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<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
]>
<book id="cpg">
<bookinfo>
<title>Custom Linux: A Porting Guide</title>
<subtitle>Porting LinuxPPC to a Custom SBC</subtitle>
<authorgroup>
<author>
<firstname>Shie</firstname>
<surname>Erlich</surname>
<affiliation>
<address>
<email>shie@myrealbox.com</email>
</address>
</affiliation>
</author>
<othercredit>
<firstname>Rafi Yanai</firstname>
<contrib>my partner in the porting process</contrib></othercredit>
<othercredit>
<firstname>Avi Rubenbach</firstname>
<contrib>without whom this wouldn't be possible</contrib></othercredit>
</authorgroup>
<revhistory id="revhistory">
<revision>
<revnumber>2.1</revnumber>
<date>2003-03-08</date>
<authorinitials>gjf</authorinitials>
<revremark>Modified code example per author</revremark>
</revision>
<revision>
<revnumber>2.0</revnumber>
<date>2002-06-13</date>
<authorinitials>tab</authorinitials>
<revremark>Added GFDL per author</revremark>
</revision>
<revision>
<revnumber>1.0</revnumber>
<date>2002-05-13</date>
<authorinitials>SE</authorinitials>
<revremark>Initial release</revremark>
</revision>
</revhistory>
</bookinfo>
<toc></toc>
<chapter>
<title>Introduction</title>
<sect1>
<title>Who needs to read this ?</title>
<para>This guide describes a work in progress, to port Linux to a custom PowerPC-based board.
This means making the operating system work on unfamiliar hardware. Anyone who is on
the same track might benefit from reading this paper, as it highlights the pitfalls
and problematic points along the way.</para>
</sect1>
<sect1><title>What do I need to know (why so much) ?</title>
<para>Before attempting to port Linux, know at least the following:
(whenever possible, a link to a proper information source is attached)</para>
<itemizedlist>
<listitem><para>Hardware: know what hardware you've got, how it works (if it works),
and how is it initialized. Get all the hardware manuals you can - you'll probably need them.
Also, never assume the hardware works the way it supposed to ! Hardware people do the
darnest things :-(</para></listitem>
<listitem><para>Basic understanding of drivers and how they work in Linux. Programming
knowledge of simple drivers is an advantage - but not a must.
<ulink url="http://www.tldp.org/HOWTO/Module-HOWTO/index.html"></ulink></para></listitem>
<listitem><para>How to work with Vision-ICE, how configure it and use it to load a binary
kernel into the target RAM. Also, at the beginning, you'll need to know how to
use ICE to debug in assembly.</para></listitem>
<listitem><para>How to compile and configure a Linux kernel.
<ulink url="http://www.tldp.org/HOWTO/Kernel-HOWTO.html"></ulink></para></listitem>
<listitem><para>The Linux boot process.
<ulink url="http://www.tldp.org/HOWTO/BootPrompt-HOWTO.html"></ulink></para></listitem>
<listitem><para>Working knowledge of C programming is a must. Some assembly is sure to
help. Also, it is best to get to know Makefiles. They tend to raise their ugly head once
in a while.</para></listitem>
<listitem><para>The Internet is your friend. All the information you need is probably on
the net. You just have to know how to find it. Google is a good way to start; mailing
lists and news groups usually keep the real gold.</para></listitem>
<listitem><para>How to install Linux, configure it, administrate it and basically take
care of everything it needs. This guide does not cover anything regarding system
administration, setting up a server etc.</para></listitem>
</itemizedlist>
</sect1>
<sect1>
<title>The tools</title>
<para>This section describes the tools we used during the process. Most are trivial to
install and use. When neccesary, consult the appropriate url or manual.</para>
<itemizedlist>
<listitem><para><emphasis>HardHat Linux</emphasis>: First and foremost, HHLinux, now known
as MontaVista Linux, is the distribution we started with. The distribution contains LSPs (same as BSPs)
for PowerPC in a number of board configurations. For porting to our board, we took the LSP which
is closest in hardware to our Artysyn PMPPC board, and started from there.</para></listitem>
<listitem><para><emphasis>LXR</emphasis>: This is THE killer tool, which allowed us to port
Linux in a very short time. LXR is a cross referencer, which means it reads a piece of code
(the Linux kernel, for example), and then allows browsing the code, searching through it and
much more. I cannot emphasize enough how important this tool is. To see what the end
result looks like, look at <ulink url="http://lxr.linux.no/source"></ulink>. LXR itself
can be downloaded at <ulink url="http://lxr.sf.net"></ulink></para></listitem>
<listitem><para><emphasis>VisionICE</emphasis>: A hardware debugger, which has the ability
to stop, run and add breakpoint straight in the CPU. VisionIce is very usuful when no operating
system is running, and allows to step in the kernel during boot process. The application can also be used to
take a binary image of a kernel, load it into the target's RAM memory and run it - useful
when you've got no boot loader.</para></listitem>
<listitem><para><emphasis>CVS</emphasis>: A version control system, allows you to keep
multiple versions of the code. Other than backing up the code, it allows diffing between
different version, and reverting to older version, when needed.</para></listitem>
<listitem><para>A terminal program, like <application>HyperTerminal</application> or <application>ProCOMM</application>
for Windows&trade;, or <application>minicom</application> for Linux.</para></listitem>
</itemizedlist>
</sect1>
<sect1>
<title>The hardware</title>
<para>The board is based on PPC750 (PowerPC) processor. It is 6U VME64 standard. The board is designed to
host two PCI Mezzanine cards (CCPMC) - Mezzanine cards that comply with Std CCPMC1386 can be attached.</para>
<itemizedlist>
<listitem><para>COP connector.</para></listitem>
<listitem><para>1 MB of L2 cache.</para></listitem>
<listitem><para>CPC700 system controller.</para></listitem>
<listitem><para>128 MB SDRAM with ECC.</para></listitem>
<listitem><para>Flash memory, divided to boot flash and user flash.</para></listitem>
<listitem><para>NVRAM memory.</para></listitem>
<listitem><para>I/O discretes.</para></listitem>
<listitem><para>RS232 channels.</para></listitem>
<listitem><para>General purpose registers.</para></listitem>
<listitem><para>PCI 2.1 local bus.</para></listitem>
<listitem><para>10/100 BaseT ethernet channel.</para></listitem>
<listitem><para>VME64 system bus.</para></listitem>
</itemizedlist>
</sect1>
<sect1>
<title>Copyright &amp; License</title>
<para>Copyright (c) 2002 Shie Elrich</para>
<para>Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in <xref linkend="gfdl"/>.
