Driver development

[NANDAKUMAR S]

Hi all,

  Iam working on an android aosp project, in which now i want to develop drivers for UART,I2C communication protocols. But iam new to this LINUX and Android platform.

Can anyone suggest me some resources or a procedure on how to learn this.

[Dave McLaughlin]

UART and I2C are simple enough. The underlying Linux drivers should be part of the kernel and you simply open them from /dev in your Android App. The only issue you may have is not having user rights access to them.

What I did was give user access to GPIO, UART, I2C, and SPI from the init.rc and then wrote a JNI to access the /dev or /sys/class/gpio etc. 

This is assuming you are developing the Android OS build yourself. That certainly makes it easier to set up access rights from init.rc.

Check your /dev directory and see if the tty or i2c devices are already present in your build. If so, you just need to check the access rights and write a JNI to use them from an Android App.

Dave...

[NANDAKUMAR S]

Thanks for your reply,

I have tty in /dev directory. I'll check with permissions. 

Can i get any readily available app for testing.

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[Dave McLaughlin]

Not likely as Android apps tend to work with phones which don't have the need for I2C or UART comms etc.

I wrote a couple of JNI libraries to handle GPIO, I2C and UART, you can find it here. This can be added to your project.

http://www.axoninstruments.biz/download/gpioserial.zip

This is just the library for I2C and UART. This is very basic code for the I2C (I omitted the required exception handling)

Open the I2C device

            fileHandle = i2c.open("/dev/i2c-0");

Read I2C

        i2c.read(fileHandle, slaveAddr, buf, 3);

Write I2C

i2c.write(fileHandle, slaveAddr, 0, buf, 1);

Close the device

i2c.close(fileHandle);

Buf is an int array. slaveAddr is the 7 bit address of the device you want to talk to.

Serial Port is pretty much similar. I used this with a Modbus library. You only need to allocate the file handle to the streams for this to work. Search for modbus4j by Serotonin. This works great with Android. 

I hope this helps?

[Ignacio Moya]

Hi, I was trying to change permissions of the i2c files in /dev/i2c-x (x: from 0 to 3) adding the lines "chmod 777 /dev/i2c-x" inside the "on early-boot"  action. But trying to run i2cdetectx from shell still throws "permission denied".

Could someone show me where should I change permissions inside the init.rc file? Or another way of doing the same but not in "init.rc"? I'm using AOSP 9.

Thanks!

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[Ignacio Moya]

Hi, I managed to solved it setting selinux enforce policy to "permissive". It is possible to do that from adb shell with the commands "su", then "setenforce 0" and then use "getenforce" to see if your change worked. This setting is only valid until reboot.