reserved-memory和memreserve 区别

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原文链接：http://www.voidcn.com/article/p-qxdokkft-bqd.html

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本文详细对比了Devicetree中reserved-memory和memreserve两种内存预留方式的区别，包括它们在dtc编译过程中的处理差异，内核中的处理流程不同，以及对操作系统内存可用性的影响。reserved-memory支持更多配置选项，如CMA分配，而memreserve的内存则不可再被系统使用。

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转载自： http://www.voidcn.com/article/p-qxdokkft-bqd.html

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 为shared-dma-pool；

（注：也可如下新加一个cma，专门给vpp用）

[dma-contiguous.c] +RESERVEDMEM_OF_DECLARE(cma1, "vpp-cma", rmem_cma_setup);

+       ion_for_vpp: ion {
+           compatible = "vpp-cma";
+           //compatible = "shared-dma-pool";
+           reg = <0x1 0x80000000 0x0 0x40000000>; // 1G
+           reusable;
+       };

heap_carveout@0 {
            compatible = "bitmain,cma_vpp";
-           memory-region = <&cma_reserved>;
+           memory-region = <&ion_for_vpp>;
        }；

）

2. 没有定义no-map属性

3. 定义了resuable属性