</para>
</sect1>
</chapter>
<chapter>
<title>Bootcamp: How To Begin ?</title>
<sect1>
<title>Creating a development environment</title>
<para>The minimum requirments are obviously a development station and a target. However,
the recommended way of working is having a third host which acts as a server. The server
runs several services such as ftp, telnet, NFS, tftp (if needed) and CVS. The main role of
the server is to run CVS and track version control, however once you can boot the target
from network, the server will also hold the target images, and filesystem,
which makes development much easier.</para>
<para>Regardless, the first step is to install a tool-chain (compiler, linker etc.) for your
target. The HardHat Linux cdrom includes all the needed files, and the installation sequence
is documented in the HardHat Linux documentation. During the installation, you must select
your LSP (basic software for the selected board), and HardHat will install a set of tools
and a kernel source tree matching your LSP.</para>
<para>We had a board that had vxWorks running on it, so we setup the target to boot
using the standard vxWorks loader. Once the loader initiated, we used visionICE to take-over
the target (so that vxWorks won't load an image file) and load a Linux image into the target.
What you need to do at this point is get an ICE, connect it to the network and to the target -
through a JTAG connection - and install the ICE software on your host.</para>
<para>What should have been done so far:</para>
<itemizedlist>
<listitem><para>A Linux host installed, and the HardHat tool-chain.</para></listitem>
<listitem><para>A working target (hardware should be functional)</para></listitem>
<listitem><para>ICE is connected to the target and the network and its software usable.</para></listitem>
<listitem><para>Optionally, a server running CVS, telnetd, NFS and FTP.</para></listitem>
</itemizedlist>
</sect1>
<sect1>
<title>Compiling the first kernel</title>
<para>If you've installed the Linux kernel that comes with HardHat, then cross-compiling
should already be enabled in the kernel <filename>Makefile</filename>. If your
kernel is not from the HardHat CD, you should enable cross-compiling in the
<filename>Makefile</filename> by defining a CROSS_COMPILE entry in the following manner:
(a code segment from the main Makefile)</para>
<programlisting>
<![CDATA[
CROSS_COMPILE = /opt/hardhat/devkit/ppc/7xx/bin/ppc_7xx-
AS = $(CROSS_COMPILE)as
LD = $(CROSS_COMPILE)ld
CC = $(CROSS_COMPILE)gcc
]]>
</programlisting>
<para>The Linux kernel is modular, and allows you to configure it and choose which <quote>blocks</quote>
should be compiled with the kernel. In order to do this, first <command>cd /usr/src/linux</command>
(assuming your kernel source code is installed at /usr/src/linux). Once there, type
<command>make xconfig</command>.After saving your options, you should <command>make vmlinux</command>
to create a kernel image suitably for using with VisionICE.</para>
<para>We will not go into more details here, as it's outside the scope of this document.
For more information, try <ulink url="http://www.tldp.org/HOWTO/Kernel-HOWTO.html"></ulink></para>
</sect1>
<sect1>
<title>Booting the machine</title>
<para>First, configure the terminal program, in our case <application>minicom</application>,
the following way: 9600 bps, 8 bits, no parity, 1 stop bit and no flow control of any kind.
The serial port in Linux should be <filename>/dev/ttyS0</filename> for <envar>COM1</envar>,
<filename>/dev/ttyS1</filename> for <envar>COM2</envar> etc.</para>
<para>Start the target. You should see the vxWorks bootloader on your terminal screen, and
should be able to stop the boot sequence by pressing the space bar.</para>
<note><para>We <emphasis>cannot use the vxWorks bootloader to load a Linux kernel</emphasis> since
it looks in the ELF header and loads the image to the address written there. However, the Linux
kernel, which uses virtual memory, is linked to a high-memory address, and vxWorks can't
handle that.</para></note>
<para>Once the target is stopped, run the <application>VisionICE</application> software
and perform the following steps:
<itemizedlist>
<listitem><para>Initialize the target by pressing <command>Target|Initialize</command></para></listitem>
<listitem><para>Press <command>File|Load Executable</command>. A dialog box will open,
asking you to choose a file. Please choose your kernel image (vmlinux). Before pressing
<command>Load</command>, don't forget to enter a value in the <parameter class="option">+/- Bias</parameter> field.
