Linux设备树（2）--memreserve和reserved-memory的区别

转载自https://blog.csdn.net/kickxxx/article/details/54631535
memreserve和reserved-memory的区别

Devicetree 提供了两种方式预留内存: reserved-memory和memreserve

memreserve示例

/memreserve/ 0x40000000 0x01000000

reserved-memory示例

    reserved-memory {
        #address-cells = <1>;
        #size-cells = <1>;
        ranges;

        ipu_cma@90000000 {
            compatible = "shared-dma-pool";
            reg = <0x90000000 0x4000000>;
            reusable;
            status = "okay";
        };

区别1

二者在dtc编译时中处理的方法不同, reserved-memory做为device tree node解析到device-tree structure中; memreserve最终会加到dtb文件的memory reserve map，如下图所示

区别2

二者在内核中的处理方式不同

1. memreserve处理流程

start_kernel                                             - init/main.c

    ->setup_arch                                         - arch/arm/kernel/setup.c

        ->arm_memblock_init                              - arch/arm/kernel/setup.c

            ->arm_dt_memblock_reserve                    - arch/arm/kernel/devtree.c

arm_dt_memblock_reserve实现如下

    /* Reserve the dtb region */
    memblock_reserve(virt_to_phys(initial_boot_params),
             be32_to_cpu(initial_boot_params->totalsize));

    /*
     * Process the reserve map.  This will probably overlap the initrd
     * and dtb locations which are already reserved, but overlaping
     * doesn't hurt anything
     */
    reserve_map = ((void*)initial_boot_params) +
            be32_to_cpu(initial_boot_params->off_mem_rsvmap);
    while (1) {
        base = be64_to_cpup(reserve_map++);
        size = be64_to_cpup(reserve_map++);
        if (!size)
            break;
        memblock_reserve(base, size);
    }

initial_boot_params实际指向dtb文件在内存中的位置, 该地址还可以指向其他类型的启动参数
initial_boot_params头中的off_mem_rsvmap指向一系列的reserve memory(地址, 尺寸)空间, 对于dtb来说, 就是memory reserve map.

2. reserved-memory处理流程

start_kernel                                                        - init/main.c

    ->setup_arch                                                    - arch/arm/kernel/setup.c

        ->arm_memblock_init                                         - arch/arm/kernel/setup.c
            ->early_init_fdt_scan_reserved_mem                      - arch/arm/mm/init.c

                ->__fdt_scan_reserved_mem,                          - drivers/of/fdt.c

                    ->__reserved_mem_reserve_reg                    - drivers/of/fdt.c

                        ->early_init_dt_reserve_memory_arch         - drivers/of/fdt.c

                            ->memblock_remove                       - mm/memblock.c

                            ->memblock_reserve                      - mm/memblock.c

            ->fdt_init_reserved_mem

reserved-memory有一些可选参数, 比如no-map, 如果使用了no-map, 那么这段区域执行memblock_remove, 反之执行memblock_reserve.

在调用完memblock_reserve后,还会执行fdt_init_reserved_mem

void __init fdt_init_reserved_mem(void)
    {
             ...
            if (rmem->size == 0)
                err = __reserved_mem_alloc_size(node, rmem->name,
                             &rmem->base, &rmem->size);
            if (err == 0)
                __reserved_mem_init_node(rmem);
            ...
    }


    /**
     * res_mem_init_node() - call region specific reserved memory init code
     */
    static int __init __reserved_mem_init_node(struct reserved_mem *rmem)
    {
        ...

        for (i = __reservedmem_of_table; i < &__rmem_of_table_sentinel; i++) {
            reservedmem_of_init_fn initfn = i->data;
            const char *compat = i->compatible;

            if (!of_flat_dt_is_compatible(rmem->fdt_node, compat))
                continue;
            if (initfn(rmem) == 0) {
                pr_info("Reserved memory: initialized node %s, compatible id %s\n",
                    rmem->name, compat);
                return 0;
            }
        }
        return -ENOENT;
    }

如果reserved-memory下节点的compatible=, 则这块内存会被用来进行Contiguous Memory Allocator for dma

initfn对应drivers/base/dma-contiguous.c下的rmem_cma_setup以及drivers/base/dma-coherent.c中的rmem_dma_setup, 由于二者的compatible相同,所以前者优先.

rmem_cma_setup会对这块内存做初始化, 把这块区域加到cma_areas[cma_area_count]中，cma_areas保存着所有的CMA区域, 稍后core_init_reserved_areas会对这个数组进行处理

static int __init cma_init_reserved_areas(void)
    {
        int i;

        for (i = 0; i < cma_area_count; i++) {
            int ret = cma_activate_area(&cma_areas[i]);
            if (ret)
                return ret;
        }

        return 0;
    }
    core_initcall(cma_init_reserved_areas);

cma_activate_area把该cma area中的所有pages都改为MIGRATE_CMA, 并加到MIGRATE_CMA的free_list上.

区别3

由于二者的处理流程不同, 导致memreserve分配的内存, 无法再被操作系统使用; 而reserved-memory内存有可能进入系统CMA, 是否做为CMA, 依赖以下几个条件:

1. compatible 必须为shared-dma-pool
2. 没有定义no-map属性
3. 定义了resuable属性