Difference between revisions of "Kernel GPIO Logic analyzer"

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This document briefly describes how to run the software based in-kernel 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/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.''
 
''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.''
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= Setup =
 
= Setup =
  
Tell the kernel which GPIOs are used as probes. For a DT based system, e.g.:
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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 {
 
     i2c-analyzer {
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     gpio-logic-analyzer -s 1500000 -t "2F" -d 15000
 
     gpio-logic-analyzer -s 1500000 -t "2F" -d 15000
  
Note that 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:
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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 <your bus number>
 
     i2cdetect -y -r <your bus number>

Latest revision as of 23:45, 29 March 2021

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 <your bus number>

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:

Kernel la demo.png