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在聊這個話題之前，我們先回憶一下單片機系統中是如何分配內存的？如果沒有bootloader，那硬件環境起來之后就直接進入主程序運行，如果有引導程序bootloader，那就需要設置好跳轉地址，否則設置不對，系統就無法啟動了。

那么對于linux系統來說，何嘗不是這樣呢？因為linux系統包含的文件比較多，啟動也分幾個階段，每個階段也都是通過跳轉執行的。如何管理好內存是系統穩定運行的前提。

原生的uboot只能管理自己所在的內存塊，無法管理整塊空間。于是rk引入了bidram和sysmem，用于管理uboot之外的地址空間，RK平臺就把系統所有內存通過sysmem + bidram + malloc管理起來了，防止出現內存沖突等問題。

bidram：管理u-boot、kernel階段不可用、需要剔除的內存塊，例如：ATF、OP-TEE 占用的空間；sysmem：管理kernel 可見、可用的內存塊。例如：fdt、ramdisk、kernel、fastboot 占用的空間。

相關代碼：

./lib/sysmem.c./lib/bidram.c./include/memblk.h./arch/arm/mach-rockchip/memblk.c

先來大概預覽一下源文件吧：

intsysmem_init(void){  struct sysmem *sysmem = &plat_sysmem;  phys_addr_t mem_start;  phys_size_t mem_size;  int ret;
  lmb_init(&sysmem->lmb);  INIT_LIST_HEAD(&sysmem->allocated_head);  INIT_LIST_HEAD(&sysmem->kmem_resv_head);  sysmem->allocated_cnt = 0;  sysmem->kmem_resv_cnt = 0;
  if (gd->flags & GD_FLG_RELOC) {    sysmem->has_initr = true;  } else {    SYSMEM_I("initn");    sysmem->has_initf = true;  }
  /* Add all available system memory */#ifdef CONFIG_NR_DRAM_BANKS  int i;
  for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {    if (!gd->bd->bi_dram[i].size)      continue;          ret = sysmem_add(gd->bd->bi_dram[i].start,         gd->bd->bi_dram[i].size);    if (ret) {      SYSMEM_E("Failed to add sysmem from bi_dram[%d]n", i);      goto fail;    }  }#else  mem_start = env_get_bootm_low();  mem_size = env_get_bootm_size();  ret = sysmem_add(mem_start, mem_size);  if (ret) {    SYSMEM_E("Failed to add sysmem from bootm_low/sizen");    goto fail;  }#endif  /* Reserved for board */  ret = board_sysmem_reserve(sysmem);  if (ret) {    SYSMEM_E("Failed to reserve sysmem for boardn");    goto fail;  }
  /* Reserved for U-boot framework: 'reserve_xxx()' */  mem_start = gd->start_addr_sp;  mem_size = gd->ram_top - mem_start;  if (!sysmem_alloc_base(MEM_UBOOT, mem_start, mem_size)) {    SYSMEM_E("Failed to reserve sysmem for U-Boot frameworkn");    ret = -ENOMEM;    goto fail;  }    /* Reserved for U-Boot stack */  mem_start = gd->start_addr_sp - CONFIG_SYS_STACK_SIZE;  mem_size = CONFIG_SYS_STACK_SIZE;  if (!sysmem_alloc_base(MEM_STACK, mem_start, mem_size)) {    SYSMEM_E("Failed to reserve sysmem for stackn");    ret = -ENOMEM;    goto fail;}return0;fail:  if (ret && !(gd->flags & GD_FLG_RELOC)) {    sysmem_dump();    SYSMEM_W("Maybe malloc size %d MiB is too large?nn",       SIZE_MB(CONFIG_SYS_MALLOC_LEN));}  return ret;}

sysmem 內存信息表：

sysmem_dump_all:--------------------------------------------------------------------// <1> 這里是sysmem可管理的總內存容量，即bidram<3>之外的可用ddr容量，對kernel可見。memory.rgn[0].addr = 0x00200000 - 0x08400000 (size: 0x08200000)memory.rgn[1].addr = 0x0a200000 - 0x80000000 (size: 0x75e00000)memory.total = 0x7e000000 (2016 MiB. 0 KiB)--------------------------------------------------------------------// <2> 這里顯示了各個固件alloc走的內存塊信息allocated.rgn[0].name = "U-Boot".addr = 0x71dd6140 - 0x80000000 (size: 0x0e229ec0)allocated.rgn[1].name = "STACK"  // 表明棧溢出.addr = 0x71bd6140 - 0x71dd6140 (size: 0x00200000)allocated.rgn[2].name = "FDT".addr = 0x08300000 - 0x08316204 (size: 0x00016204)allocated.rgn[3].name = "KERNEL"  // 表明內存塊溢出.addr = 0x00280000 - 0x014ce204 (size: 0x0124e204)allocated.rgn[4].name = "RAMDISK".addr = 0x0a200000 - 0x0a3e6804 (size: 0x001e6804)// <3> malloc_r/f的大小malloc_r: 192 MiB, malloc_f: 16 KiBallocated.total = 0x0f874acc (248 MiB. 466 KiB)--------------------------------------------------------------------// <4> 這里是核心算法對上述<2>進行的信息整理，顯示被占用走的內存塊信息LMB.reserved[0].addr = 0x00280000 - 0x014ce204 (size: 0x0124e204)LMB.reserved[1].addr = 0x08300000 - 0x08316204 (size: 0x00016204)LMB.reserved[2].addr = 0x0a200000 - 0x0a3e6804 (size: 0x001e6804)LMB.reserved[3].addr = 0x71bd6140 - 0x80000000 (size: 0x0e429ec0)reserved.core.total = 0x0f874acc (248 MiB. 466 KiB)--------------------------------------------------------------------

bidram 內存信息表：

bidram_dump_all:--------------------------------------------------------------------// <1> 這里顯示了U-Boot從前級loader獲取的ddr的總容量信息，一共有2GBmemory.rgn[0].addr = 0x00000000 - 0x80000000 (size: 0x80000000)memory.total = 0x80000000 (2048 MiB. 0 KiB)--------------------------------------------------------------------// <2> 這里顯示了被預留起來的各固件內存信息，這些空間對kernel不可見reserved.rgn[0].name = "ATF".addr = 0x00000000 - 0x00100000 (size: 0x00100000)reserved.rgn[1].name = "SHM".addr = 0x00100000 - 0x00200000 (size: 0x00100000)reserved.rgn[2].name = "OP-TEE".addr = 0x08400000 - 0x0a200000 (size: 0x01e00000)reserved.total = 0x02000000 (32 MiB. 0 KiB)--------------------------------------------------------------------// <3> 這里是核心算法對上述<2>進行的預留信息整理，例如：會對相鄰塊進行合并LMB.reserved[0].addr = 0x00000000 - 0x00200000 (size: 0x00200000)LMB.reserved[1].addr = 0x08400000 - 0x0a200000 (size: 0x01e00000)reserved.core.total = 0x02000000 (32 MiB. 0 KiB)--------------------------------------------------------------------