Classes of Devices and Modules

The Unix way of looking at devices distinguishes between three device types. Each module usually implements one of these types, and thus is classifiable as a char module, a block module, or a network module. This division of modules into different types, or classes, is not a rigid one; the programmer can choose to build huge modules implementing different drivers in a single chunk of code. Good programmers, nonetheless, usually create a different module for each new functionality they implement, because decomposition is a key element of scalability and extendability.

The three classes are the following:

Character devices

A character (char) device is one that can be accessed as a stream of bytes (like a file); a char driver is in charge of implementing this behavior. Such a driver usually implements at least the open, close, read, and write system calls. The text console (/dev/console) and the serial ports (/dev/ttyS0 and friends) are examples of char devices, as they are well represented by the stream abstraction. Char devices are accessed by means of filesystem nodes, such as /dev/tty1 and /dev/lp0. The only relevant difference between a char device and a regular file is that you can always move back and forth in the regular file, whereas most char devices are just data channels, which you can only access sequentially. There exist, nonetheless, char devices that look like data areas, and you can move back and forth in them; for instance, this usually applies to frame grabbers, where the applications can access the whole acquired image using mmap or lseek.

Block devices

Like char devices, block devices are accessed by filesystem nodes in the /dev directory. A block device is something that can host a filesystem, such as a disk. In most Unix systems, a block device can be accessed only as multiples of a block, where a block is usually one kilobyte of data or another power of 2. Linux allows the application to read and write a block device like a char device—it permits the transfer of any number of bytes at a time. As a result, block and char devices differ only in the way data is managed internally by the kernel, and thus in the kernel/driver software interface. Like a char device, each block device is accessed through a filesystem node and the difference between them is transparent to the user. A block driver offers the kernel the same interface as a char driver, as well as an additional block-oriented interface that is invisible to the user or applications opening the /dev entry points. That block interface, though, is essential to be able to mount a filesystem.

Network interfaces

Any network transaction is made through an interface, that is, a device that is able to exchange data with other hosts. Usually, an interface is a hardware device, but it might also be a pure software device, like the loopback interface. A network interface is in charge of sending and receiving data packets, driven by the network subsystem of the kernel, without knowing how individual transactions map to the actual packets being transmitted. Though both Telnet and FTP connections are stream oriented, they transmit using the same device; the device doesn’t see the individual streams, but only the data packets.

Not being a stream-oriented device, a network interface isn’t easily mapped to a node in the filesystem, as /dev/tty1 is. The Unix way to provide access to interfaces is still by assigning a unique name to them (such as eth0), but that name doesn’t have a corresponding entry in the filesystem. Communication between the kernel and a network device driver is completely different from that used with char and block drivers. Instead of read and write, the kernel calls functions related to packet transmission.

Other classes of driver modules exist in Linux. The modules in each class exploit public services the kernel offers to deal with specific types of devices. Therefore, one can talk of universal serial bus (USB) modules, serial modules, and so on. The most common nonstandard class of devices is that of SCSI[1] drivers. Although every peripheral connected to the SCSI bus appears in /dev as either a char device or a block device, the internal organization of the software is different.

Just as network interface cards provide the network subsystem with hardware-related functionality, so a SCSI controller provides the SCSI subsystem with access to the actual interface cable. SCSI is a communication protocol between the computer and peripheral devices, and every SCSI device responds to the same protocol, independently of what controller board is plugged into the computer. The Linux kernel therefore embeds a SCSI implementation (i.e., the mapping of file operations to the SCSI communication protocol). The driver writer has to implement the mapping between the SCSI abstraction and the physical cable. This mapping depends on the SCSI controller and is independent of the devices attached to the SCSI cable.

Other classes of device drivers have been added to the kernel in recent times, including USB drivers, FireWire drivers, and I2O drivers. In the same way that they handled SCSI drivers, kernel developers collected class-wide features and exported them to driver implementers to avoid duplicating work and bugs, thus simplifying and strengthening the process of writing such drivers.

In addition to device drivers, other functionalities, both hardware and software, are modularized in the kernel. Beyond device drivers, filesystems are perhaps the most important class of modules in the Linux system. A filesystem type determines how information is organized on a block device in order to represent a tree of directories and files. Such an entity is not a device driver, in that there’s no explicit device associated with the way the information is laid down; the filesystem type is instead a software driver, because it maps the low-level data structures to higher-level data structures. It is the filesystem that determines how long a filename can be and what information about each file is stored in a directory entry. The filesystem module must implement the lowest level of the system calls that access directories and files, by mapping filenames and paths (as well as other information, such as access modes) to data structures stored in data blocks. Such an interface is completely independent of the actual data transfer to and from the disk (or other medium), which is accomplished by a block device driver.

If you think of how strongly a Unix system depends on the underlying filesystem, you’ll realize that such a software concept is vital to system operation. The ability to decode filesystem information stays at the lowest level of the kernel hierarchy and is of utmost importance; even if you write a block driver for your new CD-ROM, it is useless if you are not able to run ls or cp on the data it hosts. Linux supports the concept of a filesystem module, whose software interface declares the different operations that can be performed on a filesystem inode, directory, file, and superblock. It’s quite unusual for a programmer to actually need to write a filesystem module, because the official kernel already includes code for the most important filesystem types.

[1] SCSI is an acronym for Small Computer Systems Interface; it is an established standard in the workstation and high-end server market.

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