Monday, August 30, 2010

The Kernel Entry Point

The kernel’s entry point is located in init/main.c in the routine start_kernel. The architecturespecific code jumps to this address after the hardware setup is complete. The sure way you know you’ve arrived at this step is when the following appears:

Linux version 2.6.17 ( (gcc version 4.2.1)

The code here starts the interrupt handlers, process scheduler, virtual memory management system, and power management; it also scans the PCI bus for peripherals, enables networking, and performs other housekeeping tasks. While this activity is occurring, the device drivers print out information that documents the current state of the system:

CPU: ARM926EJ-Sid(wb) [41069265] revision 5 (ARMv5TEJ)
Machine: ARM-IntegratorCP
Memory policy: ECC disabled, Data cache writeback
CPU0: D VIVT write-through cache
CPU0: I cache: 4096 bytes, associativity 4, 32 byte lines, 32 sets
CPU0: D cache: 65536 bytes, associativity 4, 32 byte lines, 512 sets
Built 1 zonelists
Kernel command line: console=ttyAMA0 mem=128M ip=

The kernel command-line printing is an important milestone: if the kernel has made it this far, then the scheduler (the part of the kernel that decides what software should be running) has started and is ready to handle threads started by the kernel itself as well as the initial user process. The output from the kernel startup process is available from the command line by using the dmesg command immediately after the kernel boots. dmesg prints out the contents of the kernel’s temporary buffer, which is overwritten with newer data as the system runs. For example, on a desktop machine, try

$ dmesg | less

That command sends the results of dmesg to the less program, which lets you control the file’s scrolling.

Hardware initialization continues, ending with the configuration of the network adapter:

eth0: link up
Sending DHCP requests ...... timed out!
IP-Config: Reopening network devices...
eth0: link up
Sending DHCP requests ., OK
IP-Config: Got DHCP answer from, my address is
IP-Config: Complete:
device=eth0, addr=, mask=, gw=,
host=, domain=, nis-domain=(none),
bootserver=, rootserver=, rootpath=

These messages show an adapter that attempts to get an address via Dynamic Host Configuration Protocol (DHCP) and succeeds on the second attempt. DHCP is a protocol in which the board sends out a packet to the entire network requesting an address and waits for a reply. Because the board doesn’t know if it will get a reply, it makes several attempts before giving up.

The kernel initialization of the hardware interface is necessary for kernels that will attempt to mount a root file system over the network using NFS or PXE, both of which were covered earlier in the book. If the kernel doesn’t need to mount a remote file system as root, it doesn’t need to configure the network adapter at this point, because this may happen later in the boot process; doing so results in the system getting to userland slightly faster, and you have more control over the assignment of the IP address if the operation is deferred until a userland scripting language can be used.

Source of Information : Pro Linux Embedded Systems

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