Linux/MIPS Porting Guide

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This document reflects what I have learned through porting several MIPS machines and other related Linux work. Hopefully it will help beginners to get started and give the experienced a reference point.

This document goes through all the major steps to port Linux to a MIPS machine. The focus can perhaps be called "MIPS machine abstraction layer", i.e., the interface between machine-specific code and, mostly, MIPS common code. Another useful document focuses on "Linux hardware abstraction layer", i.e., the interface between Linux common code and architecture-specific code. The document is written by C Hanish Menon (www.hanishkvc.com).

Overview

Prerequisites

  • Know C programming.
  • Have some knowledge of OS concepts, such as interrupt handling, system calls, memory management.
  • Know how to configure and make Linux kernel. You can find much help on this if you are not very comfortable.
  • Have some knowledge of MIPS CPU. More than likely you will need to deal with CP0 registers, enable or disable interrupts, etc..
  • You don't have to be an expert in MIPS assembly, but total ignorance of it might make you handicapped in some situations.
  • Obviously, you need a MIPS hardware to play with.
  • Finally but most importantly, you need a willing-to-learn heart and perhaps many restless debugging hours. :-)

It is also highly recommanded to read through the Linux MIPS HOWTO by Ralf Bächle, ralf@gnu.org. By the way, as part of the pre-requisite, you should also remember Ralf's name. :-)

Overall porting steps

Depending on your specific case, some (but not all!) of the following may be skipped:

  1. Setup a cross-development environment.
  2. "Hello, World!" - Get board setup, serial porting working, and print out "Hello, world!" through the serial port.
  3. Download the kernel -- this is where you'll be doing all your work.
  4. Add your own code.
  5. Get early printk working - Make the first MIPS image and see the printk output from kernel.
  6. Serial driver and serial console - Get the real printk working with the serial console.
  7. KGDB - KGDB can be enormously helpful in your development. It is highly recommended and it is not that difficult to set up.
  8. CPU support - If your MIPS CPU is not currently supported, you need to add new code that supports it.
  9. Board specific support - Create your board-specific directory. Setup interrupt routing/handling and kernel timer services.
  10. PCI subsystems - If your machine has PCI, you need to get the PCI subsystem working before you can use PCI devices.
  11. Ethernet drivers - You should already have the serial port working before attempting this. Usually the next driver you want is the ethernet driver. With ethernet driver working, you can set up a NFS root file system which gives you a fully working Linux userland.
  12. ROMFS root file system - Alternatively you can create a userland file system as a ROMFS image stored in a ramdisk.

Setup a cross-development environment

You'll need a cross-development environment since your board is not yet self-hosting (ie. unable to natively build and run executable code). In this scenario, you usually have two machines -- a host and target. It is on your host (eg. Linux on i386) that you build your code, and onto your target (MIPS board) that you download and run it. This is a preferred (and robust) methodology of doing embedded development, since most targets are resource-constrained (memory- and CPU-wise) for things like native builds of whole kernel trees.

A primary step towards setting up your environment is installing a cross-toolchain. While you may be able to roll your own, you're better off using a pre-built set (full toolchain builds have since become quite time consuming). An exception to this is when you require added support for your platform to the toolchain itself, which is out of scope for this guide.

See Toolchains for where to download pre-built tools, or instructions on how to roll your own.

"Hello, world!"

In cross development, the serial port is usually the most important interface: That is where you can see anything happening! It might be worthwhile to make sure you get serial porting work before you even start playing with Linux. You can find the sample code or gzipped tar ball of a stand-alone program that can do printf. Such a program can even be useful in later debugging staging, e.g., printing out hardware register values.

Before you rush to type 'make', check and modify the following configurations:

  1. The sample code assumes R4K style CP0 structure. It should apply to most CPUs named above number 4000 and the recent MIPS32/MIPS64.
  2. Check if you have 1MB RAM size. (You really should have at 1MB to run Linux at all.) It is recommanded you have 8MB RAM or more.
  3. Is your serial port a standard UART type? If yes, modify the serial code and parameters. If not, you will have to supply your own functions to utilize the UART.
  4. What is your cross-tool name and path? Modify the Makefile accordingly.

Now, fire your "make" command.

Depending on your downloader on your MIPS box, you may need to generate ELF image, binary image or a SREC image.

