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