前面一篇文章,简单介绍了内核启动的汇编主流程,这篇介绍其中调用的汇编子函数__lookup_processor_type
函数__lookup_processor_type介绍:
内核中使用了一个结构struct proc_info_list,用来记录处理器相关的信息,该结构定义在
kernel/include/asm-arm/procinfo.h头文件中。
/*
* Note! struct processor is always defined if we're
* using MULTI_CPU, otherwise this entry is unused,
* but still exists.
*
* NOTE! The following structure is defined by assembly
* language, NOT C code. For more information, check:
* arch/arm/mm/proc-*.S and arch/arm/kernel/head-armv.S
*/
struct proc_info_list {
unsigned int cpu_val;
unsigned int cpu_mask;
unsigned long __cpu_mmu_flags; /* used by head-armv.S */
unsigned long __cpu_flush; /* used by head-armv.S */
const char *arch_name;
const char *elf_name;
unsigned int elf_hwcap;
struct proc_info_item *info;
struct processor *proc;
};
在arch/arm/mm/proc-xscale.S文件中定义了所有和xscale有关的proc_info_list,我们使用的pxa270定义如下:
.section ".proc.info", #alloc, #execinstr
.type __bva0_proc_info,#object
__bva0_proc_info:
.long 0x69054110 @ Bulverde A0: 0x69054110, A1 : 0x69054111.
.long 0xfffffff0 @ and this is the CPU id mask.
#if CACHE_WRITE_THROUGH
.long 0x00000c0a
#else
.long 0x00000c0e
#endif
b __xscale_setup
.long cpu_arch_name
.long cpu_elf_name
.long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_EDSP|HWCAP_XSCALE
.long cpu_bva0_info
.long xscale_processor_functions
.size __bva0_proc_info, . - __bva0_proc_info
由于.section指示符,上面定义的__bva0_proc_info信息在编译的时候被放到了.proc.info段中,这是由linux的
链接脚本文件vmlinux.lds指定的,参考如下:
SECTIONS
{
. = 0xC0008000;
.init : { /* Init code and data */
_stext = .;
__init_begin = .;
*(.text.init)
__proc_info_begin = .;
*(.proc.info)
__proc_info_end = .;
这里的符号__proc_info_begin指向.proc.info的起始地址,而符号__proc_info_end指向.proc.info的结束地址。
后面就会引用这两个符号,来指向.proc.info这个段。
下面来来看看函数的源代码,为了分析方便将函数按行进行编号,其中17-18行就是前面提到的对.proc.info的引用,
第2行将17行的地址放到寄存器r5中,adr是小范围的地址读取伪指令。第3行将r5所指向的数据区的数据读出到r7,r9
r10,执行结果是r7=__proc_info_end,r9=__proc_info_begin,r10=第19行的地址,第4-6行的结果应该是r10指向
__proc_info_begin的地址,第7行读取cpu的id,这是一个协处理器指令,将processor ID存储在r9中,第8行将r10指向
的__bva0_proc_info开始的数据读出放到寄存器r5,r6,r8,结果r5=0x69054110(cpu_val),r6=0xfffffff0(cpu_mask),
r8=0x00000c0e(__cpu_mmu_flags),第9-10行将读出的id和结构中的id进行比较,如果id相同则返回,返回时r9存储
processor ID,如果id不匹配,则将指针r10增加36(proc_info_list结构的长度),如果r10小于r7指定的地址,也就是
__proc_info_end,则继续循环比较下一个proc_info_list中的id,如第11-14行的代码,如果查找到__proc_info_end
仍未找到一个匹配的id,则将r10清零并返回,如15-16行,也就是说如果函数执行成功则r10指向匹配的proc_info_list
结构地址,如果函数返回错误则r10为0。
/*
* Read processor ID register (CP#15, CR0), and look up in the linker-built
* supported processor list. Note that we can't use the absolute addresses
* for the __proc_info lists since we aren't running with the MMU on
* (and therefore, we are not in the correct address space). We have to
* calculate the offset.
*
* Returns:
* r5, r6, r7 corrupted
* r8 = page table flags
* r9 = processor ID
* r10 = pointer to processor structure
*/
1 __lookup_processor_type:
2 adr r5, 2f
3 ldmia r5, {r7, r9, r10}
4 sub r5, r5, r10 @ convert addresses
5 add r7, r7, r5 @ to our address space
6 add r10, r9, r5
7 mrc p15, 0, r9, c0, c0 @ get processor id
8 1: ldmia r10, {r5, r6, r8} @ value, mask, mmuflags
9 and r6, r6, r9 @ mask wanted bits
10 teq r5, r6
11 moveq pc, lr
12 add r10, r10, #36 @ sizeof(proc_info_list)
13 cmp r10, r7
14 blt 1b
15 mov r10, #0 @ unknown processor
16 mov pc, lr
/*
* Look in include/asm-arm/procinfo.h and arch/arm/kernel/arch.[ch] for
* more information about the __proc_info and __arch_info structures.
*/
17 2: .long __proc_info_end
18 .long __proc_info_begin
19 .long 2b
20 .long __arch_info_begin
21 .long __arch_info_end
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