Download the barebone image to your target and give it a run! Connect the serial port to your host machine. Start minicom and hopefully you can see the "Hello, world!" message.

Trouble shooting:

  • Make sure your bootloader downloads the image to uncached KSEG1 segment. If your bootloader downloads to the cached KSEG0 area, you will want to run the image from the KSEG0 area too.
  • If your bootloader has already initialized the serial port, you may want to skip your own initialization.
  • Did you set up minicom correctly? Test it with other machines.
  • Hopefully it is not the toolchain problem...

Getting the Kernel

Ideally, you should be working against the latest stable release. The older stable release, 2.4.x, is only open to bug fixes and won't be accepting any new features. Besides, countless bugs may have since been squished, APIs overhauled etc., all of which you'll greatly benefit from.

See Getting the Kernel for where to obtain the latest stable release.

Initial board support

A first step toward adding support for your board is to hook into the kernel build system (kbuild). At this point you should be worrying about getting the tree to actually build. You'll also need to provide the board-specific hooks that the architecture back-end depends on.

In the following examples, we assume a board named Aloha supplied by a vendor Hawaii.

Decide on a directory hierarchy

Your code for a new board can be classified into board-support code (or board-specific code) and drivers. Driver code should be placed under the drivers/ directory and board specific code should be placed under the arch/mips/ directory.

The easiest choice is to create a directory called arch/mips/aloha.

However, a couple of other considerations might make it slightly complicated.

  • If Aloha uses a chipset or System on a Chip (SOC) that is already supported or belongs to a bigger family, such as NEC VR41xx or gt64120, it makes sense to put Aloha code under those sub-directories. You can re-use and share a lot of common code.
  • Similarly, if Aloha is the first board that uses a chipset or SOC which is expected to be used in many other boards, you may want to create similar directory structure. However, if you are not sure, just create your own board specific directory.

In the past people have created directories based on the board manufacturer's name, such as "mips-boards". This generally is not a good idea. It is almost certain that some of these boards do not share anything common at all.

To make things worse, sometimes boards made by different companies use the same chipset or SOC. Now what are you going to do? Are you going to duplicate the common code? Or are you going stick one company's board under another company's name?

For header files, you usually create similar directory or header files under include/asm-mips/. [DEBATE] For board specific header files, I would encourage people to place them under the corresponding arch/mips/ directory if possible.

In our example, we will create arch/mips/aloha/ directory.

Hook into the build system

Add a Kconfig entry for your board so you are able to select it with any of make *config. For a description of this syntax, see Documentation/kbuild/kconfig-language.txt: (TODO: Document CONFIG_SYS_SUPPORTS_* or better yet, just do it in arch/mips/Kconfig)

--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -229,6 +229,18 @@ config MIPS_ITE8172
          either a NEC Vr5432 or QED RM5231. Say Y here if you wish to build
          a kernel for this platform.
 
+config HAWAII_ALOHA
+       bool "Hawaii Aloha"
+       select DMA_COHERENT
+       select SYS_HAS_CPU_R5000
+       select SYS_SUPPORTS_32BIT_KERNEL
+       select SYS_SUPPORTS_64BIT_KERNEL if EXPERIMENTAL
+       select SYS_SUPPORTS_LITTLE_ENDIAN
+       help
+         This is an evaluation board only available in Hawaii.
+         Before importing one, please check with your local import
+         laws on fictional hardware.
+
 config MACH_JAZZ
        bool "Support for the Jazz family of machines"
        select ARC

You'll need a corresponding Makefile entry so make may recurse into your platform sub-directory. Such a Makefile would look like this:

+#
+# Makefile for the Hawaii Aloha specific parts of the kernel
+#
+
+obj-y := irq.o serial.o setup.o time.o
+
+obj-$(CONFIG_PCI)               += pci.o
+obj-$(CONFIG_EARLY_PRINTK)      += console.o

You will also need a specific Platform file so the Linux kernel build system knows about your platform specific starting address, where to look for header files, and how to recurse into your platform specific Makefile.

