GDB调试linux内核出现问题

刚开始学习使用GDB调试工具，按照书上的方法调试内核，在U-Boot上设置命令行参数、启动内核。

u-boot> set bootargs noinitrd root=/dev/mtdblock 2 console=kgdb kgdbwait
u-boot> tftp 0x30000000 uImage.img      (下载完毕)
u-boot> bootm 0x30000000

启动信息如下：
Starting Kernel ...

Uncompressing
Linux....................................................................
..................................done,booting the kernel.


然后在主机上进入内核目录，启动arm-linux-gdb,

[root@friendlyarm linux-2.6.22.6]# sudo arm-linux-gdb ./vmlinux
GNU gdb 5.2.1
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "--host=i686-pc-linux-gnu --target=arm-linux"...
(gdb) set remotebaud 115200
(gdb) target remote /dev/ttyS0
Remote debugging using /dev/ttyS0
Ignoring packet error, continuing...
Ignoring packet error, continuing...
Ignoring packet error, continuing...
Couldn't establish connection to remote target
Malformed response to off