<tip><para>The bias field makes it possible to tell ICE to load a certain image in a different
address than what's stated in the ELF binary. We wanted to load the kernel into address
<envar>0x300000</envar>, and since the binary was linked to <envar>0xC0000000</envar>, we entered
<envar>-0xBFD00000</envar>.</para></tip></para></listitem>
<listitem><para>Once the image is loaded succesfully, you can press <command>Run</command>
or <command>Step</command> to start executing your kernel.</para></listitem>
</itemizedlist>
After pressing the <command>Run</command> button, nothing happened. At that moment, and for some time after, it
seemed that nothing was happening and the kernel was stuck. We used ICE to step through the
initialization code of the kernel and rule out some potential problems, like virtual memory
errors, only to finally discover that the problem was simple: the kernel was indeed booting
but since the console (tty) driver had problems, we couldn't see anything!
<caution><para>VisionICE is not the correct tool to use when debugging Linux. ICE doesn't know
about virtual memory and protected mode (at least the version we had), and since the Linux
kernel turns on virtual memory very early, ICE is only useful for debugging the first assembler
statements. After VM is turned on, ICE starts crashing and giving wierd results. </para></caution>
</para>
</sect1>
</chapter>
<chapter>
<title>Booting In The Dark</title>
<sect1>
<title>Debugging with print_str()</title>
<para>As stated in the previous chapter, the machine starts to boot, but nothing happens. At least, nothing
that we can see. The screen is blank and no kernel messages appear. At this point, you have to ask yourself,
is it really booting?</para>
<para>Since the console wouldn't start, and ICE died real fast, we had no choice. We had to debug somehow,
and the oldest way is good here - printing to the screen. Obviously, we couldn't use <function>printk()</function>, so we wrote a short function which pushes characters straight into the serial port. We used the boot process <quote>map</quote> shown in the previous section, and inserted some prints along the way. This helped us to know at what stage we are completing and where we're dying. The following piece of code prints a single character to the serial port, by polling it and waiting for it to be free.
<programlisting>
<![CDATA[
/* tx holding reg empty or tx */
#define LSR_THREMPTY 0x20 /* fifo is empty (in fifo mode ) */
#define THR_REG 0x00 /* Transmit holding reg */
#define LSR_REG 0x05 /* Line status reg */
#define COM1_ADDRESS 0xFF600300 /* == replace with your UART address */
void print_char (char ch) {
volatile unsigned char status = 0;
/* wait until txempty */
while ((status & LSR_THREMPTY ) == 0)
status = *((volatile unsigned char *)(COM1_ADDRESS + LSR_REG));
*((volatile unsigned char *)(COM1_ADDRESS + THR_REG)) = ch;
}
]]>
</programlisting>
<note><para>There's a better code for printing directly to the serial port, however, it's a bit
more complicated. You can find it in arch/ppc/boot/common/misc-common.c, using <function>puts()</function> or
<function>putc()</function>.</para></note>
</para>
</sect1>
<sect1>
<title>Modifying code using compiler flags</title>
<para>Although it is not a porting issue, the way you modify your code matters. It's easier if you do it right the
first time. The Linux kernel uses standard configuration flags CONFIG_XXXX (like CONFIG_PPC, CONFIG_ISA etc),
which are used to mark a certain machine, architecture or device. We defined ourselves a new flag (let's call
it CONFIG_TESTMACH), and surrounded our new/modified code with these flags:
<programlisting>
<![CDATA[
....original code....
#ifdef CONFIG_TESTMACH
....modified code....
#else
....original code....
#endif /* CONFIG_TESTMACH */
]]>
</programlisting>
To <quote>activate</quote> our code, we added the new flag to the kernel configuration file - <filename>.config</filename> -
by adding <command>CONFIG_TESTMACH=y</command> to it. In the first stage, this solution allows you a quick
way to find the code you changed, but later the flag you chose will allow you to add your code into the
kernel tree and into the configuration program (<command>make xconfig</command>).</para>
</sect1>
<sect1>
<title>Getting the console to work</title>
<sect2>
<title>Forcing the kernel to boot our-way</title>
<para>Once we discovered the kernel was indeed booting, but the console wasn't printing, it was time to begin.
First, we forced the kernel to boot using a specified configuration for the serial port,
in our case <envar>9600n1</envar>, and did not allow any command line options or boot time considerations etc.</para>
<para>The first place to go is <filename>drivers/char/tty_io.c</filename>, to <function>console_init()</function>.
This function determines the console configuration at startup. Here's a small part of it:
<programlisting>
<![CDATA[
memset(, 0, sizeof(struct termios));
memcpy(tty_std_termios.c_cc, INIT_C_CC, NCCS);
tty_std_termios.c_iflag = ICRNL | IGNPAR;
tty_std_termios.c_oflag = OPOST | ONLCR;
tty_std_termios.c_cflag = CLOCAL | B9600 | CS8 | CREAD;
tty_std_termios.c_cflag &= ~(CRTSCTS);
tty_std_termios.c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHOCTL | ECHOKE | IEXTEN;
tty_std_termios.c_iflag = ICRNL | IXON;
tty_std_termios.c_oflag = OPOST | ONLCR;
tty_std_termios.c_cflag = B38400 | CS8 | CREAD | HUPCL;
tty_std_termios.c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHOCTL | ECHOKE | IEXTEN;
]]>
</programlisting>
The first (naive) thing we tried, was to configure the console the way we wanted.
