async fn write_reg<E: WriteReg>(&mut self, i2c_addr: u8, val: E::Val) -> Result<(), Error> {
    let mut buf = [0u8; size_of::<E::AddrTy>() + size_of::<E::Val>()];
    /* ... fill the bytes with right endianness ... */
    i2c.write(i2c_addr, &buf).await
}