printk打印调试信息都以距启动开始的时间开头

1. make ARCH=arm menuconfig

2. Kernel hacking  --->

            printk and dmesg options  --->

                            [*] Show timing information on printks

     选中，方括号内出现*，就会打印出以下字样

[  121.833082] ^A3F:drivers/dma/imx-sdma.c [sdma_run_channel0]: L:681           

[  121.839183] ^A3F:drivers/dma/imx-sdma.c [sdma_enable_channel]: L:670         

[  121.845468] ^A3F:drivers/dma/imx-sdma.c [sdma_load_context]: L:1041,ret:0, bd

[  121.845475] ^A3F:drivers/dma/imx-sdma.c [sdma_int_handler]: L:837            

[  121.860489] ^A3F:drivers/dma/imx-sdma.c [sdma_prep_memcpy]: L:1537