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The second problem, that of dynamically assigning ethernet devices to the standard /dev/ethN device special files is solved more elegantly. There are eight standard entries in the devices list; one for eth0, eth1 and so on toeth7. The initialization routine is the same for all of them, it tries each ethernet device driver built into the kernel in turn until one finds a device. When the driver finds its ethernet device it fills out the ethN device data structure, which it now owns.
If the request list is being processed, the buffer cache need not do anything else; otherwise it must nudge the SCSI disk subsystem to go and process its request queue. Each SCSI disk in the system is represented by a Scsi_Disk data structure. These are kept in the rscsi_disks vector that is indexed using part of the SCSI disk partition’s minor device number. For exmaple, /dev/sdb1 has a major number of 8 and a minor number of 17; this generates an index of 1. Each Scsi_Disk data structure contains a pointer to the Scsi_Device data structure representing this device. That in turn points at the Scsi_Host data structure which “owns” it.
For example a system with two NCR810 PCI SCSI controllers would have two Scsi_Host entries in the list, one per controller. Each Scsi_Host points at the Scsi_Host_Template representing its device driver.
If the driver could find a device it fills out the rest of the device data structure with information about the device and the addresses of the support functions within the network device driver. Once Linux has initialized the SCSI subsystem, the SCSI devices may be used. Each active SCSI device type registers itself with the kernel so that Linux can direct block device requests to it. There can be buffer cache requests via blk_dev or file operations via blkdevs. Taking a SCSI disk driver that has one or more EXT2 filesystem partitions as an example, how do kernel buffer requests get directed to the right SCSI disk when one of its EXT2 partitions is mounted? Each request to read or write a block of data to or from a SCSI disk partition results in a newrequest structure being added to the SCSI disks current_request list in the blk_devvector.
It is also at this time that the network device driver initializes the physical hardware that it is controlling and works out which IRQ it is using, which DMA channel and so on. A driver may find several instances of the network device that it is controlling and, in this case, it will take over several of the /dev/ethN device data structures. Once all eight standard /dev/ethN have been allocated, no more ethernet devices will be probed for.
This device tells the hard drive what to do and how to move its components. When the operating system needs to read or write information, it examines the hard drive’s FAT to determine file location and available write areas. Once they determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information. A hard drive can store any data, including pictures, music, videos, text documents, and any files created or downloaded. Also, hard drives store files for the operating system and software programs that run on the computer.
These will be processed by the individual SCSI device driver once the appropriate usb devices drivers from Driversgeeks data blocks have been read or written. First, Linux finds out which of the SCSI host adapters, or controllers, that were built into the kernel at kernel build time have hardware to control. These routines are called by the SCSI subsystem as it configures itself and they are part of the SCSI device driver supporting this host type. Each detected SCSI host, those for which there are real SCSI devices attached, has itsScsi_Host_Template data structure added to the scsi_hosts list of active SCSI hosts. Each instance of a detected host type is represented by a Scsi_Host data structure held in thescsi_hostlist list.
Deciding Upon Rapid Systems Of Driver Support
For PCI interrupts Linux supports standard PCI BIOS callbacks to determine information about the devices in the system, including their IRQ numbers. Although most hard drives are internal, there are also stand-alone devices called external hard drives or portable hard drives that backup data on computers and expand the available space. External drives are often stored in an enclosure that helps protect the drive and allows it to interface with the computer, usually over USB, eSATA, or FireWire. An excellent example of an external backup device that supports multiple hard drives is the Drobo. Data sent to and read from the hard drive is interpreted by the disk controller.
This requesting of interrupt resources is done at driver initialization time. Some of the interrupts in the system are fixed, this is a legacy of the IBM PC’s architecture. So, for example, the floppy disk controller always uses interrupt 6. Other interrupts, for example the interrupts from PCI devices are dynamically allocated at boot time. In this case the device driver must first discover the interrupt number of the device that it is controlling before it requests ownership of that interrupt.
There are two problems to be solved for network device drivers. Firstly, not all of the network device drivers built into the Linux kernel will have devices to control. Secondly, the ethernet devices in the system are always called /dev/eth0, /dev/eth1 and so on, no matter what their underlying device drivers are. The problem of “missing” network devices is easily solved. As the initialization routine for each network device is called, it returns a status indicating whether or not it located an instance of the controller that it is driving. If the driver could not find any devices, its entry in the device list pointed at bydev_base is removed.