<emphasis>Of course, this didn't help us much ;-)</emphasis></para>
<para>Disappointed but not discouraged, we remembered that we didn't have a bootloader yet, and that we didn't
really know if any option was being passed on to the kernel. <quote>Maybe the kernel gets some garbage for command line?</quote>
we (again, naively) thought. So we tried to stop the kernel from parsing command-line options, and manually
inserted our command line. <emphasis>This didn't help us much ;-)</emphasis></para>
</sect2>
<sect2>
<title>Non-standard hardware - just say no!</title>
<para>At that point, we didn't have a console, but we had time. So we dove a bit deeper into the console
issues. Looking at <filename>drivers/char/serial.c</filename>,
we came across <function>serial_console_setup()</function>. This function, apart from parsing command-line
options, also configures the serial port by writing directly to it. Our hardware people
decided it was a good time to let us know that our serial port wasn't standard. The lines that are used for
flow control were not connected. We decided to remark-out the following line, which sets the RTS and DTR
lines high, because we just didn't have them.
<programlisting><![CDATA[serial_out(info, UART_MCR, UART_MCR_DTR | UART_MCR_RTS);]]></programlisting>
<emphasis>Ofcourse, this didn't help us much :-(</emphasis>
The lesson learned here was <emphasis>check, check, check your hardware!</emphasis>. Custom boards might
not be standard, and the porting will go a lot quicker if you know about it.</para>
</sect2>
<sect2>
<title>Let there be light: calculating baud rate</title>
<para>Finally, we decided to check the baudrate. Did Linux mean what we thought it meant when it said
9600? Possibly not, since we didn't know how it computed that value. We've noticed that the file(s)
<filename>include/asm-ppc/pmppc_serial.h</filename> (replace pmppc with your board name) included a definition
of <envar>BAUDBASE</envar>, which is later used for everything regarding serial ports. It was computed
using the board's local bus frequency, bus clock to system clock ratio etc. This seemed wrong, so we checked
out what the base baud was in a vxWorks system we had running on the board, and changed it to:
<programlisting>
<![CDATA[
/*
* system clock = 33Mhz, serial clock = system clock / 4
* the following must hold: (divisor * BaudRate) == (System clock / 64)
*/
#define BASE_BAUD (33000000 / 4 / 16)
]]>
</programlisting>
A quick compilation, and a reboot later we had a booting kernel visible through our serial port. Success!
</para>
</sect2>
</sect1>
</chapter>
<chapter>
<title>Linux Still Isn't Booting</title>
<sect1>
<title>Memory probing, RTC and decrementors</title>
<para> Now that the console was working, we could see the real problems. The system wasn't booting yet. Since we were working with C code, we traced the code, and found that a function called <function>sdram_size()</function> wasn't completing correctly. The function probed a register for the size of the RAM, a register our board doesn't have. We made the function return a given value of 128MB, it's an ugly hack, but our board doesn't have a way of knowing the amount of RAM.</para>
<para>Wwe had the same problems with a bunch of functions called todc_XXXX, mainly
<function>todc_get_rtc_time()</function>, <function>todc_set_rtc_time()</function>, and <function>time_init()</function> since we don't have a RTC (real-time clock) chip on our board, and those functions were using it. For the time being, we made the <function>todc_XXX</function> function only set and get a constant date and time, since our board doesn't have a bios battery and so cannot keep time when powered off.</para>
<para>Once all this was done, we found <function>todc_calibrate_descr()</function>, which again uses the RTC chip. We had to replace that function with our own:
<programlisting>
<![CDATA[
void calibrate_decr() {
int freq, divisor;
freq = bus_freq();
divisor = 4;
tb_ticks_per_jiffy = freq / HZ / divisor;
tb_to_us = mulhwu_scale_factor(freq / divisor, 1000000);
}
]]>
</programlisting></para>
</sect1>
<sect1>
<title>Big-little endian (we should have known)</title>
<sect2> <!-- Begin of Head 3 -->
<title>Probing the CPC700</title>
<para>Finally, we reached the PCI-probing part of the boot process, only to discover that it didn't work. We tried communicating with the CPC700 using <function>cpc700_read_local_pci_cfgb()</function>, which was supplied along with the PMPPC's LSP, and tried to read CPC's config register. We should have gotten <envar>0x1014</envar>, which is the vendor ID, but we didn't. We realized that we were talking little-endian and the CPC was listening in big-endian. We made a small patch to the functions, so that we spoke big-endian to the CPC700. We could then read the vendor ID correctly, but the rest of it still didn't work. We didn't want to alter the code so that everything would be done big-endian style.</para>
</sect2> <!-- end of Head 3 -->
<sect2> <!-- Begin of Head 3 -->
<title>Making CPC700 speak little-endian</title>
<para>We discovered that the CPC700 can be initialized to do automatic byte-swapping, which does little-to-big endian convertion on the fly. As it seems, our board was initialized to do just that. We added a small code segment in <function>setup_arch()</function>, which checks if byte-swapping is enabled, and if so, disables it:
<programlisting>
<![CDATA[
while (cnt<2) {
cpc700_read_local_pci_cfgb(0, );
cpc700_read_local_pci_cfgb(1, );
if (l == 0 && h == 0) {
if (cnt == 0) {
printk("CPC700 byte swaping enabled - trying to disable ... ");
cpc700_write_pifcfg_be(0x18, 0); /* disable byte-swapping */
} else {
printk("FAILED !!\n");
break;
}
} else {cd
printk("byte swapping disabled.\n");
break;
}
++cnt;
}
]]>
</programlisting>
A short compilation later, PCI probing was working! We got some beer and partied ;-)</para>
</sect2> <!-- End of Head 3 -->
</sect1>
<sect1>
<title>Ethernet: our first PCI device</title>
<para>Our board uses an Intel ethernet chip, called i82559er, which has a module called <emphasis>eepro100</emphasis>. After compiling the module and booting, we discovered that the module isn't working, although an ethernet device was found. We guessed that it was an irq problem, and that the devices don't get the IRQs they need. We modified a function called <function>pmppc_map_irq()</function> to map our ethernet devices:
<programlisting>
<![CDATA[
XXXX_map_irq(struct pci_dev *dev, unsigned char idsel, unsigned char pin) {
