PFN相关宏说明：
/* kernel/include/linux/pfn.h */PFN ： Page Frame Number（物理页帧）
/*
* PFN_ALIGN:返回地址x所在那一页帧的下一页帧的起始地址。
* 例如：PFN_ALIGN(0x00000800) = 0x00001000 ； PFN_ALIGN(0x00001800) = 0x00002000;
* 理解：假如我们认为一页大小是0x0f，那么当前地址是0x08，如何通过0x08获得0x10呢？ 0x08 + （0x10 - 1） = 0x17, 然后再把低位抹掉，不就刚好是10.
* 问题：这样做有一个问题，如果x=0x0，那么返回的是0x0，而不是0x10，这是为什么呢？
*/
#define PFN_ALIGN(x) (((unsigned long)(x) + (PAGE_SIZE - 1)) & PAGE_MASK)/*
* PFN_UP：获取地址x所在物理页帧的的后一个PFN值，即x如果属于page 0， 则返回1.
* 问题：如果 x = 0x0, 那么返回的不是1，而是0；如果 x = 0x00001000,返回的是1，而不是2.为什么？
*/
#define PFN_UP(x) (((x) + PAGE_SIZE-1) >> PAGE_SHIFT)/*
* PFN_DOWN：获取地址x所在物理页帧的前一个PFN值，即如果x属于page 1，则返回0.
*/
#define PFN_DOWN(x) ((x) >> PAGE_SHIFT)
/*
* PFN_PHYS：返回PFN值为x时，对应的物理页帧的起始地址
*/
#define PFN_PHYS(x) ((phys_addr_t)(x) << PAGE_SHIFT)关于上述问题，其实如果看看它的实际应用场景就会明白的。要记住：PFN是一个从0开始的页帧编号
打印结果记录：
initrd_start = 0x81a0000, initrd_end = 0x81b2e720
我们继续看 start_kernel—>setup_arch—>arch_mem_init—>bootmem_init。
/* kernel/arch/mips/kernel/setup.c */
static void __init bootmem_init(void)
{
unsigned long reserved_end;
unsigned long mapstart = ~0UL;
unsigned long bootmap_size;
int i;
/*
* Init any data related to initrd. It's a nop if INITRD is
* not selected. Once that done we can determine the low bound
* of usable memory.
*/
/*
* 初始化所有和initrd相关的数据。如果没有INITRD，这将是一个空操作。一旦该操作完成，我们就可以确定可用内存的边界。
*/
// 计算需要为initrd保留的内存区域，那么剩余的内存就是可用内存
reserved_end = max(init_initrd(),
(unsigned long) PFN_UP(__pa_symbol(&_end)));
// reserved_end = max(0x1b2f, 0x936) = 0x1b2f
/*
* Redo reserved_end because there is no need to reserve so much memory(about 20MB).
*/
reserved_end = (unsigned long) PFN_UP(__pa_symbol(&_end));
// reserved_end = 0x936 = 2358（PFN）
/*
* 上述步骤中得 PFN_UP(__pa_symbol(&end)) 不太懂，没有继续深入，而且上述步骤计算reserved_end做的一些事也是不太理解！！
*/ /*
* max_low_pfn is not a number of pages. The number of pages
* of the system is given by 'max_low_pfn - min_low_pfn'.
* 系统物理页帧总数 = max_low_pfn - min_low_pfn
*/
min_low_pfn = ~0UL;
max_low_pfn = ; /*
* Find the highest page frame number we have available.
*/
/*
* 寻找最大可用PFN。
* 代码很简单，记录下我们的打印，助于分析：
* memory: 0e000000 @ 00000000 (usable)
* memory：10000000 @ 30000000 (usable)
* start = 0, end = 53744
* start = 196608, end = 262144
* mapstart = 2358
*/
for (i = ; i < boot_mem_map.nr_map; i++) {
unsigned long start, end; if (boot_mem_map.map[i].type != BOOT_MEM_RAM)
continue; start = PFN_UP(boot_mem_map.map[i].addr);
end = PFN_DOWN(boot_mem_map.map[i].addr
+ boot_mem_map.map[i].size);
if (end > max_low_pfn)
max_low_pfn = end;
if (start < min_low_pfn)
min_low_pfn = start;
if (end <= reserved_end)
continue;
if (start >= mapstart)
continue;
mapstart = max(reserved_end, start);
} if (min_low_pfn >= max_low_pfn)
panic("Incorrect memory mapping !!!");
if (min_low_pfn > ARCH_PFN_OFFSET) {
pr_info("Wasting %lu bytes for tracking %lu unused pages\n",
(min_low_pfn - ARCH_PFN_OFFSET) * sizeof(struct page),
min_low_pfn - ARCH_PFN_OFFSET);
} else if (min_low_pfn < ARCH_PFN_OFFSET) {
pr_info("%lu free pages won't be used\n",
ARCH_PFN_OFFSET - min_low_pfn);
}
min_low_pfn = ARCH_PFN_OFFSET; // 就是0 /*
* Determine low and high memory ranges
*/
/*
HIGHMEM_START：定义在kernel/arch/include/asm/mach-generic/spaces.h
对于32位系统， #define HIGHMEM_START _AC(0x20000000, UL) 【就是512M，从512M开始的物理地址认为是高端内存】
经过下边的计算：highstart_pfn = PFN_DOWN(HIGHMEM_START) = 131072, hightend_pfd = 262144
min_low_pfn = 0, max_low_pfn = 131072
*/
max_pfn = max_low_pfn;
if (max_low_pfn > PFN_DOWN(HIGHMEM_START)) {
#ifdef CONFIG_HIGHMEM
highstart_pfn = PFN_DOWN(HIGHMEM_START);
highend_pfn = max_low_pfn;
#endif
max_low_pfn = PFN_DOWN(HIGHMEM_START);
} /*
* Initialize the boot-time allocator with low memory only.
