Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Devices and Drivers (Chapter 7) Khattab Alhabashi UNIX System Administration
Introduction • Definition: A device driver is a program that manages the system’s interaction with a particular piece of hardware. • The driver translates between the hardware commands understood by the device and the stylized programming interface used by the kernel. • The existence of the driver layer helps to keep UNIX reasonably device-independent. • Device drivers are part of the kernel; they are not user processes. However, a driver can be accessed both from within the kernel and from user space. • User-level access to devices is provided through special device file that live in the /dev directory.
Introduction Contd. • Most devices in the system will fall into three categories: 1) SCSI: • Devices that attach to a SCSI bus are easy to configure. • Most systems have one driver that allows direct access to the SCSI bus, plus additional drivers for special types of devices such as disks and tapes. 2) Vendor: • Most devices you can buy from your hardware vendor will already be supported by the operating system. • Unless you have explicitly removed drivers, the kernel will usually recognize a new device as soon as you install it. • The appropriate device files in /dev will already have been created for you, or the OS will create them on the fly.
Introduction Contd. 3) Third-party: • When you purchase a piece of hardware from a third party (someone other than your hardware vendor), it will usually come with an installation script that installs the device, configures the kernel if necessary, and makes devices entries. • On systems with loadable drivers, kernel configuration may not be necessary.
Device Numbers and Jump Tables • Device files are mapped to devices via their “major and minor device numbers”, values that are stored in the file’s inode structure. • The major device number identifies the driver that the file is associated with. • The minor device number identifies which particular device of a given type is to be addressed. It is often called the unit number or “instance” of the device. • There are two types of device files: • Block device files: it is read or written a block (a group of bytes, usually multiple of 512) at a time. • Character device files: it can be read or written one byte at a time.
Device Numbers and Jump Tables Contd. • Some devices support access via both block and character device files. • Each driver has routines for performing some or all of the following functions: probe attach open close read reset stop select strategy dump psize write timeout process a transmit interrupt process a receive interrupt ioctl (input / output control) • Inside the kernel, the addresses of these functions for each driver are stored in a structure called a jump table.
Device Numbers and Jump Tables Contd. • There are actually two tables: - One for character device and - One for block devices. • The jump tables are indexed by major device numbers. • When a program performs an operation on a device file, the kernel automatically catches the reference, looks up the appropriate function name in the jump table, and transfers control to it. • To perform an unusual operation that doesn’t have a direct analog in filesystem model (for example, ejecting floppy disk), the ioctl system call is used to pass a message directly from user space into the driver.
Device Numbers and Jump Tables Contd. • Three examples were discussed in this chapter about adding device drivers: - Adding a BSD Device Driver: Adding a completely new device driver to BSD machine involves adding it to a couple of configuration files and editing the kernel source code to include references to the driver’s routine. - Adding an HP-UX Device Driver: On HP-UX systems, the jump tables are constructed from specifications in a text file called master. - Adding an IRIX Device Driver: IRIX uses a directory of files (/var/sysgen/master.d) to perform the same function as the single master file in HP-UX.
Device Numbers and Jump Tables Contd. • There are some common steps in all three examples that have to be done after adding and modifying the files specified for each example: - Building a new kernel. - Copying the old kernel aside and installing the new kernel. - Rebooting and testing the new kernel. - Creating device files and test the device itself.
Device Files • By convention, device files are kept in the /dev directory. • ATT systems handle device files quite nicely by using separate subdirectory of /dev for each type of device: disk, tape, terminal, etc. • Device files are created with the mknod command, which has the syntax: mknodfilename type major minor where - filename is the device file to be created. - type is c for a character device or b for a block device. - major and minor are the major and minor device numbers.
Device Files Contd. • A shell script called MAKEDEV is sometimes provided (in /dev) to automatically supply default values to mknod. • You need to scan through the script to find the arguments needed for your device. For example, to make PTY entries on a SunOS system, you would use the following command: # cd /dev # ./MAKEDEV pty
Naming Conventions For Devices • Devices that have both block and character identities usually - preface the character device name with the letter r for “row” (e.g., /dev/sd0 and /dev/rsd0), or - place it in a subdirectory with a name that starts with r (e.g., /dev/dsk/dks0d3s0 vs. /dev/rdsk/dks0d3s0). • Serial devices are usually named tty followed by a sequence of letters that identify the interface the port is attached to (See chapter 8 for more information about serial ports).
Naming Conventions For Devices Contd. • BSD disk names often begin with a two-letter abbreviation for either the drive of the controller, followed by the drive number and partition name. - Example: sd0a is the block device representing the partition of the first disk drive on a SCSI controller; rsd0a is the corresponding character device. • The names of tapes and devices often include not only a reference to the drive itself, but also an indication of whether the device rewinds after each tape operation and the density at which it reads and writes.
Loadable Kernel Modules • Loadable modules allow device drivers to be linked into and removed from the kernel while it is running. - This makes the installation of drivers much easier, since the kernel binary does not have to be changed. - It also allows the kernel to be smaller, because drivers are not loaded unless they are needed. • Loadable modules are implemented by providing one or more documents “hooks” into the kernel where additional device drivers can grab on. • A user-level command communicates with the kernel and tells it to load new modules into memory and to make entries for them in the system’s jump tables.
Loadable kernel Modules Contd. • Although loadable drivers are convenient, they are not entirely safe. • Any time you load or unload a module, you risk causing a kernel panic. • Like other aspects of device and driver management, the implementation of loadable modules is operating system dependent. • There are four of the example systems that support loadable modules: - Solaris: In Solaris, virtually everything is a loadable module. The modinfo command lists the currently-loaded modules. You can add a driver with the add_drv command.
Loadable Kernel Modules Contd. - HP-UX: Generic HP-UX does not support loadable modules, but there is an option that allows the loading of STREAMS modules. - IRIX: IRIX 5.2 supports loadable modules; earlier versions did not. Modules are manipulated with the ml command. ml with the list option catalogs the modules that the kernel is currently aware of. - SunOS: SunOS versions 4.1.2 and later contain support for loadable modules. Currently-loaded modules can be listed with modstat.
END OF CHAPTER QUESTIONS???