$(load-y) is the starting address for your Linux image when it is loaded into RAM. Note that the first 0x200 bytes are used by the exception vectors on most CPUs. Some CPUs will require a larger space, so modify $(load-y) accordingly. Due to the linker's addressing limit, the start address is aligned on a 8KB boundary, so setting $(load-y) to 0xffffffff80002000 should be reasonable:

--- a/arch/mips/aloha/Platform
+++ b/arch/mips/aloha/Platform
+#
+# Hawaii Aloha board
+#
+platform-$(CONFIG_HAWAII_ALOHA) += aloha/
+cflags-$(CONFIG_HAWAII_ALOHA)   += -I$(srctree)/arch/mips/include/asm/mach-aloha/
+load-$(CONFIG_HAWAII_ALOHA)     += 0xffffffff80002000

Provide board-specific hooks

These are the hooks you need to get a tree that actually builds. Actual implementation depends on your hardware:

  • get_system_type()
/*
 * arch/mips/aloha/init.c
 */

const char *get_system_type(void)
{
        return "Hawaii Aloha";
}
  • arch_init_irq()
/*
 * arch/mips/aloha/irq.c
 */

#include <linux/init.h>

void __init arch_init_irq(void)
{
        /* do board-specific irq initialization */
}
  • prom_init(), prom_free_prom_memory()
/*
 * arch/mips/aloha/prom.c
 */

#include <linux/init.h>

void __init prom_init(void)
{
        /* get whatever info we need from the PROM */
}

unsigned long prom_free_prom_memory(void)
{
        return 0; /* "Freeing unused memory: %ldk freed\n" */
}
  • plat_mem_setup()
/*
 * arch/mips/aloha/setup.c
 */

#include <linux/init.h>

void __init plat_mem_setup(void)
{
        /* do board-specific setup.. */
}

Configure and build a kernel image

Now you are ready to run your favorite configuration tool. Since we do not have much code added yet, do not be too greedy with selecting options. Just pick a couple of simple options such as the serial and serial console.

  • If you denoted the Aloha board support to be EXPERIMENTAL, select 'Prompt for development and/or incomplete code/drivers' under 'Code maturity level options'.
  • Select 'Support for Hawaii Aloha board' and unselect all other machines under 'Machine selection'.
  • Select the right CPU. Under 'CPU selection' select your CPU. If there is no entry for the CPU on your board, you will need to add support for it. Most recent CPUs can generally run to some degree with CPU_R4X00.
  • Under 'Character devices', select 'Standard/generic (8250/16550 and compatible UARTs) serial support' and 'Support for console on serial port'. Unselect the 'Virtual terminal' option.
    • Note that some platforms may not use a standard 8250/16550 compatible UARTs but something entirely custom (refer to the hardware manual or reference code in that case)
    • With some recent platforms you may find more and more AMBA-compatible peripherals such as the AMBA PL010/PL011 UART blocks
  • Under the 'Kernel hacking' option, select 'Are you using a crosscompiler'.
  • For other options either take the default or select 'no'.

Here is a sample minimum config for our Aloha board.

Before you type make, double-check the arch/mips/Makefile and make sure the cross-toolchain program names are correct and in your execution path i.e. your PATH environment variable.

Now type make dep and make. Then wait for a miracle to happen!

Early printk()

Assuming you are lucky and actually generate an image from the last chapter, don't bother running it because you won't see anything. This is not strange because all our board-specific code is empty and we have not told Linux kernel anything about our serial port or I/O devices yet.

The sign of a live Linux kernel comes from the output of printk, which is routed to the first console. Since we have configured a serial console, we should be able to see something on the serial wire if we have set it up correctly.

Unfortunately, setup of the serial console happens much later during the kernel startup process. (See Appendix A: Linux startup sequence in 2.4.x for a chart of the kernel start-up sequence). Chances are your new kernel probably dies even before that. That is where the early printk patch comes in handy. It allows you to see printk as early as the first line of C code.

In order to properly make use of early printk, your platform should select SYS_HAS_EARLY_PRINTK and implement a prom_putchar() routine which will be used by the kernel to output characters via the UART.

Serial driver and console

While early printk is rather useful, you still need to get the real serial driver working. Serial Driver and Console has all the details you'll need in order to implement a device driver for your serial port.

kgdb

For many Linux kernel developers, KGDB is a life-saving tool. With KGDB, you can debug the kernel while it runs! You can set breakpoints or do single stepping at the source code level.

