Writing Device Drivers

Testing Specific Types of Drivers

Because each type of device is different, it is difficult to describe how to test them all specifically. This section provides some information about how to test certain types of standard devices.

Tape Drivers

Tape drivers should be tested by performing several archive and restore operations. The cpio(1) and tar(1) commands may be used for this purpose. The dd(1M) command can be used to write an entire disk partition to tape, which can then be read back and written to another partition of the same size, and the two copies compared. The mt(1) command will exercise most of the I/O controls that are specific to tape drivers (see mtio(7I)); all the options should be attempted. The error handling of tape drivers can be tested by attempting various operations with the tape removed, attempting writes with the write protect on, and removing power during operations. Tape drivers typically implement exclusive-access open(9E) calls, which should be tested by having a second process try to open the device while a first process already has it open.

Disk Drivers

Disk drivers should be tested in both the raw and block device modes. For block device tests, a new file system should be created on the device and mounted. Multiple file operations can be performed on the device at this time.

Note -

The file system uses a page cache, so reading the same file over and over again will not really exercise the driver. The page cache can be forced to retrieve data from the device by memory-mapping the file (with mmap(2)), and using msync(2) to invalidate the in-memory copies.

Another (unmounted) partition of the same size can be copied to the raw device and then commands such as fsck(1M) can be used to verify the correctness of the copy. The new partition can also be mounted and compared to the old one on a file-by-file basis.

Asynchronous Communication Drivers

Asynchronous drivers can be tested at the basic level by setting up a login line to the serial ports. A good start is if a user can log in on this line. To sufficiently test an asynchronous driver, however, all the I/O control functions must be tested, and many interrupts at high speed must occur. A test involving a loopback serial cable and high data transfer rates will help determine the reliability of the driver. Running uucp(1C) over the line also provides some exercise; however, since uucp(1C) performs its own error handling, it is important to verify that the driver is not reporting excessive numbers of errors to the uucp(1C) process.

These types of devices are usually STREAMS based.

Network Drivers

Network drivers may be tested using standard network utilities. ftp(1) and rcp(1) are useful because the files can be compared on each end of the network. The driver should be tested under heavy network loading, so that various commands can be run by multiple processes. Heavy network loading means:

Network cables should be unplugged while the tests are executing to ensure that the driver recovers gracefully from the resulting error conditions. Another important test is for the driver to receive multiple packets in rapid succession (back-to-back packets). In this case, a relatively fast host on a lightly loaded network should send multiple packets in quick succession to the test machine. It should be verified that the receiving driver does not drop the second and subsequent packets.

These types of devices are usually STREAMS based.