No expected output in console window with printk

Question

Hello, SDK: ti-processor-sdk-linux-am335x-evm-05.03.00.07. I want to have more debug ouputs in the console window during the Linux kernel booting so that I can trace for example MMC driver loading process. Added some printk() into the kernel, for example printk("this function is called 
"). After successful compiling and loading the kernel unfortunately I can't see the exptected output message "this function is called" in the console window. I also tried echo 8 > /proc/sys/kernel/printk dmesg after login but the same results. Is there any configuration missing in .config file? I also see brcmf_dbg() in kernel files, how can I activate it? Thanks Xi

marisol · Accepted Answer

Hi Xi Fan, Please see my answers in bold Hope that helps! I want to have more debug ouputs in the console window during the Linux kernel booting so that I can trace for example MMC driver loading process. Added some printk() into the kernel, for example printk("this function is called 
"). After successful compiling and loading the kernel unfortunately I can't see the exptected output message "this function is called" in the console window. It is generally easier to use the "pr_" helper functions: https://elinux.org/Debugging_by_printing pr_err() uses KERN_ERR level should always print regardless of the printk log level. KERN_ERR is not a good practice for debug output, but it does eliminate the chance that the log level is filtering out your print statements. Try adding this: pr_err("DEBUG: start name_of_function()"); If you don't see that in dmesg, then it is likely that the function you put that in is never actually called. I also tried echo 8 > /proc/sys/kernel/printk dmesg after login but the same results. Is there any configuration missing in .config file? level 8 should be sufficient but it is easier if you just set the level in the .config before you compile your kernel. CONFIG_PRINTK_TIME=y CONFIG_CONSOLE_LOGLEVEL_DEFAUL T=7 CONFIG_CONSOLE_LOGLEVEL_QUIET= 7 CONFIG_MESSAGE_LOGLEVEL_DEFAUL T=7 You probably also want to make sure dynamic debugging is enabled: CONFIG_DYNAMIC_DEBUG=y Dynamic debugging is a neat feature where you can chose to see more debug information for a particular source code file: https://linux-kernel-labs.github.io/refs/heads/master/labs/kernel_modules.html?highlight=config_dynamic_debug#dynamic-debugging Sometimes you don't even need to add in your own debug print statements, you just need to enable the dynamic debug output that is already built-in to a driver or subsystem. I also see brcmf_dbg() in kernel files, how can I activate it? Are you trying to debug a Broadcom wifi driver? Taking a look at Elixir to search the kernel code: https://elixir.bootlin.com/linux/latest/ident/brcmf_dbg It looks like CONFIG_BRCM_TRACING will enable debug output: https://elixir.bootlin.com/linux/latest/source/drivers/net/wireless/broadcom/brcm80211/brcmfmac/debug.h#L63 Try setting that in your .config: https://cateee.net/lkddb/web-lkddb/BRCM_TRACING.html While it is technically to enable function tracing for the driver, it looks like it will cause brcmf_dbg() to be defined. Now the individual invocations of brcmf_dbg() will have a log level associated with them. It appears that level can be set as kernel module parameter: https://elixir.bootlin.com/linux/latest/source/drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.c#L42 This forum post has more information: https://forum.openwrt.org/t/getting-output-of-brcmf-dbg/31094/4 It indicates this can be set while the module is loaded via: echo 6 > /sys/module/brcmfmac/parameter s/debug Best Regards, Marisol