/* 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

/* 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 = 0;    /*     * 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 = 0; 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(0), mapstart,                     min_low_pfn, max_low_pfn);    for (i = 0; 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(0, 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 = 0; 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(0, 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();}

unsigned long initrd_start, initrd_end;

initrd_start = 0x81a00000initrd_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*/

/* 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 = 0;    initrd_end = 0;    return 0;}