static char pci_irq_table[][4] =
/*
* PCI IDSEL/INTPIN->INTLINE
* A B C D
*/
{
{22, 0, 0, 0},/* IDSEL 3 - Ethernet */
{0, 0, 0, 0},/* IDSEL 4 - unused */
{0, 0, 0, 0},/* IDSEL 5 - unused */
{0, 0, 0, 0},/* IDSEL 6 - ???? */
{0, 0, 0, 0},/* IDSEL 7 - unused */
{0, 0, 0, 0},/* IDSEL 8 - unused */
{0, 0, 0, 0},/* IDSEL 9 - unused */
};
const long min_idsel = 3, max_idsel = 9, irqs_per_slot = 4;
return PCI_IRQ_TABLE_LOOKUP;
}
]]>
</programlisting>
The function maps IRQs according to IDselects, which means in the order on the PCI bus by which the devices are set. This structure is a bit tricky: <emphasis>min_idsel</emphasis> denotes the topleft corner of the array, and <emphasis>max_idsel</emphasis> is the bottomleft corner. <emphasis>irqs_per_slot</emphasis> is the number of IRQs per line. The structure is as follows:
<programlisting>
<![CDATA[
each cell contains (IDSEL, SLOT#, IRQ)
+----------------------------------------+
| (3,0,22) | (3,1,0) | (3,2,0) | (3,3,0) |
+----------------------------------------+
| (4,0,0) | (4,1,0) | (4,2,0) | (4,3,0) |
+----------------------------------------+
..........
..........
+----------------------------------------+
| (9,0,0) | (9,1,0) | (9,2,0) | (9,3,0) |
+----------------------------------------+
]]>
</programlisting>
As you can see, our i8559er needs IRQ 22, and is seated in IDselect 3. Of course, we didn't know that at the start, so we wrote a small piece of code that read all the vendor IDs in all the IDselects. Once done we compiled, but the ethernet device still didn't work.</para>
<para>The next problem was that the module couldn't decide on a MAC address for the device. The MAC address should be written on an EEPROM chip (connected to the device), but we discovered that the hardware guys decided that i82559 doesn't need the EEPROM, so they removed it. After hardcoding a MAC address inside <filename>eepro100.c</filename>, the ethernet device finally worked. The final solution was to make the module read the MAC address from NVRAM memory, and if no other choice was available, to fall back to a default MAC address.
<note><para>The next step was to mount a NFS root filesystem. For details see the documentation in Documentation/nfsroot.txt</para></note>
</para>
</sect1>
<sect1>
<title>Some Miscellaneous Issues</title>
<para>We had new problems, some would say good problems. We didn't have a bootloader yet, however we needed to pass a command line to the kernel at boot time. We hard-coded the command line into the kernel inside the <function>parse_options()</function>. After that was finished, we made <function>console_init()</function> and <function>serial_console_setup()</function> work the way they should. They no longer ignored the command line, but still RTS and DTR stay low.</para>
<para>Another important issue was memory mapping. The file <filename>arch/ppc/mm/init.c</filename> contains a function called <function>MMU_init()</function>. This function is actually a big <command>switch</command> statment, divided by the machine type. Each machine maps its memory using the <function>setbat()</function> and <function>ioremap()</function> functions. The BAT mechanism is a way of translating virtual addresses into physical ones. Thus, <function>setbat()</function> is used by specifying a virtual address, a physical address and a page size. Not every size can be used here; you should use one of the finite set of sizes, ranging from 128KB to 256MB. We mapped our IO memory so that virtual equalled physical.</para>
<para>As mentioned, there is another way of mapping memory - <function>ioremap()</function>. <function>ioremap()</function> is used to map physical addresses into virtual ones, making them available to the kernel. The function <emphasis>does not allocate any memory</emphasis>, simply returns a virtual address by which one can access the memory region. The following is a snippet from <function>MMU_init()</function>:
<programlisting>
<![CDATA[
case _MACH_mymachine:
setbat(0, LOW_IO_VIRT_BASE, LOW_IO_PHYS_BASE, LOW_IO_SIZE, IO_PAGE);
ioremap(UNIVERSE_BASE,UNIVERSE_SIZE); /* Universe VME */
ioremap(EEPRO100_BASE,EEPRO100_SIZE); /* Ethernet EEPRO100 */
break;
]]>
</programlisting>
As you can see, we don't take the return value of <function>ioremap()</function>. We don't need it, since at this stage the kernel maps the addresses so that virtual address == physical address.</para>
</sect1>
</chapter>
<chapter>
<title>Linux Is Booting ... What Now ?</title>
<sect1>
<title>The 64 bit barrier</title>
<para>The CPC700 has a <quote>feature</quote> which is supposed to make some memory access use 64 bit wide. This is a problem since some test-and-set registers on our board might get set unintentionally, because we were trying to read something 16 bits lower. In order to solve this situation, we set the memory controller to 64 bit wide intervals. If you try to access those areas in another manner (8 or 16 bit access), the CPC700 simply throws them away. We had to be able to read/write those areas, since important <quote>discretes</quote> (controlled by an Altera device) were mapped there.</para>
<para>In order to access those areas, we needed a function that does a 64 bit write. As far as I know, doing a 64 bit write on a PowerPC is possible in two ways: using cache lines and using a floating point register. The floating point register is a 64 bit sized register, so when we write it, the whole 64 bit get written. The problem is that you can't do floating point in the kernel. Since the kernel doesn't save the floating point registers during context switch, it doesn't allow FP, and will throw an exception if done in the kernel.</para>
<para>After messing with cache lines, we decided to go the FP way, and added the following function:
<programlisting>
<![CDATA[
void out64(__u32 addr, long long *pVal) {
__u32 flags, tmp_msr;
save_flags(flags);
cli();
tmp_msr = __get_MSR();
tmp_msr |= MSR_FP;
tmp_msr &= ~(MSR_FE0 | MSR_FE1);
__put_MSR(tmp_msr);
sysOut64(addr, pVal);
__put_MSR(flags & ~(MSR_EE));
restore_flags(flags);
}
]]>
</programlisting>
The function adds a floating point to the PowerPC MSR register, and makes sure that no exceptions will be generated as a result of doing FP. Once done, it uses an assembly code, described below in the <function>sysOut64()</function> to do the actual floating-point operation. Note that the function turns off interrupts, but this is acceptable here, since we use the function on rare occasion.
