Linux Device Drivers

Prithvi N. Rao

January 12th, 2012

Agenda

● Overview of Linux Device Driver Architecture (Desktop)

● Overview of Linux Device Driver Architecture (Android)

● Device Driver Development Checklist

● How much do you really need to know about Hardware?

● Writing a Linux/Android Device Driver – Getting Started

● Debugging a Linux Device Driver in Android

● Driver Event propagation to the Android Application

● Accessing Events from Android User Space

● Using CCSv5 for debugging Linux kernel in Android

!Agenda

● Dinner – You're on your own :-)!

● History of Linux

● Details of how Linux works

● Android Tutorial

● How to Build Linux kernel for Android

● How to create a NAND flash image

● Shrink Wrapped Driver Development Kit !

Acknowledgements

● Oleg Perelet● Tom Olson● Jeff Janis● Roger Yang

Overview of Linux Device Driver Architecture (Desktop)

Overview of Linux Device Driver Architecture (Android)

Overview of Linux Device Driver Architecture (Android)

Device Driver Development Checklist

● Machine Running Linux or Equivalent with sources installed (Git)

● EVM for Development, RS232 Cable, USB Cable for EVM

● JTAG Emulator/Trace 32 Lauterbach

● Hardware Reference manuals

– Board (Mistral AM/DM37x OMAP 3730 TRM)

– Device (Avago ADBS-A350 Optical Finger Navigation Data Sheet)

● Code Composer Studio version 5

● Putty (for Unix connectivity)

● SD Card and reader/writer

How Much Do You Really Need to Know About Hardware?

● The more you know the better

– Be nice to the hardware chaps...you will need them :-)!

– Bringing food and other goodies helps (Android Tablets, Android Phones)

● Seriously you need to spend some time understanding

– How the device is hooked up to the EVM

– How to read a schematic

– How interrupts work (edge triggered, level triggered)

– How to use an oscilloscope

– How to convince the hardware chaps that it is their problem :-)!

– I2C Protocol and SMBUS

MOST OF ALL DON'T BE INTIMIDTED BY MOST OF ALL DON'T BE INTIMIDTED BY HARDWAREHARDWARE

Writing a Linux Device Driver

TIME TO GET REALTIME TO GET REAL

Optical Finger Navigation Driver

● First determine what is the purpose of this device

– Input Only (in our case for OFN)

– Output Only

– Both

● Decide where in the kernel sources it should reside

– kernel/drivers/input/mouse/avagoADBMA350.c

– kernel/drivers/input/mouse/avagoADBMA350.h

● Edit the Kconfig and Makefile

– Each directory in “drivers” has a Kconfig and Makefile

– Enter the details of the driver code in here

– Top level kernel build picks all this up and handles the build

Optical Finger Navigation Driver

● Draw a block diagram of the hardware and control and data flows

– K3-K4->Deliverables->K3-OpticalTouchPad

● Follow the “guidelines” on how to write in-kernel device drivers

– Look at ”safari books on line”

– Use Google

– kernel/Documentation has lots of stuff but requires filtering

– Look at other in-kernel drivers in Linux - REALLY!REALLY!

!PANIC!PANIC

Debugging a Linux Device Driver

● Panicking can be good

– If it is in the kernel – you know where the PC is

– printk can get you a long way

– U-boot is very useful but requires some work

– Debugfs is absolutely a must (in my opinion)

● .config in kernel directory allows you to output various kernel driver messages

● Look for #interrupts in /proc/interrupts

● Look for events in /dev/input/eventX (from your device)

● CCSv5 is useful for kernel debug in general

– Talk to me if you want to know how to do this

Driver Event Propagation in Linux/Android

● Linux exposes externally a uniform input event interface for each device

– /dev/input/eventX

– Each device driver has to register/unregister itself

– Each device driver publishes input events that it receives

● Sequence of events for device driver in the kernel

– Probe detects if the device is connected

– input-register-device invoked if device found

– request_irq is used to associate a handler with interrupt

– set up work_queue inside handler (linux version dependent)

It's more fun to look at code :-)!It's more fun to look at code :-)!

Driver Event Propagation in Linux/Android

● On receiving an interrupt (user pressed touchpad or moved it)

– IRQ handler is invoked

– Schedule work to propagate event to Android Framework

● input_report_rel● input_sync● input_report_key

● Android Framework provides a JNI interface to the upper layers.

Accessing Events from Android User Space

● Android GUI initiates access to events

– Instance of WindowManagerService.java is created

– Utilize MotionEvent.java, InputDevice.java, InputEvent.java,InputQueue.java, EventHub.java

– Android FrameWork intercepts events and provides a way to access event queues (/dev/input/eventX)

Overview of Linux Device Driver Architecture (Android)

Key Take Aways● Writing a Linux Device Driver is one of the most rewarding experiences

● Don't be intimidated – it's only kernel programming :-)!

● Respect the fact that you ARE working with a serious OS kernel

– The driver must play well with the rest of the system

– Linux has made it simpler than Unix

● Understand the hardware as much as you can.

– You will do so eventually...trust me :-)!

● Share information with the hardware folks

– Don't blind side them at the last minute with tons of debug info

● Be aware that driver architecture has changed in Linux over time

ACCEPT THE CHALLENGE – YOU WON'T REGRET IT!