the background. Unless this has finished, you will not find a
83result file in the current or specified directory. For the above example, we
84will then need to trigger I2C communication::
85
86    i2cdetect -y -r <your bus number>
87
88Result is a .sr file to be consumed with PulseView or sigrok-cli from the free
89`sigrok`_ project. It is a zip file which also contains the binary sample data
90which may be consumed by other software. The filename is the logic analyzer
91instance name plus a since-epoch timestamp.
92
93.. _sigrok: https://sigrok.org/
94