* 在系统启动过程中，内存管理尚未初始化，但内核需要分配内存 以创建各种数据结构，bootmem分配器用于在启动阶段早起的内存分配。
* 所以，init_bootmem_node计算 bootmem allocator 所需内存大小，该部分内存是作为 reserved memory。
*/
bootmap_size = init_bootmem_node(NODE_DATA(), mapstart,
min_low_pfn, max_low_pfn); for (i = ; i < boot_mem_map.nr_map; i++) {
unsigned long start, end; start = PFN_UP(boot_mem_map.map[i].addr);
end = PFN_DOWN(boot_mem_map.map[i].addr
+ boot_mem_map.map[i].size); if (start <= min_low_pfn)
start = min_low_pfn;
if (start >= end)
continue;#ifndef CONFIG_HIGHMEM
if (end > max_low_pfn)
end = max_low_pfn; /*
* ... finally, is the area going away?
*/
if (end <= start)
continue;
#endif
// start 和 end 没有发生改变：
// start = 0, end = 57344
// start = 196608, end = 262144
add_active_range(, start, end); // 记录物理内存的PFN信息
} /*
* Register fully available low RAM pages with the bootmem allocator.
* 注册所有的可用低端内存给bootmem allocator【bootmem allocator可以操作低端内存】
*/
for (i = ; i < boot_mem_map.nr_map; i++) {
unsigned long start, end, size; /*
* Reserve usable memory.
*/
if (boot_mem_map.map[i].type != BOOT_MEM_RAM)
continue; start = PFN_UP(boot_mem_map.map[i].addr);
end = PFN_DOWN(boot_mem_map.map[i].addr
+ boot_mem_map.map[i].size);
/*
* We are rounding up the start address of usable memory
* and at the end of the usable range downwards.
*/
if (start >= max_low_pfn)
continue;
if (start < reserved_end)
start = reserved_end;
if (end > max_low_pfn)
end = max_low_pfn; /*
* ... finally, is the area going away?
*/
if (end <= start)
continue; /* start = 2358, end = 53744 */
size = end - start; /* Register lowmem ranges 【完成注册】*/
free_bootmem(PFN_PHYS(start), size << PAGE_SHIFT);
memory_present(, start, end);
} /*
* Reserve the bootmap memory.【保留bootmap memory，mapstart = 2358, PFN_PHYS(mapstart) = 0x936000, bootmap_size = 16384Byte】
*/
reserve_bootmem(PFN_PHYS(mapstart), bootmap_size, BOOTMEM_DEFAULT); /*
* Reserve initrd memory if needed.【为initrd保留内存】 Initial ramdisk at：0x81a00000 (1238816 bytes)
*/
finalize_initrd();
}
2. init_initrd
在 kernel/init/do_mounts_initrd.c中定义了变量：
unsigned long initrd_start, initrd_end;
在arch_mem_init —> parse_early_param 中，不仅调用了early_parse_mem函数，同时也调用了：rd_start_early和rd_size_early。
这两个函数就是解析 command line 中指定的 initrd 的内存信息，代码我就贴了，调用这两个函数的结果就是：
initrd_start = 0x81a00000
initrd_end = 0x81b2e720
/*
* command line : console=ttyS3,115200n8 mem=224M@0x0 mem=256M@0x30000000 ip=off root=/dev/ram0 rw rdinit=/init rd_start=0x81A00000 rd_size=0x0012E720
*/
此时， arch_mem_init—>bootmem_init—>init_initrd：
/* it returns the next free pfn after initrd */
static unsigned long __init init_initrd(void)
{
unsigned long end; /*
* Board specific code or command line parser should have
* already set up initrd_start and initrd_end. In these cases
* perfom sanity checks and use them if all looks good.
*/
if (!initrd_start || initrd_end <= initrd_start)
goto disable; if (initrd_start & ~PAGE_MASK) {
pr_err("initrd start must be page aligned\n");
goto disable;
}
if (initrd_start < PAGE_OFFSET) {
pr_err("initrd start < PAGE_OFFSET\n");
goto disable;
} /*
* Sanitize initrd addresses. For example firmware
* can't guess if they need to pass them through
* 64-bits values if the kernel has been built in pure
* 32-bit. We need also to switch from KSEG0 to XKPHYS
* addresses now, so the code can now safely use __pa().
*/
end = __pa(initrd_end);
initrd_end = (unsigned long)__va(end);
initrd_start = (unsigned long)__va(__pa(initrd_start)); ROOT_DEV = Root_RAM0;
return PFN_UP(end);
disable:
initrd_start = ;
initrd_end = ;
return ;
}
