A function to check for the dht11 overlay and only load it if it isn’t already loaded is easy enough with just some string handling and the use of the Popen method:

import subprocess 
import io
import fcntl
def checkDht11():
    indicator = "dht11  gpiopin=4"
    command =["sudo", "dtoverlay", "dht11","gpiopin=4"]
    temp = subprocess.Popen(["sudo", "dtparam", "-l"],
                              stdout = subprocess.PIPE) 
    output = str(temp.communicate())
    print(output)
    if output.find(indicator)==-1:
        temp = subprocess.Popen(command,
                             stdout = subprocess.PIPE) 
        output = str(temp.communicate())
        print(output)
    return
checkDht11()
fdr = io.open("/sys/bus/iio/devices/iio:device0/name",
                                      "rb", buffering=0)
name = fdr.readline().decode("utf-8")
print("name=", name)
fdr.close()
fdr = io.open(
      "/sys/bus/iio/devices/iio:device0/in_temp_input",
                                      "rb", buffering=0)
temp = fdr.readline().decode("utf-8")
print("temp=", int(temp)/1000, "C")
fdr.close()
fdr = io.open("/sys/bus/iio/devices/iio:device0/
         in_humidityrelative_input", "rb", buffering=0)
hum = fdr.readline().decode("utf-8")
print("Humidity=",int(hum)/1000,"%")
fdr.close()

The indicator string is used to specify the string that is in the overlay listing when the driver is loaded and this isn’t necessarily the same as the string in the command string, which is the command to install the driver with any parameters that are needed.

You can run this program without having to change the config.txt file. The driver remains loaded until the next reboot and is reinstalled when you next run the program. Notice that the program has to have root permissions to be able to run sudo, even if the program isn’t being run as root.

Summary

• The Device Tree is the modern way to specify the hardware configuration of a machine and to load and configure drivers.

• The boot disk contains device trees for each of the different versions of the Pi and the loader selects the appropriate device tree for the Pi model that is booting.

• The loader merges the device tree with the overlays listed in the config.txt file to produce a customized final device tree used to boot the system.

• Overlays are fragments of the device tree which can be used to modify and add to the basic device tree.

• Overlays can themselves be customized by specifying parameters.

• When the loader parses the DT and applies all the overlays you have specified, the final DT is represented by the folders in /proc/overlay.

• The DHT22 Humidity and Temperature Sensor provides an example of loading and customizing a driver.

• A fairly recent innovation is the ability to load and customize drivers after the system has booted using the dtoverlay and dtparam commands.

• You can easily write a program to load and configure missing drivers at runtime.

• Dynamic loading of overlays doesn’t always work and unloading overlays, while possible, isn’t a good idea.

Raspberry Pi IoT In PythonUsing Linux Drivers
Second Edition

By Harry Fairhead & Mike James

Buy from Amazon.

Contents

1.  Choosing A Pi For IoT
2.  Getting Started With Python
3.   Drivers: A First Program
4.  The GPIO Character Driver ***NEW!!
5.  GPIO Using I/O Control
6.  GPIO Events
7.  The Device Tree
      Extract: The DHT22 * 
8.  Some Electronics
9.  Pulse Width Modulation
      Extract: PWM *
10. SPI Devices
11. I2C Basics
       Extract: I2C *
12. The I2C Linux Driver
13. Advanced I2C
14. Sensor Drivers
15. 1-Wire Bus
       Extract 1-Wire And The DS18B20 *
16. Going Further With Drivers
17. Appendix I

*From the first edition waiting for update.

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