To do this, you will need a dedicated serial port on your target, and use a crossover-cable (also known as a null-modem) to connect it to your development host. If you are also using a serial console, this implies you will need two serial ports on your target. It is possible to do both kernel debug and serial console through a single serial port. This will be mentioned later in this chapter.

When you configure the kernel, select the Remote GDB kernel debugging which is listed under Kernel hacking. Do a make clean and recompile the kernel so that debugging symbols are compiled into your kernel image. Try to make a new image. You will soon discover two missing symbols in the final linking stage:

arch/mips/kernel/kernel.o: In function `getpacket':
arch/mips/kernel/kernel.o(.text+0x85ac): undefined reference to `getDebugChar'
arch/mips/kernel/kernel.o(.text+0x85cc): undefined reference to `getDebugChar'
arch/mips/kernel/kernel.o(.text+0x8634): undefined reference to `getDebugChar'
arch/mips/kernel/kernel.o(.text+0x864c): undefined reference to `getDebugChar'
arch/mips/kernel/kernel.o(.text+0x8670): undefined reference to `putDebugChar'
arch/mips/kernel/kernel.o(.text+0x8680): undefined reference to `putDebugChar'
arch/mips/kernel/kernel.o(.text+0x8698): undefined reference to `putDebugChar'
arch/mips/kernel/kernel.o(.text+0x86a0): undefined reference to `putDebugChar'

You need to supply these two functions for your own boards:

int putDebugChar(uint8 byte)
uint8 getDebugChar(void)

As an example, here is the dbg_io.c for DDB5476 board. DDB5476 uses a standard UART serial port to implement those two functions.

After supplying those two functions, you are ready to debug the kernel. Run the new kernel image. If you also use the early printk patch, you should be able to see something like this on your console:

Wait for gdb client connection ...

Assuming you have already connected a cross-over serial cable between the dedicated serial port on the target and a serial port on your host (say, COM0), you can then set the appropriate baud rate and start the cross gdb on your host:

stty -F /dev/ttyS0 38400
mipsel-linux-gdb vmlinux

At the gdb prompt, type

target remote /dev/ttyS0

And, voila! You should be talking to the kernel through KGDB - if you are lucky enough!

A couple of tips on using KGDB:

  • Any functions preceded with the __init label will not break very well with breakpoints. Sometimes it will screw up the line numbers of other functions in the same file. Try undefining __init to be an empty macro in your include/linux/init.h file. Refer to the patch. [NOTE: This problem is fixed in the latest gdb version, at least in gdb 5.2]
  • Sometimes if you break on a function, you cannot see the correct value in variables and cannot do back-tracing. This is probably because certain registers are still not initialized [HELP: because kernel is compiled with -O2 flag?]. Step into the function a couple of lines, and you should see the variable and back-tracing fine.

What if the board only has one serial port?

Some boards only have one serial port. If you use it as serial console, you cannot really use it for KGDB - unless you do some tricks to it.

There are two solutions. One is GDB console, and the other is to use a KGDB demuxing script.

It is easy to use the GDB console. When you select 'Remote GDB kernel debugging' under the 'Kernel hacking' sub-menu, you are also prompted for 'Console output to GDB'. Simply selecting that choice will work! In fact, this option is so easy to use you might want to use it even if you have a second serial port.

However, this option has a limit. When the kernel goes to userland, the console stops working. This is because the KGDB stub in the kernel and GDB are not designed to provide interactive output. [HELP: any volunteers?]

The second option uses a script called 'kgdb_demux' written by Brian Moyle. It creates two virtual ports, typically ttya0 and ttya1. It then listens to the real serial port (such as ttyS0). It will forward console traffic to ttya0 and KGDB traffic to ttya1. All you have to do then is to start minicom on /dev/ttya0 (port setting does not matter) and KGDB on /dev/ttya1. With 2.6 kernel pseudo terminals can be /dev/pts/1-7

You can download the tarball here. A couple of usage tips.

  • Untar the file to some place.
  • Copy kgdb_demux script to your execution path, and modify it properly.
  • Set the port parameters properly before you start kgdb_demux.

A newer C version of kdmx can be downloaded from this post.

