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GPIO monitoring daemon
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README.md

mausberry-switch

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This is a daemon for Raspberry Pi devices that monitors GPIO pins 23 and 24, waiting for a low signal from a Mausberry Circuits switch in order to poweroff the system safely. It is intended to replace the official setup script.

Why not just use the official script available from from their website?

For reference, we're talking about this code:

while [ 1 = 1 ]; do
    cat /sys/class/gpio/gpio$GPIOpin1/value
    sleep 1
done

In my opinion, if there's a wrong way to monitor GPIO pins, it is a bash script. I do have to give credit for strong adherence to the KISS principle - 21 lines of bash is hard to beat.

However, on a Raspberry PI device, we're dealing with a processor that is a close relative to a piece of tinfoil. Just think about all the poor, wasted clock cycles that occur while polling the GPIO state repeatedly.

Alright, what about one of those Python scripts?

With raw access to GPIO and the ability to wait for GPIO events without wasting CPU time, Python is definitely a better way to go.

So let's say you have a Model B rev 2 board with 512MB of RAM. After you give some to the GPU, you only really have 400MB or 500MB left. Each instance of the Python interpreter takes up roughly 4MB, and I don't consider that to be a small loss.

Okay then. Prove that yours really is better!

There's some fine documentation on Linux kernel gpio/sysfs that tells us how the kernel can alert us when the value of a GPIO has changed. This eliminates the need for constant sampling of the GPIO. Here's how it works:

  1. Export the pins to userspace with /sys/class/gpio/export and set their direction with /sys/class/gpio/gpioN/direction.
  2. Set the value of /sys/class/gpio/gpioN/edge to setup an interrupt-generating pin.
  3. poll(2) on /sys/class/gpio/gpioN/value for the events POLLPRI and POLLERR
  4. Wait for poll to return and you've got a winner!

In the context of Linux system calls, poll means "wait for events to occur", as opposed to the traditional definition of polling, which is more along the lines of "actively sampling".

Here's the output of top showing the CPU and RAM usage of this program:

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
22682 root      20   0  1504  284  228 S   0.0  0.1   0:00.00 mausberry-switch

Alright, I'm convinced. How do I install this thing?

Please see doc/building.md for instructions on compiling and installing this package.

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