Christopher Hallinan - Embedded Linux Primer - A Practical, Real-World Approach

If no bi_record data is found, the PowerPC architecture expects this data in the form of U-Boot board information structure, or bd_info. It is the bootloader's responsibility to construct this data structure and pass the address in register r3. Currently, many bits of hardware information are available in the bd_info structure, including information on DRAM, FLASH, SRAM, processor clock rates, bus frequencies, serial port baud rate setting, and more.

The bi_record structure can be examined in .../include/asm-ppc/bootinfo.h, and the bd_info structure can be found in .../include/asm-ppc/ppcboot.h.

It is the responsibility of the platform-initialization routines to make use of any of the data that might be necessary to complete the hardware setup, or to communicate it to the kernel. For example, platform_init() sets up a pointer to a function whose name reveals its purpose. The code from Listing 16-8 is reproduced here:

ppc_md.find_end_of_memory = mpc52xx_find_end_of_memory;

Looking at the function mpc52xx_find_end_of_memory(), which is found in .../arch/ppc/syslib/mpc52xx_setup.c, we find the following:

u32 ramsize = __res.bi_memsize;

if (ramsize == 0) {

... /* Find it another way */

}

return ramsize;

The __res data structure above is the board information structure, whose address was passed to us from the bootloader in register r3 above. As you can see, the generic setup code stored the residual data (as it is often called) passed in by the bootloader, but it's up to the machine or platform-specific code to make use of it.

Many common routines that the kernel needs either for initialization or for operation are architecture and machine (CPU) dependent. From the platform_init() function reproduced in Listing 16-8, we saw the following:

...

/* Setup the ppc_md struct */

ppc_md.setup_arch = lite5200_setup_arch;

ppc_md.show_cpuinfo = lite5200_show_cpuinfo;

ppc_md.show_percpuinfo = NULL;

ppc_md.init_IRQ = mpc52xx_init_irq;

ppc_md.get_irq = mpc52xx_get_irq;

#ifdef CONFIG_PCI

ppc_md.pci_map_irq = lite5200_map_irq;

#endif

ppc_md.find_end_of_memory = mpc52xx_find_end_of_memory;

ppc_md.setup_io_mappings = mpc52xx_map_io;

ppc_md.restart = mpc52xx_restart;

ppc_md.power_off = mpc52xx_power_off;

ppc_md.halt = mpc52xx_halt;

...

Lines similar to these make up the rest of the platform_init() function. Here the bulk of the platform-specific needs are communicated to the Linux kernel. The global variable ppc_md, of type struct machdep_calls, provides the hooks to easily customize the Linux kernel for a PowerPC platform. This variable is declared in .../arch/ppc/kernel/setup.c. Many places in the PowerPC-specific kernel branch call functions indirectly through this structure. For example, Listing 16-10 reproduces a portion of .../arch/ppc/kernel/setup.c, which contains support for the restart, power-off, and halt functions:

Listing 16-10. Generic PowerPC Machine Functions

void machine_restart(char *cmd) {

#ifdef CONFIG_NVRAM

nvram_sync();

#endif

ppc_md.restart(cmd);

}

void machine_power_off(void) {

#ifdef CONFIG_NVRAM

nvram_sync();

#endif

ppc_md.power_off();

}

void machine_halt(void) {

#ifdef CONFIG_NVRAM

nvram_sync();

#endif

ppc_md.halt();

}

These functions are called via the ppc_md structure and contain the machine- or platform-specific variants of these functions. You can see that some of these functions are machine specific and come from mpc52xx_* variants of the functions. Examples of these include mpc52xx_restart and mpc52xx_map_io. Others are specific to the hardware platform. Examples of platform-specific routines include lite5200_map_irq and lite5200_setup_arch.

Now that we have a reference from which to proceed, we can create the necessary files and functions for our own custom board. We copy the Lite5200 platform files for our baseline and modify them for our custom PowerPC platform. We'll call our new platform PowerDNA . The steps we will perform for this custom port are as follows:

1.Add a new configuration option to ...arch/ppc/Kconfig.

2.Copy lite5200.* to powerdna.* as a baseline.

3.Edit new powerdna.* files as appropriate for our platform.

4.Edit .../arch/ppc/Makefile to conditionally include powerdna.o.

5.Compile, load, and debug!

You learned how to add a configuration option to Kconfig in Chapter 4. The configuration option for our new PowerDNA port is detailed in Listing 16-11.

Listing 16-11. Configuration Option for PowerDNA

config POWERDNA

bool "United Electronics Industries PowerDNA"

select PPC_MPC52xx

help

Support for the UEI PowerDNA board

This Kconfig entry is added just below the entry for LITE5200 because they are related. [114] To preserve space, we temporarily removed many machine types in Figure 16-4 prior to LITE5200. Figure 16-4 illustrates the results when the configuration utility is invoked.

Figure 16-4. Machine type option for PowerDNA

Notice that when the user selects POWERDNA, two important actions are performed:

1.The CONFIG_PPC_MPC52xx configuration option is automatically selected. This is accomplished by the select keyword in Listing 16-11.

2.A new configuration option, CONFIG_POWERDNA, is defined that will drive the configuration for our build.

The next step is to copy the files closest to our platform as the basis of our new platform-initialization files. We have already decided that the Lite5200 platform fits the bill. Copy lite5200.c to powerdna.c, and lite5200.h to powerdna.h. The difficult part comes next. Using the hardware specifications, schematics, and any other data you have on the hardware platform, edit the new powerdna.* files as appropriate for your hardware. Get the code to compile, and then proceed to boot and debug your new kernel. There is no shortcut here, nor any substitute for experience. It is the hard work of porting, but now at least you know where to start. Many tips and techniques for kernel debugging are presented in Chapter 14, "Kernel Debugging Techniques."

To summarize our porting effort, Listing 16-12 details the files that have been added or modified to get Linux running on the PowerDNA board.

Listing 16-12. PowerDNA New or Modified Kernel Files

linux-2.6.14/arch/ppc/configs/powerdna_defconfig

linux-2.6.14/arch/ppc/Kconfig

linux-2.6.14/arch/ppc/platforms/Makefile

linux-2.6.14/arch/ppc/platforms/powerdna.c

linux-2.6.14/arch/ppc/platforms/powerdna.h

linux-2.6.14/drivers/net/fec_mpc52xx/fec.c

linux-2.6.14/drivers/net/fec_mpc52xx/fec.h