Supporting a new CPU

In the MIPS world, there are different families of MIPS cores. For example Toshiba TX49, SiByte SB1, NEC VR41XX, MIPS32, and MIPS64 to name a few. These different families may have different cache architectures, may be 32-bit or 64-bit, and may or may not have a FP unit. A complete list of families is found in the arch/mips/Kconfig and include/asm-mips/cpu.h files. When you are configuring your kernel the complete list will appear under the CPU type menu selection. If you are adding support for an entirely new family of MIPS cores, you will need to change the following files:

  • arch/mips/Kconfig: You will need to add your processor family to the list under CPU type.
  • include/asm-mip/cpu.h: You will need to add a PRID_COMP and PRID_IMP entries for your processor family if they do not already exist. You will also need to add the CPU types that you will support in that family. Adding CPU types is covered below.
  • arch/mips/kernel/cpu-probe.c: For each processor family, there is typically a cpu_probe_XXX() function that probes and fills in the struct cpuinfo_mips which contains information about the CPU type, whether a FP unit exists, the number of TLBs available, etc. Your function is responsible for properly probing and filling in this information for the entire family of processors. Finally, if your family does something special for CPU idle, then you will have to define a wait() function. Most likely, you will be able to use one that already exists. If not, add your wait() function and enable detection of it in check_wait().
  • arch/mips/mm: If your processor family has a unique cache architecture or anything out of the ordinary, you will need to either add or modify files in this directory.

Within each family of MIPS cores, there are multiple processors. For example, Alchemy processors and Broadcom processors are in the MIPS32 family, the Sibyte 1250 is in the SB1 family, and so on. Once you have decided what family your processor is in, you need to verify that a unique CPU identification exists for it. include/asm-mips/cpu.h contains all of the currently supported CPU types for the Linux/MIPS kernel.

If your processor is already listed in include/asm-mips/cpu.h, the existing Linux/MIPS code should auto-detect it. Otherwise, you'll need to change three files to properly detect your processor:

  • include/asm-mips/cpu.h: Add your processor at the end of the CPU_XXX definitions. Make sure you update the CPU_LAST value to match that of the processor you added.
  • arch/mips/kernel/proc.c: Add an entry to the cpu_name array so that your CPU name can be properly displayed in /proc/cpuinfo.
  • arch/mips/kernel/cpu-probe.c: Add your processor into the case statement in the function check_wait() if your CPU can do some sort of wait or idle operation. Add the necessary code in the cpu_probe_XXX() function to detect and set your CPU type.

Once you have support for your processor family and specific CPU type, you should be able to take full advantage of its capabilities.

Interrupts

If you followed the previous steps, most likely you will see kernel hanging at the BogusMIPS calibration step. The reason is simple: The interrupt code is not there and jiffies are never updated. Therefore the calibration can never be done.

Before you start writing interrupt code, it really pays to study the hardware first. Pay particular attention to identify all interrupt sources, their corresponding controllers and how they are routed.

Then you need to come up with a strategy, which typically includes:

  • a static interrupt routing map
  • a list of interrupt sources
  • a list of their corresponding controllers
  • how interrupt controllers cascade from each other

Interrupt code overview

To completely service an interrupt, four different pieces of code work together:

  • IRQ detection/dispatching:

This is typically assembly code in a file called 'int_handler.S'. Sometimes there is also secondary-level dispatching code written in C for complicated IRQ detection. The end result is that we identify and select a single IRQ source, represented by an integer, and then pass it to function 'do_IRQ()'.

  • do_IRQ()

do_IRQ() is provided in the 'arch/mips/kernel/irq.c' file. It provides a common framework for IRQ handling. It invokes the individual IRQ controller code to enable/disable a particular interrupt. It calls the driver supplied interrupt handling routine that does the real processing.

  • hw_irq_handler

It is a structure associated with each IRQ source. The structure is a collection of function pointers, which tells do_IRQ() how it should deal with this particular IRQ.

  • driver interrupt handling code

The code that does the real job.

Obviously for our porting purposes we need to write IRQ detection/disptaching code and the hw_irq_handler code for any new IRQ controller used in the system. In addition, there is also IRQ setup and initialization code.

CPU as an IRQ controller

MIPS processors include a simple interrupt controller. In it's simplest case as implemented in the R2000 it implements two software interrupts. This are interrupts that can only be raised by software setting the bit in the cause register and needs to be cleared by the interrupt handler. Otherwise they will behave just like hardware interrupts.

