Using dbus in pi

Note on getting dbus interfaces

Lot of the followings have been actually updated, check https://github.com/kkibria/dbus-dev

the xml files were generated in raspberry pi

dbus-send --system --dest=fi.w1.wpa_supplicant1 \
  --type=method_call --print-reply=literal /fi/w1/wpa_supplicant1 \
  org.freedesktop.DBus.Introspectable.Introspect > wpa.xml

dbus-send --system --dest=org.freedesktop.timedate1 \
  --type=method_call --print-reply=literal /org/freedesktop/timedate1 \
  org.freedesktop.DBus.Introspectable.Introspect > timedate.xml

and copied in the project.

then on wsl we can use the xml files

dbus-codegen-rust -s -f org.freedesktop.timedate1 < timedate.xml > src/timedate.rs
dbus-codegen-rust -s -f fi.w1.wpa_supplicant1 < wpa.xml > src/wpa.rs

alternatively we can use dbus-codegen-rust on pi to generate rust files directly and copy to rust project

put it all together in get-pi-rs.sh

#assuming RPI is already exported in .bashrc
ssh $RPI 'bash -s' <<-"EOF"
 export PATH=$HOME/.cargo/bin:$PATH
 rm -rf temp-wsl
 mkdir temp-wsl
 pushd temp-wsl
 dbus-codegen-rust -s -d org.freedesktop.timedate1 -p "/org/freedesktop/timedate1" -f org.freedesktop.timedate1 > timedate.rs
 dbus-codegen-rust -s -d fi.w1.wpa_supplicant1 -p "/fi/w1/wpa_supplicant1" -f fi.w1.wpa_supplicant1 > wpa.rs
 popd
EOF
rcp $RPI:temp-wsl/*.rs src

Python example for dbus

• wpas-dbus-new.py

Setting up dbus for interacting with WPA_SUPPLICANT

I was trying to write some code that is to use dbus api to access wpa_supplicant. My understanding from reading various posts that wpa_supplicant must be started with -u flag to fully expose it's apis to dbus. So I edited, /lib/dhcpcd/dhcpcd-hooks/10-wpa_supplicant to by adding the -u flag to the invocation of the wpa_supplicant daemon in the wpa_supplicant_start().

At this point I couldn't use wpa_cli to connect to wlan0 anymore. I checked the processes with ps and got,

pi@raspi:~ $ ps -aux | grep wpa_sup
root       306  0.0  1.0  10724  4732 ?        Ss   21:21   0:00 /sbin/wpa_supplicant -u -s -O /run/wpa_supplicant

So, I edited /lib/dhcpcd/dhcpcd-hooks/10-wpa_supplicant again to remove -u flag, rebooted etc. and again checked the processes. This time I got,

pi@raspi:~ $ ps -aux | grep wpa_sup
root       260  0.3  1.0  10724  4640 ?        Ss   21:25   0:00 /sbin/wpa_supplicant -u -s -O /run/wpa_supplicant
root       350  0.1  0.9  10988  4052 ?        Ss   21:25   0:00 wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0 -Dnl80211,wext

and now I can use wpa_cli to connect to wlan0.

This is confusing to me. I am not sure why.

After some digging, it appears that wpa_supplicant.service should be disabled as it was preventing wpa_cli from connecting to wlan0.

After doing,

sudo systemctl disable wpa_supplicant
sudo reboot

I was able to connect.

I am still not sure why.

https://raspberrypi.stackexchange.com/questions/132998/confusing-wpa-supplicant-processes-while-trying-to-use-dbus-api

this has an explanation. https://github.com/mark2b/wpa-connect

cross compile dbus

• https://github.com/diwic/dbus-rs/blob/master/libdbus-sys/cross_compile.md
• https://serverfault.com/questions/892465/starting-systemd-services-sharing-a-session-d-bus-on-headless-system headless dbus.
• https://raspberrypi.stackexchange.com/questions/114739/how-to-install-pi-libraries-to-cross-compile-for-pi-zero-in-wsl2.
• https://airtower.wordpress.com/2010/07/20/using-gvariant-tuples/
• https://fosdem.org/2020/schedule/event/rust_dbus_library/

Working with dbus

How do I get properties using dbus

I have listed the properties that I am interested in using timedatectl which uses systemd dbus,

$ timedatectl
               Local time: Tue 2020-07-28 19:37:00 PDT
           Universal time: Wed 2020-07-29 02:37:00 UTC
                 RTC time: n/a
                Time zone: America/Los_Angeles (PDT, -0700)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

Next, I checked timedatectl.c in systemd source code to get bus endpoint and method using which I went ahead and introspected,

$ dbus-send --system --dest=org.freedesktop.timedate1 --type=method_call --print-reply /org/freedesktop/timedate1 org.freedesktop.DBus.Introspectable.Introspect

method return time=1595997538.869702 sender=:1.30 -> destination=:1.29 serial=3 reply_serial=2
   string "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
 ...
 <interface name="org.freedesktop.DBus.Properties">
  ...
  <method name="GetAll">
   <arg name="interface" direction="in" type="s"/>
   <arg name="properties" direction="out" type="a{sv}"/>
  </method>
  ... 
 </interface>
 <interface name="org.freedesktop.timedate1">
  <property name="Timezone" type="s" access="read">
  </property>
  <property name="LocalRTC" type="b" access="read">
  </property>
  ...
 </interface>
</node>
"

Next I tried to use the method GetAll,

$ dbus-send --system --dest=org.freedesktop.timedate1 --type=method_call --print-reply /org/freedesktop/timedate1 org.freedesktop.DBus.Properties.GetAll string:org.freedesktop.timedate1

method return time=1595997688.111555 sender=:1.33 -> destination=:1.32 serial=4 reply_serial=2
   array [
      dict entry(
         string "Timezone"
         variant             string "America/Los_Angeles"
      )
      dict entry(
         string "LocalRTC"
         variant             boolean false
      )
      dict entry(
         string "CanNTP"
         variant             boolean true
      )
      dict entry(
         string "NTP"
         variant             boolean true
      )
      dict entry(
         string "NTPSynchronized"
         variant             boolean true
      )
      dict entry(
         string "TimeUSec"
         variant             uint64 1595997688110070
      )
      dict entry(
         string "RTCTimeUSec"
         variant             uint64 0
      )
   ]

and we get our desired result same as timedatectl.