<programlisting>
<![CDATA[
_GLOBAL(sysOut64)
stwu r1, -DEPTH(r1)
mflr r0
stw r31, FP_LOC(r1)
stw r0, LR_LOC(r1)
mr r31, r1
stfd fr0, FPR_SAVE(r31) /* save floating point reg contents */
lfd fr0,0(r4)
stfd fr0,0(r3)
eieio
lfd fr0, FPR_SAVE(r31) /* restore floating point value */
lwz r4, 0(r1) /* now restore the stack frame */
lwz r0, 4(r4)
mtlr r0
lwz r31, -4(r4)
mr r1, r4
blr
]]>
</programlisting></para>
</sect1>
<sect1>
<title>Booting from flash</title>
<para>While Linux was booting using an NFS filesystem, this was not enough. For an actual field product, we needed Linux to boot from an independent device, without the need for a network at all. We decided to create a special kind of image, called <emphasis>initrd</emphasis>, which is basically a Linux kernel with a compressed file. The compressed file includes a Linux filesystem. The filesystem is unpacked to a ramdisk on boot, and mounted as the root filesystem.</para>
<para>During the boot process, the bootloader relocated the kernel image to address zero - which was fine, and the initrd part to a higher address. The area to which initrd was relocated was not mapped in our kernel's memory, and all we got was a kernel error (access to bad area). After modifying the bootloader to relocate initrd to a different address, all was fine and Linux booted succesfully.</para>
<tip><para>If your board has some NVRAM memory, it would be a good idea to use it for bootloader purposes. After writing a module for our NVRAM memory (out of scope for this paper), we modified the bootloader, so that the kernel command-line, and MAC address were saved in NVRAM. When the bootloader starts, it checks NVRAM and if it is initialized (by a certain magic number), the bootloader uses the command line written there. Otherwise, the bootloader reverts to a default command line, allowing the user to edit it.</para></tip>
</sect1>
</chapter>
<appendix id="gfdl">
<title>GNU Free Documentation License</title>
<para>Version 1.1, March 2000</para>
<blockquote>
<para>Copyright (C) 2000 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.</para>
</blockquote>
<sect1 id="gfdl-0">
<title>PREAMBLE</title>
<para>The purpose of this License is to make a manual, textbook,
or other written document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by
others.</para>
<para>This License is a kind of "copyleft", which means that
derivative works of the document must themselves be free in the
same sense. It complements the GNU General Public License, which
is a copyleft license designed for free software.</para>
<para>We have designed this License in order to use it for manuals
for free software, because free software needs free documentation:
a free program should come with manuals providing the same
freedoms that the software does. But this License is not limited
to software manuals; it can be used for any textual work,
regardless of subject matter or whether it is published as a
printed book. We recommend this License principally for works
whose purpose is instruction or reference.</para>
</sect1>
<sect1 id="gfdl-1">
<title>APPLICABILITY AND DEFINITIONS</title>
<para>This License applies to any manual or other work that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. The "Document",
below, refers to any such manual or work. Any member of the
public is a licensee, and is addressed as "you".</para>
<para>A "Modified Version" of the Document means any work
containing the Document or a portion of it, either copied
verbatim, or with modifications and/or translated into another
language.</para>
<para>A "Secondary Section" is a named appendix or a front-matter
section of the Document that deals exclusively with the
relationship of the publishers or authors of the Document to the
Document's overall subject (or to related matters) and contains
nothing that could fall directly within that overall subject.
