本文共 5471 字，大约阅读时间需要 18 分钟。

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,

见下图

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=<shared-dma-pool>, 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