In case of the R4000 and most newer processors newer than the R4000 the IP7 bit in the cp0 status and cause registers doubles to serve the timer interrupt also. That is the IP7 interrupt will be raised whenever the cop0 count and compare registers have the same value. The exact mode of operation of the count register and the timer interrupt are often configurable by the CPU's mode bits. Using IP7 as the timer interrupt typically also means this interrupt cannot sensibly used for any other purpose.

So here's how to initialize the timer interrupt in a typical system. First define "CONFIG_IRQ_CPU" for your machine to enable compile the support code into the kernel. Then in your arch_init_irq() just call mips_cpu_irq_init(x) where for x you put the first of the 8 interrupt numbers you're going to assign to the CPU's interrupt controller.

Set up cascading interrupts

More than likely you will have more interrupt sources than those that can directly connect to the CPU interrupt pins. A second or even third-level interrupt controller may connect to one or more of those CPU pins. In that case, you have cascading interrupts.

There are plenty of examples of how cascading interrupt works, such as the DDB5477. Here is a short summary:

  • Assign blocks of IRQ numbers to various interrupt controllers in the whole system. For example, in the case of Vr4181 systems, CPU interrupts occupy IRQ 0 to 7. Vr4181 system interrupts occupy IRQ 8 to 39, and GPIO interrupts occupy 40 to 56. In most cases, the actual IRQ numbers do not matter, as long as the driver knows which IRQ number it should use. However, if you have an i8259 interrupt controller and an ISA bus, you should try to assign IRQ number 0 to 16 for the i8259 interrupts because it will make the legacy PC drivers happy. (Please note before ~12/08/2001 in version 2.4.16 of the Linux kernel, the 'i8259.c' file set the base vector to be 0x20. If you use the IRQ acknowledgement cycle to obtain the interrupt vector, you will get an IRQ number from 0x20 to 0x2f. You will then need to substract 0x20 from the return value to get the correct IRQ number.)
  • Write the 'hw_irq_controller' member functinos for your specific controllers. Note that CPU and i8259 already have their code written. You just need to define appropriate CONFIG options for your board. See the next sub-section for more details about writing 'hw_irq_controller' member functions.
  • In your IRQ setup routine, initialize all the controllers, usually by calling 'interrupt_controller_XXX_init()' functions.
  • In your IRQ setup routine, setup the cascading IRQs. This setup will enable interrupts for the upper interrupt controller so that the lower-level interrupts can cascade through once they are enabled. A typical way of doing this is to have a dummy 'irqaction struct' and setup as follows:
static struct irqaction cascade =
        { no_action, SA_INTERRUPT, 0, "cascade", NULL, NULL };

extern int setup_irq(unsigned int irq, struct irqaction *irqaction);

void __init <my>_irq_init(void)
{
	....
	setup_irq(CPU_IP3, &cascade);
}
  • You need to expand your interrupt dispatching code properly to identify the added interrupt sources. If the code is simple enough, you can do it in the same int_handler.S file. If it is more complicated, you may do it in a separate C function (such as in the DDB5476 board).

The hw_irq_controller struct

The hw_irq_controller structure' is a defined in the 'include/linux/irq.h' file as an alias for the 'hw_interrupt_type' structure.

struct hw_interrupt_type {
        const char * typename;
        unsigned int (*startup)(unsigned int irq);
        void (*shutdown)(unsigned int irq);
        void (*enable)(unsigned int irq);
        void (*disable)(unsigned int irq);
        void (*ack)(unsigned int irq);
        void (*end)(unsigned int irq);
        void (*set_affinity)(unsigned int irq, unsigned long mask);
};

The 'arch/mips/kernel/irq_cpu.c' is a good sample code to write 'hw_irq_controller' member functions. Here are some more programming notes for each of these functions:

  • const char * typename;

Controller name. Will be displayed under /proc/interrupts.

  • unsigned int (*startup)(unsigned int irq);

Invoked when request_irq() or setup_irq are called. You need to enable this interrupt here. Other than that you may also want to do some IRQ-specific initialization (such as turning on power for this interrupt, perhaps).

  • void (*shutdown)(unsigned int irq);

Invoked when free_irq() is called. You need to disable this interrupt and perhaps some other IRQ-specific cleanup.

