Книга: Embedded Linux Primer: A Practical, Real-World Approach

16.3.3. Machine-Dependent Calls

16.3.3. Machine-Dependent Calls

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.

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