這幾天一直在除錯atmel at91sam9x25的串列埠，用著用著總會導致Oops，Oops內容如下：

[code language=”c”]
[ 1023.510000] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[ 1023.520000] pgd = c0004000
[ 1023.520000] [00000000] *pgd=00000000
[ 1023.520000] Internal error: Oops: 17 [#1]
[ 1023.520000] last sysfs file: /sys/devices/virtual/misc/at91flash/dev
[ 1023.520000] Modules linked in: at91flash at91gpio at91mc323 ds18b20 at91adc
[ 1023.520000] CPU: 0 Tainted: G W (2.6.39 #35)
[ 1023.520000] PC is at atmel_tasklet_func+0x104/0x690
[ 1023.520000] LR is at atmel_tasklet_func+0x10/0x690
[ 1023.520000] pc : [<c01a33ac>] lr : [<c01a32b8>] psr: 20000013
[ 1023.520000] sp : c7825f58 ip : 60000093 fp : 00000000
[ 1023.520000] r10: 00000006 r9 : 00000000 r8 : 0000000a
[ 1023.520000] r7 : 00000000 r6 : c7824000 r5 : c78a2484 r4 : c03c0cb8
[ 1023.520000] r3 : 0000004c r2 : 0000004c r1 : 60000013 r0 : 00000001
[ 1023.520000] Flags: nzCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment kernel
[ 1023.520000] Control: 0005317f Table: 27b40000 DAC: 00000017
[ 1023.520000] Process ksoftirqd/0 (pid: 3, stack limit = 0xc7824270)
[ 1023.520000] Stack: (0xc7825f58 to 0xc7826000)
[ 1023.520000] 5f40: 00000001 c7824000
[ 1023.520000] 5f60: 00000100 0000000a 00000000 00000006 c7825f8c 00000000 00000001 c7824000
[ 1023.520000] 5f80: 00000100 0000000a 00000006 c0045cf8 c03b995c c00461d8 c7aa6ae0 00000000
[ 1023.520000] 5fa0: 60000093 00000000 c7824000 c0046274 00000013 00000000 00000000 c00462e0
[ 1023.520000] 5fc0: 00000000 c7819f70 00000000 c00570e0 00000000 00000000 00000000 00000000
[ 1023.520000] 5fe0: c7825fe0 c7825fe0 c7819f70 c0057060 c0030b14 c0030b14 ffffffff ffffffff
[ 1023.520000] [<c01a33ac>] (atmel_tasklet_func+0x104/0x690) from [<c0045cf8>] (tasklet_action+0x84/0xe8)
[ 1023.520000] [<c0045cf8>] (tasklet_action+0x84/0xe8) from [<c00461d8>] (__do_softirq+0x88/0x124)
[ 1023.520000] [<c00461d8>] (__do_softirq+0x88/0x124) from [<c00462e0>] (run_ksoftirqd+0x6c/0x128)
[ 1023.520000] [<c00462e0>] (run_ksoftirqd+0x6c/0x128) from [<c00570e0>] (kthread+0x80/0x88)
[ 1023.520000] [<c00570e0>] (kthread+0x80/0x88) from [<c0030b14>] (kernel_thread_exit+0x0/0x8)
[ 1023.520000] Code: 1a000002 e59f057c e59f157c ebfa3d49 (e5973000)
[ 1023.710000] —[ end trace 786b41cd25d3b661 ]—
[ 1023.710000] Kernel panic – not syncing: Fatal exception in interrupt
[ 1023.720000] [<c0034b10>] (unwind_backtrace+0x0/0xe0) from [<c02a8af8>] (panic+0x50/0x170)
[ 1023.720000] [<c02a8af8>] (panic+0x50/0x170) from [<c0032e00>] (die+0x184/0x1c4)
[ 1023.730000] [<c0032e00>] (die+0x184/0x1c4) from [<c0035aa8>] (__do_kernel_fault+0x64/0x84)
[ 1023.740000] [<c0035aa8>] (__do_kernel_fault+0x64/0x84) from [<c0035c7c>] (do_page_fault+0x1b4/0x1c8)
[ 1023.750000] [<c0035c7c>] (do_page_fault+0x1b4/0x1c8) from [<c002a240>] (do_DataAbort+0x30/0x98)
[ 1023.760000] [<c002a240>] (do_DataAbort+0x30/0x98) from [<c002f86c>] (__dabt_svc+0x4c/0x60)
[ 1023.770000] Exception stack(0xc7825f10 to 0xc7825f58)
[ 1023.770000] 5f00: 00000001 60000013 0000004c 0000004c
[ 1023.780000] 5f20: c03c0cb8 c78a2484 c7824000 00000000 0000000a 00000000 00000006 00000000
[ 1023.790000] 5f40: 60000093 c7825f58 c01a32b8 c01a33ac 20000013 ffffffff
[ 1023.790000] [<c002f86c>] (__dabt_svc+0x4c/0x60) from [<c01a33ac>] (atmel_tasklet_func+0x104/0x690)
[ 1023.800000] [<c01a33ac>] (atmel_tasklet_func+0x104/0x690) from [<c0045cf8>] (tasklet_action+0x84/0xe8)
[ 1023.810000] [<c0045cf8>] (tasklet_action+0x84/0xe8) from [<c00461d8>] (__do_softirq+0x88/0x124)
[ 1023.820000] [<c00461d8>] (__do_softirq+0x88/0x124) from [<c00462e0>] (run_ksoftirqd+0x6c/0x128)
[ 1023.830000] [<c00462e0>] (run_ksoftirqd+0x6c/0x128) from [<c00570e0>] (kthread+0x80/0x88)
[ 1023.840000] [<c00570e0>] (kthread+0x80/0x88) from [<c0030b14>] (kernel_thread_exit+0x0/0x8)
[/code]

注意上述第 8 和第 9 行。

下面就來顯示如何定位出出錯程式碼行:

1. 首先，編譯時開啟complie with debug info選項，步驟如下

make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- menuconfig

進入 Kernel hacking

選擇 Compile the kernel with debug info
然後，儲存，退出。

接著 make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi-
編譯， 等編譯完成。

2. 利用arm-none-linux-gnueabi-gdb 除錯，如下：
arm-none-linux-gnueabi-gdb vmlinux

對應著Oops 訊息裡面的這一行

[code language=”c”]
[ 1023.520000] LR is at atmel_tasklet_func+0x10/0x690
[/code]

在gdb下鍵入命令 ： list *atmel_tasklet_func+0x10

這樣就找到了出錯的程式碼行。在這裡鄙視一下atmel提供的核心，竟然還有bug！

從這裡可以看出是由於串列埠的dma導致Oops的，於是我去掉了串列埠的dma傳輸。方法如下：

去掉之後還沒有發現上述的Oops出現。

本文轉自[http://blog.chinaunix.net/uid-23046336-id-3220727.html]