I’m building a custom Nerves system for a CM4-based board and running into a timing issue where the main Elixir application starts before all kernel drivers and hardware are fully initialized.

My firmware uses NervesTime with an RTC (PCF8563) on an I2C bus that’s created by a device tree overlay. During boot, NervesTime tries to initialize the RTC, but the I2C bus doesn’t exist yet:

22:43:22.173 [info] i2c i2c-22: Added multiplexed i2c bus 10

22:43:22.218 [debug] [NervesTime] starting /usr/sbin/ntpd with: ["-n", "-S", "/srv/erlang/lib/nerves_time-0.4.9/priv/ntpd_script", "-p", "0.pool.ntp.org", "-p", "1.pool.ntp.org", "-p", "2.pool.ntp.org", "-p", "3.pool.ntp.org"]

22:43:22.237 [error] [NervesTime] Cannot initialize rtc '{NervesTime.RTC.NXP.PCF8563, [bus_name: "i2c-10", address: 81]}': :bus_not_found

When I try to initialize the RTC later via iex, it works with no issues:

iex(2)> {:ok, ref} = NervesTime.RTC.NXP.PCF8563.init(bus_name: "i2c-10", address: 0x51)
{:ok,
 %{
   address: 81,
   bus_name: "i2c-10",
   i2c: %Circuits.I2C.I2CDev{
     ref: #Reference<0.3964485155.1160904719.209822>,
     retries: 0,
     flags: [:supports_empty_write]
   }
 }}

I am considering adding an erlinit pre-run script to explicitly wait for the the hardware to be fully initialized, but am curious if there is a recommended Nerves pattern for waiting on hardware initialization? Ideally this would be baked into the Nerves system.