Device Driver Concept Intro

• Kernel Mode ==> Physical Address Space. Access to HW registers is possible
• User Mode ==> Virtual Addresses. Direct access to physical address space is not possible 
• But, the file system is a shared name space useful for
  • Accessing shared system resources such as devices and shared memory
  • Performing inter-process communication (named pipes)
  • Naming of semaphores
• A file is an interface that exports:
  •     open():  convert public name space to local name, established "proper" access to the file
  •         proper does not necessarily mean exclusive, just that read, write operations perform properly
  •     read()  returns bytes from the file/device (advances file position)
  •     write(): appends bytes to the file/device (at current position)
  •     close(): relinquish access to the device (invalidates the local file descriptor)
  •     lseek(): move the position in the file
• A device can be hidden behind a file. Any call to open, read, write, close will cause the associated device driver to perform appropriate actions on the device and return appropriate results to the caller. This is basically how processes can share memory, access hardware, etc. Users do not have the power to install device drivers, so protection is maintained. 

Example: cat

          cat does this (not showing a lot of stuff, like error checking, end of file, etc).

            main(argc, argv) {

                while (1) {

                    s = read(stdin, buf, 1);

                    s = write(stdout, buf, 1);

            }

'cat < /dev/console >/dev/console' is equivalent to 'cat'

both will simply echo what you type. /dev/console is special file in the file system that has a "device driver" associated with it. When you call write() on this file, you are really calling a function in the device driver package associated with /dev/console. In this case, the device driver, running in kernel space, can send characters to the user's display and get them from the keyboard by accessing the physical memory space.

Device Drivers

• A set of functions that run in Kernel Mode that present file-like interface to user programs. The device needs to act like a file in some way so that users can say open("/dev/motor",...)...
• To use a device driver you must:
  • Create a special file (node) on the disc and tell the OS which device driver is associated with it. (mknod, insmod)
  • Write the device driver, which is really just a collection of functions that are allowed to run in kernel mode so that direct access to the HW resources is possible. The key point is that you must define open(), read(), write(), close(), etc. so that a user can treat your device like a file. Not all operations must be defined for all devices. Some devices are "write" only and some are "read" only. In this case, open should fail (return -1) in the case that the user opens the device in an unsupported mode.
  • Install the device driver. This can be done either when you compile the operating system or you can install a device driver when the OS is already running (in Linux). When it is properly installed, the OS passes the open("/dev/motor") system call to the open_motor() function in your device driver (as an example).

Example

    With the proper device driver, you should be able to get sonar data from a sonar device using

    cat < /dev/sonar

    The device driver has to deal either with the sonar directly, or maybe it communicates with an 8051 that is driving the sonar.
    All of this is now hidden from the user.

Homework Assignment

Suppose you are building a robot with the Cerfboard as the central controller. The robot has two wheels and two motors to drive them (right and left). You can independently control the direction and speed of each motor. Specify a device driver interface that you think would be appropriate for the motors. All I want you do to is answer these questions:

• What should the user program be responsible for? 
• What should the device driver be responsible for? 
• Define functionality of open(), read(), write(), and close() based on the decomposition that you proposed above. I'm talking a sentence or two on each!

This week's lab assignment has a template for a device driver.