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问题现象：

[    2.120000] VFS: Cannot open root device "mtdblock5" or unknown-block(0,0): error -6
[    2.136000] Please append a correct "root=" boot option; here are the available partitions:
[    2.152000] 1f00            4096 mtdblock0  (driver?)
[    2.164000] 1f01             192 mtdblock1  (driver?)
[    2.172000] 1f02              64 mtdblock2  (driver?)
[    2.184000] 1f03              64 mtdblock3  (driver?)
[    2.192000] 1f04            3776 mtdblock4  (driver?)
[    2.204000] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

 

导致该问题的原因：flash分区不对

 

第一种情况是根本没有做分区，没有出现下面的启动信息

[    0.460000] Creating 4 MTD partitions on "spi32766.0":
[    0.470000] 0x000000000000-0x000000030000 : "u-boot"
[    0.480000] 0x000000030000-0x000000040000 : "u-boot-env"
[    0.490000] 0x000000040000-0x000000050000 : "factory"
[    0.500000] 0x000000050000-0x000000400000 : "firmware"

像MTK的就需要在dts文件中添加flash分区

target/linux/ramips/dts/MT7621.dts

       palmbus@1E000000 {
                spi@b00 {
                        status = "okay";

                        m25p80@0 {
                                #address-cells = <1>;
                                #size-cells = <1>;
                                compatible = "w25q128";
                                reg = <0 0>;
                                linux,modalias = "m25p80";
                                spi-max-frequency = <10000000>;

                                partition@0 {
                                        label = "u-boot";
                                        reg = <0x0 0x30000>;
                                        read-only;
                                };

                                partition@30000 {
                                        label = "u-boot-env";
                                        reg = <0x30000 0x10000>;
                                        read-only;
                                };

                                factory: partition@40000 {
                                        label = "factory";
                                        reg = <0x40000 0x10000>;
                                        read-only;
                                };

                                partition@50000 {
                                        label = "firmware";
                                        reg = <0x50000 0xfb0000>;
                                };

                        };
                };

第二种情况是dts文件中添加了flash分区，flash大小没配置正常

                                        label = "firmware";
                                       reg = <0x50000 0xfb0000>;   // 16M = 0x50000 + 0xfb0000

　　要是板子是4M的，4M = 0x50000 + 0x3b0000，需要调整为reg = <0x50000 0x3b0000>;

 

第三种情况是分区没有分完整，rootfs分区没出来

[    0.460000] Creating 4 MTD partitions on "spi32766.0":
[    0.470000] 0x000000000000-0x000000030000 : "u-boot"
[    0.480000] 0x000000030000-0x000000040000 : "u-boot-env"
[    0.490000] 0x000000040000-0x000000050000 : "factory"
[    0.500000] 0x000000050000-0x000000400000 : "firmware"

在uboot启动阶段有以下打印

## Booting image at bc030000 ...

此种情况下"firmware"的分区起始地址要放在0x000000030000，即 0x000000030000-0x000000400000 : "firmware"

或者

## Booting image at bc050000 ...

此种情况下"firmware"的分区起始地址要放在0x000000050000，即 0x000000050000-0x000000400000 : "firmware"

 

配置正确后可出现如下分区信息：

[    1.608000] 2 uimage-fw partitions found on MTD device firmware
[    1.620000] 0x000000030000-0x000000165801 : "kernel"
[    1.632000] mtd: partition "kernel" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
[    1.656000] 0x000000165801-0x000000400000 : "rootfs"
[    1.668000] mtd: partition "rootfs" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
[    1.696000] 1 squashfs-split partitions found on MTD device rootfs
[    1.708000] 0x000000330000-0x000000400000 : "rootfs_data"

最后，要出现以上分割firmware分区的效果，需要配置kernel

执行make kernel_menuconfig

 

MTK openwrt 3.10.14的SDK，有的flash分区不在dts文件中,需要在源码中调整，譬如以下

vi build_dir/target-mipsel_24kec+dsp_uClibc-0.9.33.2/linux-ramips_mt7628/linux-3.10.14-p112871/drivers/mtd/ralink/ralink_bbu_spi.c