Porting Linux to U-Boot based systems:

---------------------------------------

U-Boot cannot save you from doing all the necessary modifications to
configure the Linux device drivers for use with your target hardware
(no, we don't intend to provide a full virtual machine interface to
Linux :-).

But now you can ignore ALL boot loader code (in arch/ppc/mbxboot).

Just make sure your machine specific header file (for instance
include/asm-ppc/tqm8xx.h) includes the same definition of the Board
Information structure as we define in include/u-boot.h, and make
sure that your definition of IMAP_ADDR uses the same value as your
U-Boot configuration in CFG_IMMR.


Configuring the Linux kernel:
-----------------------------

No specific requirements for U-Boot. Make sure you have some root
device (initial ramdisk, NFS) for your target system.


Building a Linux Image:
-----------------------

With U-Boot, "normal" build targets like "zImage" or "bzImage" are
not used. If you use recent kernel source, a new build target
"uImage" will exist which automatically builds an image usable by
U-Boot. Most older kernels also have support for a "pImage" target,
which was introduced for our predecessor project PPCBoot and uses a
100% compatible format.

Example:

make TQM850L_config
make oldconfig
make dep
make uImage

The "uImage" build target uses a special tool (in 'tools/mkimage') to
encapsulate a compressed Linux kernel image with header information,
CRC32 checksum etc. for use with U-Boot. This is what we are doing:

* build a standard "vmlinux" kernel image (in ELF binary format):

* convert the kernel into a raw binary image:

${CROSS_COMPILE}-objcopy -O binary \
-R .note -R .comment \
-S vmlinux linux.bin

* compress the binary image:

gzip -9 linux.bin

* package compressed binary image for U-Boot:

mkimage -A ppc -O linux -T kernel -C gzip \
-a 0 -e 0 -n "Linux Kernel Image" \
-d linux.bin.gz uImage


The "mkimage" tool can also be used to create ramdisk images for use
with U-Boot, either separated from the Linux kernel image, or
combined into one file. "mkimage" encapsulates the images with a 64
byte header containing information about target architecture,
operating system, image type, compression method, entry points, time
stamp, CRC32 checksums, etc.

"mkimage" can be called in two ways: to verify existing images and
print the header information, or to build new images.

In the first form (with "-l" option) mkimage lists the information
contained in the header of an existing U-Boot image; this includes
checksum verification:

tools/mkimage -l image
-l ==> list image header information

The second form (with "-d" option) is used to build a U-Boot image
from a "data file" which is used as image payload:

tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
-n name -d data_file image
-A ==> set architecture to 'arch'
-O ==> set operating system to 'os'
-T ==> set image type to 'type'
-C ==> set compression type 'comp'
-a ==> set load address to 'addr' (hex)
-e ==> set entry point to 'ep' (hex)
-n ==> set image name to 'name'
-d ==> use image data from 'datafile'

Right now, all Linux kernels for PowerPC systems use the same load
address (0x00000000), but the entry point address depends on the
kernel version:

- 2.2.x kernels have the entry point at 0x0000000C,
- 2.3.x and later kernels have the entry point at 0x00000000.

So a typical call to build a U-Boot image would read:

-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
> -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux.gz \
> examples/uImage.TQM850L
Image Name: 2.4.4 kernel for TQM850L
Created: Wed Jul 19 02:34:59 2000
Image Type: PowerPC Linux Kernel Image (gzip compressed)
Data Size: 335725 Bytes = 327.86 kB = 0.32 MB
Load Address: 0x00000000
Entry Point: 0x00000000

To verify the contents of the image (or check for corruption):

-> tools/mkimage -l examples/uImage.TQM850L
Image Name: 2.4.4 kernel for TQM850L
Created: Wed Jul 19 02:34:59 2000
Image Type: PowerPC Linux Kernel Image (gzip compressed)
Data Size: 335725 Bytes = 327.86 kB = 0.32 MB
Load Address: 0x00000000
Entry Point: 0x00000000

NOTE: for embedded systems where boot time is critical you can trade
speed for memory and install an UNCOMPRESSED image instead: this
needs more space in Flash, but boots much faster since it does not
need to be uncompressed:

-> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux.gz
-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
> -A ppc -O linux -T kernel -C none -a 0 -e 0 \
> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/ppc/coffboot/vmlinux \
> examples/uImage.TQM850L-uncompressed
Image Name: 2.4.4 kernel for TQM850L
Created: Wed Jul 19 02:34:59 2000
Image Type: PowerPC Linux Kernel Image (uncompressed)
Data Size: 792160 Bytes = 773.59 kB = 0.76 MB
Load Address: 0x00000000
Entry Point: 0x00000000


Similar you can build U-Boot images from a 'ramdisk.image.gz' file
when your kernel is intended to use an initial ramdisk:

-> tools/mkimage -n 'Simple Ramdisk Image' \
> -A ppc -O linux -T ramdisk -C gzip \
> -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
Image Name: Simple Ramdisk Image
Created: Wed Jan 12 14:01:50 2000
Image Type: PowerPC Linux RAMDisk Image (gzip compressed)
Data Size: 566530 Bytes = 553.25 kB = 0.54 MB
Load Address: 0x00000000
Entry Point: 0x00000000

GCC - packed structures

GCC allows you to specify attributes of variables and structures using the keyword__attribute__, the syntax of which is __attribute__((attribute list)). One such attribute is __packed__ which specifies that

a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.
GCC Manual
which means that GCC will not add any of the zero's for padding (for memory alignement) and make variables or fields immediately next to each other. For example, here are some things I tried out -- I created a C source file - test.c
struct test_t {
int a;
char b;
int c;
} ;

struct test_t test = { 10, 20, 30};
And compiled it with the -S option (ie to generate the assembly equivalent of the code generated).
$gcc -c test.c -S -o test.s
The file test.s -
.file "test.c"
.globl test
.data
.p2align 2
.type test,@object
.size test,12
test:
.long 10
.byte 20
.zero 3
.long 30
.ident "GCC: (GNU) 3.2.2 [FreeBSD] 20030205 (release)"
Notice the emphasized code. You can see that the structure "test" is being declared. First the field "a" (int) as .long 10 followed by "b" (char) as .byte 20. To keep the fields' word alignment, notice that GCC has added 3 zero bytes (.zero 3) before field "c" (int) which is declared as .long 30. This makes the effective sizeof struct test_t as 12 instead of the expected 9. Then I tried with the __packed__ attribute -
struct test_t {
int a;
char b;
int c;
} __attribute__((__packed__));

struct test_t test = { 10, 20, 30};
and the "-S" output I got after compiling was
.file "test.c"
.globl test
.data
.type test,@object
.size test,9
test:
.long 10
.byte 20
.long 30
.ident "GCC: (GNU) 3.2.2 [FreeBSD] 20030205 (release)"
in which the zeros are missing making the sizeof structure test_t = 9. Always remember that memory alignment is *good* even if it compromises space, so think twice before using this attribute. It is generally useful when you want to assign a structure to a block of memory and manipulate it through the fields of a structure.

Recovering root password

I forgot the root password of my linux machine and googled to get this solution that worked for me

boot with the linux rescue disk

give the command
chroot /mnt/sysimage
passwd (this changes the root password!)