(For example, if the Document is in part a textbook of
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The relationship could be a matter of historical connection with
the subject or with related matters, or of legal, commercial,
philosophical, ethical or political position regarding
them.</para>
<para>The "Invariant Sections" are certain Secondary Sections
whose titles are designated, as being those of Invariant Sections,
in the notice that says that the Document is released under this
License.</para>
<para>The "Cover Texts" are certain short passages of text that
are listed, as Front-Cover Texts or Back-Cover Texts, in the
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<para>A "Transparent" copy of the Document means a
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<para>Examples of suitable formats for Transparent copies include
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Opaque formats include PostScript, PDF, proprietary formats that
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<para>The "Title Page" means, for a printed book, the title page
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For works in formats which do not have any title page as such,
"Title Page" means the text near the most prominent appearance of
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text.</para>
</sect1>
<sect1 id="gfdl-2">
<title>VERBATIM COPYING</title>
<para>You may copy and distribute the Document in any medium,
either commercially or noncommercially, provided that this
License, the copyright notices, and the license notice saying this
License applies to the Document are reproduced in all copies, and
that you add no other conditions whatsoever to those of this
License. You may not use technical measures to obstruct or
control the reading or further copying of the copies you make or
distribute. However, you may accept compensation in exchange for
copies. If you distribute a large enough number of copies you
must also follow the conditions in section 3.</para>
<para>You may also lend copies, under the same conditions stated
above, and you may publicly display copies.</para>
</sect1>
<sect1 id="gfdl-3">
<title>COPYING IN QUANTITY</title>
<para>If you publish printed copies of the Document numbering more
than 100, and the Document's license notice requires Cover Texts,
you must enclose the copies in covers that carry, clearly and
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cover, and Back-Cover Texts on the back cover. Both covers must
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copies. The front cover must present the full title with all
words of the title equally prominent and visible. You may add
other material on the covers in addition. Copying with changes
limited to the covers, as long as they preserve the title of the
Document and satisfy these conditions, can be treated as verbatim
copying in other respects.</para>
<para>If the required texts for either cover are too voluminous to
fit legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto
adjacent pages.</para>
<para>If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a
machine-readable Transparent copy along with each Opaque copy, or
state in or with each Opaque copy a publicly-accessible
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begin distribution of Opaque copies in quantity, to ensure that
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location until at least one year after the last time you
distribute an Opaque copy (directly or through your agents or
retailers) of that edition to the public.</para>
<para>It is requested, but not required, that you contact the
authors of the Document well before redistributing any large
number of copies, to give them a chance to provide you with an
updated version of the Document.</para>
</sect1>
<sect1 id="gfdl-4">
<title>MODIFICATIONS</title>
<para>You may copy and distribute a Modified Version of the
Document under the conditions of sections 2 and 3 above, provided
that you release the Modified Version under precisely this
License, with the Modified Version filling the role of the
Document, thus licensing distribution and modification of the
Modified Version to whoever possesses a copy of it. In addition,
you must do these things in the Modified Version:</para>
<orderedlist numeration="upperalpha">
<listitem><para>Use in the Title Page
(and on the covers, if any) a title distinct from that of the
Document, and from those of previous versions (which should, if
there were any, be listed in the History section of the
Document). You may use the same title as a previous version if
the original publisher of that version gives permission.</para>
</listitem>
<listitem><para>List on the Title Page,
as authors, one or more persons or entities responsible for
authorship of the modifications in the Modified Version,
together with at least five of the principal authors of the
Document (all of its principal authors, if it has less than
five).</para>
</listitem>
<listitem><para>State on the Title page
the name of the publisher of the Modified Version, as the
publisher.</para>
</listitem>
<listitem><para>Preserve all the
copyright notices of the Document.</para>
</listitem>
<listitem><para>Add an appropriate
copyright notice for your modifications adjacent to the other
copyright notices.</para>
</listitem>
<listitem><para>Include, immediately
after the copyright notices, a license notice giving the public
permission to use the Modified Version under the terms of this
License, in the form shown in the Addendum below.</para>
</listitem>
<listitem><para>Preserve in that license
notice the full lists of Invariant Sections and required Cover
Texts given in the Document's license notice.</para>
</listitem>
<listitem><para>Include an unaltered
copy of this License.</para>
</listitem>
<listitem><para>Preserve the section
entitled "History", and its title, and add to it an item stating
at least the title, year, new authors, and publisher of the
Modified Version as given on the Title Page. If there is no
section entitled "History" in the Document, create one stating
the title, year, authors, and publisher of the Document as given
on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.</para>
</listitem>
<listitem><para>Preserve the network
location, if any, given in the Document for public access to a
Transparent copy of the Document, and likewise the network
locations given in the Document for previous versions it was
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may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.</para>
</listitem>
<listitem><para>In any section entitled
"Acknowledgements" or "Dedications", preserve the section's
title, and preserve in the section all the substance and tone of
each of the contributor acknowledgements and/or dedications
given therein.</para>
</listitem>
<listitem><para>Preserve all the
Invariant Sections of the Document, unaltered in their text and
in their titles. Section numbers or the equivalent are not
considered part of the section titles.</para>
</listitem>
<listitem><para>Delete any section
entitled "Endorsements". Such a section may not be included in
the Modified Version.</para>
</listitem>
<listitem><para>Do not retitle any
existing section as "Endorsements" or to conflict in title with
any Invariant Section.</para>
</listitem>
</orderedlist>
<para>If the Modified Version includes new front-matter sections
or appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option
designate some or all of these sections as invariant. To do this,
add their titles to the list of Invariant Sections in the Modified
