2 - Raspberry PI Community Projects

BACK TO BASICS
Master boot record
The Master Boot Record (MBR) occupies the first 512 bytes of the first hard
disk, and is the first thing loaded by the BIOS to hand over control to a program
capable of booting the desired operating system. In general, a bootloader
is installed in the MBR, removing its previous content.
8.8.1. Identifying the Disks
CULTURE
udev, devfs and /dev/
The /dev/ directory traditionally houses so-called “special” files, intended to
represent system peripherals (see sidebar “Device access permissions” (page
158)). /dev/hda1 thus always corresponds to the first partition of the the first
IDE hard drive. This static structure does not allow dynamic seing of “major”
and “minor” numbers for these files, which forces kernel developers to
limit their number, since a priori assignment of the identifiers prohibits adding
more once these conventions are established.
To take into account the characteristics of more and more dynamic, modern
computers, the kernel has, at some time, offered an implementation of /dev/
by a virtual filesystem called devfs. In some cases, this makes it easier to
find files, since the naming convention uses a hierarchical structure: the first
partition of the master hard drive on the first IDE bus was then represented
by the file, /dev/ide/host0/bus0/target0/lun0/part1. Not only were these
naming conventions not very intuitive, but they were also hard-coded into the
kernel which presented problems for hot-pluggable drives because the corresponding
special file name would vary.
The current solution is the second incarnation of the process, udev, with which
the kernel delegates the choice of the device file names to be created to a
program in user space. This program (udevd) can then have all of the flexibility
of user space to decide what actions to take, naming of peripherals, etc.
With udev (user-space /dev/), a filesystem is stored in RAM and generated
automatically by udevd (and it hides the content of any /dev/ that may be
stored on-disk). udevd collaborates with the kernel's hotplug sub-system (see
Section 9.11, “Hot Plugging: hotplug” (page 213)) to detect the appearance
(hotplugging) of devices, then dynamically creates the corresponding special
files in /dev/. The content of /dev/ is, thus, lost on each reboot, but udev
recreates it systematically.
This mechanism allows the machine to dynamically choose the file name. You
can thus keep the same name for a given device, regardless of the connector
used or the connection order, which is especially useful when you use various
USB peripherals. The partition system on the first IDE hard drive can then be
called /dev/hda1 for backwards compatibility, or /dev/root-partition if you
prefer, or even both at the same time since udevd can be configured to automatically
create a symbolic link. Furthermore, /dev/ no longer contains useful
files at this time. Previously, some kernel modules did not automatically load
when you tried to access the corresponding peripheral; henceforth, the peripheral's
special file no longer exists prior to loading the module, which is no
big deal, since most modules are loaded on boot thanks to automatic hardware
detection. But for undetectable peripherals (such as older disk drives or
PS/2 mice), this doesn't work. Consider adding the modules, floppy, psmouse
and mousedev to /etc/modules in order to force loading them on boot.
Chapter 8 — Basic Configuration: Network, Accounts, Printing…
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