  • void (*enable)(unsigned int irq) and void (*disable)(unsigned int irq)

They are used to implement enable_irq(), disable_irq() and disable_irq_nosync(), which in turn are used by driver code.

  • void (*ack)(unsigned int irq)

ack() is invoked at the beginning of do_IRQ() when we want to acknoledge an interrupt. I think you need also to disable this interrupt here so that you don't get recursive interrupts on the same interrupt source. [HELP: can someone confirm?]

  • void (*end)(unsigned int irq)

This is called by do_IRQ() after it has handled this interrupt. If you disabled interrupt in ack() function, you should enable it here. [HELP: generally what else we should do here?]

  • void (*set_affinity)(unsigned int irq, unsigned long mask)

This is used in SMP machines to set up interrupt handling affinity with certain CPUs. [TODO] [HELP]

The IRQ initialization code

The IRQ initialization is done in 'init_IRQ()'. Currently it is supplied by each individual board. In the future, it will probably be a MIPS common routine, which will further invoke a board-specific function, board_irq_init(). board_irq_init will be a function pointer that <my_board>_setup() function needs to assign propoer value.

In any case, the following is a skeleton code for a normal init_IRQ() routine.

extern asmlinkage void vr4181_handle_irq(void);
extern void breakpoint(void);
extern int setup_irq(unsigned int irq, struct irqaction *irqaction);
extern void mips_cpu_irq_init(u32 irq_base);
extern void init_generic_irq(void);

static struct irqaction cascade =
        { no_action, SA_INTERRUPT, 0, "cascade", NULL, NULL };
static struct irqaction reserved =
        { no_action, SA_INTERRUPT, 0, "reserved", NULL, NULL };
static struct irqaction error_irq =
        { error_action, SA_INTERRUPT, 0, "error", NULL, NULL };

void __init init_IRQ(void)
{
        int i;
        extern irq_desc_t irq_desc[];

	/* hardware initialization code */
	....

	/* this is the routine defined in int_handler.S file */
       set_except_vector(0, my_irq_dispatcher);

	/* setup the default irq descriptor */
	init_generic_irq();

       /* init all interrupt controllers */
       mips_cpu_irq_init(CPU_IRQ_BASE);
	....

       /* set up cascading IRQ */
        setup_irq(CPU_IRQ_IP3, &cascade);

	/* set up reserved IRQ so that others can not mistakingly request
	 * it later.
 	 */
        setup_irq(CPU_IRQ_IP4, &reserved);

#ifdef CONFIG_DEBUG
	/* setup debug IRQ so that if that interrupt happens, we can 
	 * capture it.
	 */
        setup_irq(CPU_IRQ_IP4, &error_irq);
#endif

#ifdef CONFIG_REMOTE_DEBUG
        printk("Setting debug traps - please connect the remote debugger.\n");
        set_debug_traps();
        breakpoint();
#endif
}

Final notes

What is described in this chapter is what is so-called new style interrupt handling. We used to have three different ways to handle interrupts: new style (CONFIG_NEW_IRQ), the old style (CONFIG_ROTTEN_IRQ) and board-private ad hoc routines. New style is now the only valid method since October-2002.

System time and timer

Linux relies on an RTC (real-time clock) device to obtain the real calendar data and time when it boots up. It relies on a system timer to advance the tick count, jiffies. If you don't provide proper time and timer code, Linux won't run. In fact it will stick in 'calibrate_delay()' during the startup process because jiffies is never incremented. (See Appendix B for more details about Linux/MIPS startup sequence).

There is an excellent document (I becomes a little shameless. :-0) under 'Documentations/mips/time.README' that should be read to further understand timekeeping for Linux/MIPS. It is a must read.

Here are some comments on implementing time and timer services:

  • If your system has a CPU counter and another hardware timer, use the hardware timer over the CPU counter, even though CPU counter might be easier to setup and use. This is because CPU counter relies on the CPU frequency which is more likely to change in the future. In addition, performance-critical code may need to access the CPU counter for its own measurements. Some CPUs may have a variable CPU frequency which makes CPU counter not usable as a timer source. [DEBATE: 03/12/04, I changed my preference on this issue. Linux in the future will have richer and higher resolution time support. If all MIPS boards use CPU counter as the system timer, we can maximize the code sharing.]
  • Unless you have to use interrupts to calibrate the CPU frequency, you can generally avoid implementing the 'board_time_init()' function. Most of its work can be done in the board setup routine.
  • When you implement 'rtc_set_time()', more than likely you need to call the 'to_tm()' function which converts a single jiffy value to a full 'struct rtc_time'. This function is provided in 'arch/mips/kernel/time.c' and declared in 'include/asm-mips/time.h'.