Version's license notice. These titles must be distinct from any
other section titles.</para>
<para>You may add a section entitled "Endorsements", provided it
contains nothing but endorsements of your Modified Version by
various parties--for example, statements of peer review or that
the text has been approved by an organization as the authoritative
definition of a standard.</para>
<para>You may add a passage of up to five words as a Front-Cover
Text, and a passage of up to 25 words as a Back-Cover Text, to the
end of the list of Cover Texts in the Modified Version. Only one
passage of Front-Cover Text and one of Back-Cover Text may be
added by (or through arrangements made by) any one entity. If the
Document already includes a cover text for the same cover,
previously added by you or by arrangement made by the same entity
you are acting on behalf of, you may not add another; but you may
replace the old one, on explicit permission from the previous
publisher that added the old one.</para>
<para>The author(s) and publisher(s) of the Document do not by
this License give permission to use their names for publicity for
or to assert or imply endorsement of any Modified Version.</para>
</sect1>
<sect1 id="gfdl-5">
<title>COMBINING DOCUMENTS</title>
<para>You may combine the Document with other documents released
under this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination
all of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice.</para>
<para>The combined work need only contain one copy of this
License, and multiple identical Invariant Sections may be replaced
with a single copy. If there are multiple Invariant Sections with
the same name but different contents, make the title of each such
section unique by adding at the end of it, in parentheses, the
name of the original author or publisher of that section if known,
or else a unique number. Make the same adjustment to the section
titles in the list of Invariant Sections in the license notice of
the combined work.</para>
<para>In the combination, you must combine any sections entitled
"History" in the various original documents, forming one section
entitled "History"; likewise combine any sections entitled
"Acknowledgements", and any sections entitled "Dedications". You
must delete all sections entitled "Endorsements."</para>
</sect1>
<sect1 id="gfdl-6">
<title>COLLECTIONS OF DOCUMENTS</title>
<para>You may make a collection consisting of the Document and
other documents released under this License, and replace the
individual copies of this License in the various documents with a
single copy that is included in the collection, provided that you
follow the rules of this License for verbatim copying of each of
the documents in all other respects.</para>
<para>You may extract a single document from such a collection,
and distribute it individually under this License, provided you
insert a copy of this License into the extracted document, and
follow this License in all other respects regarding verbatim
copying of that document.</para>
</sect1>
<sect1 id="gfdl-7">
<title>AGGREGATION WITH INDEPENDENT WORKS</title>
<para>A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of
a storage or distribution medium, does not as a whole count as a
Modified Version of the Document, provided no compilation
copyright is claimed for the compilation. Such a compilation is
called an "aggregate", and this License does not apply to the
other self-contained works thus compiled with the Document, on
account of their being thus compiled, if they are not themselves
derivative works of the Document.</para>
<para>If the Cover Text requirement of section 3 is applicable to
these copies of the Document, then if the Document is less than
one quarter of the entire aggregate, the Document's Cover Texts
may be placed on covers that surround only the Document within the
aggregate. Otherwise they must appear on covers around the whole
aggregate.</para>
</sect1>
<sect1 id="gfdl-8">
<title>TRANSLATION</title>
<para>Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires
special permission from their copyright holders, but you may
include translations of some or all Invariant Sections in addition
to the original versions of these Invariant Sections. You may
include a translation of this License provided that you also
include the original English version of this License. In case of
a disagreement between the translation and the original English
version of this License, the original English version will
prevail.</para>
</sect1>
<sect1 id="gfdl-9">
<title>TERMINATION</title>
<para>You may not copy, modify, sublicense, or distribute the
Document except as expressly provided for under this License. Any
other attempt to copy, modify, sublicense or distribute the
Document is void, and will automatically terminate your rights
under this License. However, parties who have received copies, or
rights, from you under this License will not have their licenses
terminated so long as such parties remain in full
compliance.</para>
</sect1>
<sect1 id="gfdl-10">
<title>FUTURE REVISIONS OF THIS LICENSE</title>
<para>The Free Software Foundation may publish new, revised
versions of the GNU Free Documentation License from time to time.
Such new versions will be similar in spirit to the present
version, but may differ in detail to address new problems or
concerns. See <ulink
url="http://www.gnu.org/copyleft/">http://www.gnu.org/copyleft/</ulink>.</para>
<para>Each version of the License is given a distinguishing
version number. If the Document specifies that a particular
numbered version of this License "or any later version" applies to
it, you have the option of following the terms and conditions
either of that specified version or of any later version that has
been published (not as a draft) by the Free Software Foundation.
If the Document does not specify a version number of this License,
you may choose any version ever published (not as a draft) by the
Free Software Foundation.</para>
</sect1>
<sect1 id="gfdl-11">
<title>How to use this License for your documents</title>
<para>To use this License in a document you have written, include
a copy of the License in the document and put the following
copyright and license notices just after the title page:</para>
<blockquote><para>
Copyright (c) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
</para></blockquote>
<para>If you have no Invariant Sections, write "with no Invariant
Sections" instead of saying which ones are invariant. If you have
no Front-Cover Texts, write "no Front-Cover Texts" instead of
"Front-Cover Texts being LIST"; likewise for Back-Cover
Texts.</para>
<para>If your document contains nontrivial examples of program
code, we recommend releasing these examples in parallel under your
choice of free software license, such as the GNU General Public
License, to permit their use in free software.</para>
</sect1>
</appendix>
<appendix id="trademarks">
<title>Trademarks</title>
<para><trademark class="registered">Linux</trademark> is a registered trademark of Linus Torvalds.</para>
<para><trademark>MontaVista</trademark> is a trademark of MontaVista Software Inc.</para>
<para><trademark class="registered">PowerPC</trademark> is a registered trademark of IBM Corporation.</para>
<para><trademark class="registered">Windows</trademark> is a registered trademark of Microsoft Corporation.</para>
<para><trademark>vxWorks</trademark> and <trademark>Vision ICE</trademark> are trademarks of Wind River Systems Inc.</para>
<para><trademark>ProCOMM</trademark> is a trademark of Symantec Corporation.</para>
</appendix>
</book>
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