I'm working on rewriting part of Section 7.4 - Device and Module Handling on an LFS System. This is what I have right now, but would like comments.
-- Bruce 7.4.1. History (unchanged) 7.4.2. Udev Implementation 7.4.2.1. Sysfs The sysfs filesystem was mentioned briefly above. One may wonder how sysfs knows about the devices present on a system and what device numbers should be used for them. Drivers that have been compiled into the kernel directly register their objects with a sysfs (devtmpfs internally) as they are detected by the kernel. For drivers compiled as modules, this registration will happen when the module is loaded. Once the sysfs filesystem is mounted (on /sys), data which the drivers register with sysfs are available to userspace processes and to udevd for processing (including modifications to device nodes). 7.4.2.2. Device Node Creation In recent version of udev, udevd no longer creates device files. Instead, this is handled in the kernel by the devtmpfs filesystem. Any driver that wishes to register a device node will go through devtmpfs (via the driver core) to do it. When a devtmpfs instance is mounted on /dev, the device node will initially be created with a fixed name, permissions, and owner. A short time later, the kernel will send a uevent to udevd. Based on the rules specified in the files within the /etc/udev/rules.d, /lib/udev/rules.d, and /run/udev/rules.d directories, udevd will create additional symlinks to the device node, change its permissions, owner, or group, or modify the /sys filesystem entry (name) for that object. The rules in these three directories are numbered in a similar fashion to the LFS-Bootscripts package, and all three directories are merged together. If udevd can't find a rule for the device it is creating, it will leave the permissions and ownership at whatever devtmpfs used initially. 7.4.2.3. Udev Bootscripts The first LFS bootscript, /etc/init.d/mountvirtfs will copy any devices located in /lib/udev/devices to /dev. This is necessary because some devices, directories, and symlinks are needed before the dynamic device handling processes are available during the early stages of booting a system, or are required by udevd itself. Creating static device nodes in /lib/udev/devices also provides an easy workaround for any devices that may not be supported by the dynamic device handling infrastructure. The /etc/rc.d/init.d/udev initscript starts udevd, triggers any "coldplug" devices that have already been created by the kernel and waits for any rules to complete. The script also unsets the uevent handler from the default of /sbin/hotplug. This is done because the kernel no longer needs to call out to an external binary. Instead udevd will listen on a netlink socket for uevents that the kernel raises. The /etc/rc.d/init.d/udev_retry initscript takes care of re-triggering events for subsystems whose rules may rely on filesystems that are not mounted until the mountfs script is run (in particular, /usr and /var may cause this). This script runs after the mountfs script, so those rules (if re-triggered) should succeed the second time around. It is configured from the /etc/sysconfig/udev_retry file; any words in this file other than comments are considered subsystem names to trigger at retry time. To find the subsystem of a device, use 'udevadm info --attribute-walk <device>' where <device> is a an absolure path in /dev or /sys such as /dev/sr0 or /sys/class/rtc. 7.4.2.4. Module Loading (unchanged) ... -- http://linuxfromscratch.org/mailman/listinfo/lfs-dev FAQ: http://www.linuxfromscratch.org/faq/ Unsubscribe: See the above information page