PCI support

The PCI subsystem is perhaps the most complex code you have to deal with during the porting process. Thanks to the similarity of PCI, HyperTransport, PCI-X, Cardbus and other bus systems the time for understanding it well invested - and the key to making the PCI subsystem work properly is a good understanding of the PCI bus itself, the code layout, and the execution flow in Linux. Like many other parts of porting, you will find in the end, the actual code writen is minimal.

See PCI Subsystem for a discussion on the architecture-specific implementation and board-specific hooks you'll need to implement.

Debugging

Probably the most important chapter. In this chapter, I will list some commonly used debugging tips and tricks.

Kernel Oops

ksymoops is a program that deciphers all the secret numbers in a kernel core dump. Old versions of ksymoops may not work well for MIPS.

I sometimes use a script, call2sym, written by Phil Hollenback. You just cut and past the call trace part to feed the script and it will display a possible function call stack at the time crash. Note it is likely some symbols are bogus, but the real ones should be displayed. You will have to use your own judgement.

Linux 2.6 will automatically convert symbols if CONFIG_KALLSYMS is enabled. It may bloat kernels but it's a true life saver, so highly recommended.

Appendix A: Linux startup sequence in 2.4.x

kernel_entry() - arch/mips/kernel/head.S
   set stack;
   prepare argc/argp/envp;
   jal init_arch - arch/mips/kernel/setup.c
        cpu_probe() -
        prom_init(...) - arch/mips/ddb5074/prom.c
        loadmmu()
        start_kernel()  - init/main.c
                setup_arch(&commaind_line); - arch/mips/kernel/setup.c
                        ddb_setup()         - arch/mips/ddb5074/setup.c
                parse_options(command_line);
                trap_init();
                init_IRQ();                 - arch/mips/kernel/irq.c
                sched_init();
                softirq_init();
                time_init();
                        if (board_time_init) board_time_init();
                        set xtime by calling rtc_get_time();
			 pick appropriate do_gettimeoffset()
			 board_timer_setup(&timer_irqaction);
                console_init();
                init_modules();
                kmem_cache_init();
                sti();			/* interrupt first open */
                calibrate_delay();
                mem_init();
                kmem_cache_sizes_init();
		 pgtable_cache_init();
                fork_init();
                proc_caches_init();
                vfs_caches_init();
                buffer_init();
                page_cache_init();
                signals_init();
                smp_init();
                kernel_thread(init, ...)
                cpu_idle();


init() - init/main.c
  - lock_kernel();
    do_basic_setup();
      - [MTRR] mtrr_init();
        [SYSCTL] sysctl_init();
	 [S390] s390_init_machine_check();
        [ACPI] acpi_init();
        [PCI] pci_init();
        [SBUS] sbus_init();
        [PPC] ppc_init();
        [MCA] mca_init();
        [ARCH_ACORN] ecard_init();
        [ZORRO] zorro_init();
        [DIO] dio_init();
        [MAC] nubus_init();
        [ISAPNP] isapnp_init();
        [TC] tc_init();
        sock_init();
	 start_context_thread();
        do_initcalls();
        [IRDA] irda_proto_init();
        [IRDA] irda_device_init();
        [PCMCIA] init_pcmcia_ds();

    prepare_namespace();
    free_initmem();
    unlock_kernel();

    files = current->files;
    if(unshare_files())
            panic("unshare");
    put_files_struct(files);

    if (open("/dev/console", O_RDWR, 0) < 0)
                printk("Warning: unable to open an initial console.\n");
    (void) dup(0);
    (void) dup(0);

    if (execute_command)
                run_init_process(execute_command);
    run_init_process("/sbin/init");
    run_init_process("/etc/init");
    run_init_process("/bin/init");
    run_init_process("/bin/sh");

    panic("No init found.  Try passing init= option to kernel.");

Appendix B: Credits

People in the following list have generously given their feedback to me. In spite of my effort to keep the list as complete as possible, I am afraid many people are still missing here.