/**
 * add_active_range - Register a range of PFNs backed by physical memory 【注册物理内存的PFN范围】
 * @nid: The node ID the range resides on
 * @start_pfn: The start PFN of the available physical memory
 * @end_pfn: The end PFN of the available physical memory
 * @id : 　　    该物理内存所属的内存结点。由于我们是UMA系统，只用一个内存结点，所以我们在调用时，传入的参数为0.
 * @start_pfn：  可用物理内存的开始PFN号
 * @end_pfn:    可用屋里内存的结束PFN号
 * These ranges are stored in an early_node_map[] and later used by
 * free_area_init_nodes() to calculate zone sizes and holes. If the
 * range spans a memory hole, it is up to the architecture to ensure
 * the memory is not freed by the bootmem allocator. If possible
 * the range being registered will be merged with existing ranges.
 * 传入的物理内存信息存储到一个数组中：early_node_map[]， 稍后会在 free_area_init_nodes() 函数中使用，用来计算 zone sizes 和 holes。
 * 如果物理内存的范围包含一个 memory hole，it is up to the architecture to ensure the memory is not freed by the bootmem allocator.
 * 新注册的物理内存范围会Merge到已经存在的物理内存范围中去。
 */
void __init add_active_range(unsigned int nid, unsigned long start_pfn,
                        unsigned long end_pfn)
{
    int i;

    mminit_dprintk(MMINIT_TRACE, "memory_register",
            "Entering add_active_range(%d, %#lx, %#lx) "
            "%d entries of %d used\n",
            nid, start_pfn, end_pfn,
            nr_nodemap_entries, MAX_ACTIVE_REGIONS);

    mminit_validate_memmodel_limits(&start_pfn, &end_pfn);  // 对start_pfn 和 end_pfn 做某些验证，我们不Care

    /* Merge with existing active regions if possible */
    for (i = 0; i < nr_nodemap_entries; i++) {
        if (early_node_map[i].nid != nid)
            continue;

        /* Skip if an existing region covers this new one */
        if (start_pfn >= early_node_map[i].start_pfn &&
                end_pfn <= early_node_map[i].end_pfn)
            return;

        /* Merge forward if suitable */
        if (start_pfn <= early_node_map[i].end_pfn &&
                end_pfn > early_node_map[i].end_pfn) {
            early_node_map[i].end_pfn = end_pfn;
            return;
        }

        /* Merge backward if suitable */
        if (start_pfn < early_node_map[i].start_pfn &&
                end_pfn >= early_node_map[i].start_pfn) {
            early_node_map[i].start_pfn = start_pfn;
            return;
        }
    }

    /* Check that early_node_map is large enough */ // earlay_node_map数组有上限
    if (i >= MAX_ACTIVE_REGIONS) {
        printk(KERN_CRIT "More than %d memory regions, truncating\n",
                            MAX_ACTIVE_REGIONS);
        return;
    }

    early_node_map[i].nid = nid;
    early_node_map[i].start_pfn = start_pfn;
    early_node_map[i].end_pfn = end_pfn;
    nr_nodemap_entries = i + 1;
　　/*
　　　　early_node_map[0].nid = 0;
　　　　early_node_map[0].start_pfn = 0;
　　　　early_node_map[0].end_pfn = 57344;

　　　　early_node_map[1].nid = 0;
　　　　early_node_map[1].start_pfn = 196608;
　　　　early_node_map[1].end_pfn = 262144;

　　　　很好理解，不啰嗦了！！！

　　*/
}