x-loader 以及 如何向nand中写入x-loader
x-loader is the primary boot loader. It is loaded by ROM boot loader into the internal RAM. x-loader is responsiblefor initializing the external memory and loading the u-boot from the selected boot device. x-loader supports boot from NAND, MMC/SD and OneNAND.
The generated x-loader.bin needs to be signed before it can be used by the ROM bootloader.
x-loader是一个初级的bootloader，它被ROM boot loader加载到内部的RAM，x-loader用来初始化外部memory，以及从用户选择的启动项中加载u-boot，x-loader支持从nand、mmc/sd和三星的OneNand启动。
编译产生的x-loader.bin需要被转换成x-loader.bin.ift，这样才可以作用ROM bootloader。
这个文件和x-loader.bin的区别在于增加了一个4个字节的header。header的内容是4个字节的x-loader的长度和x-loader被load到什么地方的地址(4个字节)。由内部的ROM boot loader决定。
x-loader是在OMAP3的内部RAM中运行的（OMAP3530内部有64K SRAM、112K ROM），并没有被拷贝到外部的ddr中运行，当x-loader初始化完memory controler后，加载正式的bootloader： u-boot 来进一步初始化外围硬件，并且启动linux kernel。
启动顺序：bootrom:ROM boot loader(内部ram) -> x-loader(内部ram) -> uboot(外部ddr) ->kernel(外部ddr)
问题：怎样将x-loaer写入nandflash？
X-loader is the first stage boot loader and is primarily responsible for initializing the DRAM controller and a few other essential hardware blocks. Because x-loader is loaded by the OMAP ROM boot loader, it has some special requirements in how it is written.
The OMAP ROM boot loader expects to find x-loader in the first block of NAND. If the first block is detective, the OMAP ROM boot loader will skip it and attempt to read x-loader from the second block. If that block is defective, the third block will be read, and if that block is defective, the fourth block. If all four are defective, a very rare occurance, the boot wil fail.
Another OMAP ROM boot loader requirement is the method of computing ECC. Hardware ECC is required for these first 4 blocks.
These unique requirements can currently only be met by using u-boot to write x-loader to NAND.
The following instructions assume that you are using a bootable SD card prepared from the GNOME or console images available on this site.
Power up or reset your OMAP board and break into u-boot by pressing a key on your serial console when you see the message:
Hit any key to stop autoboot:Next have u-boot scan to find your bootable SD card and load the x-load binary from the FAT partition of the bootable SD card. The x-loader binary is called MLO.
# mmc rescan 0
# fatload mmc 0 ${loadaddr} MLO
reading MLO
22540 bytes read
At this point the x-loader binary has been copied into RAM. We now specify the use of hwecc and erase the first 4 blocks of NAND:
# nandecc hw HW ECC selected # nand erase 0 80000 NAND erase: device 0 offset 0x0, size 0x80000 Erasing at 0x60000 -- 100% complete. OKAnd as the final step we write the x-loader binary to each of the first 4 blocks in NAND:
# nand write ${loadaddr} 0 ${filesize}
NAND write: device 0 offset 0x0, size size 0x580c
22540 bytes written: OK
# nand write ${loadaddr} 20000 ${filesize}
NAND write: device 0 offset 0x20000, size size 0x580c
22540 bytes written: OK
# nand write ${loadaddr} 40000 ${filesize}
NAND write: device 0 offset 0x40000, size size 0x580c
22540 bytes written: OK
# nand write ${loadaddr} 60000 ${filesize}
NAND write: device 0 offset 0x60000, size size 0x580c
22540 bytes written: OK
因为x-loader是被ROM boot loader 加载的，所以flash它需要一些特别的条件，TI的OMAP ROM boot loader 期待在nandflash的第一个block中发现x-loader（x-loader很小，一般十几k），如果第一个block中未发现，则继续在第二个block中找，一直到第四个block，如果仍为发现，则标记启动失败，上面的code部分展示了如何向nandflash中写入x-loader，这里的前提是，板子可以从sd启动。
假如nandfalsh中的数据坏掉了，并且无法从sd卡启动，咋办？