Where does dev_dbg writes log to? - linux-device-driver

Question

In a device driver source in the Linux tree, I saw dev_dbg(...) and dev_err(...), where do I find the logged message?
One reference suggest to add #define DEBUG . The other reference involves dynamic debug and debugfs, and I got lost.

Answer 1

dev_dbg() expands to dynamic_dev_dbg(), dev_printk(), or no-op depending on the compilation flags.
#if defined(CONFIG_DYNAMIC_DEBUG)
#define dev_dbg(dev, format, ...) \
do { \
dynamic_dev_dbg(dev, format, ##__VA_ARGS__); \
} while (0)
#elif defined(DEBUG)
#define dev_dbg(dev, format, arg...) \
dev_printk(KERN_DEBUG, dev, format, ##arg)
#else
#define dev_dbg(dev, format, arg...) \
({ \
if (0) \
dev_printk(KERN_DEBUG, dev, format, ##arg); \
})
#endif
dynamic_dev_dbg() and dev_printk() call dev_printk_emit() which calls vprintk_emit().
This very same function is called in a normal mode when you just do a printk(). Just note here, that the rest functions like dev_err() will end up in the same function.
Thus, obviously, the buffer is all the same, i.e. kernel intrenal buffer.
The logged message at the end is printed to
Current console if kernel loglevel value (can be changed via kernel command line or via procfs) is high enough for certain message, here KERN_DEBUG.
Internal buffer which can be read by running dmesg command.
Note, data in 2 is kept as long as there still room in the buffer. Since it's limited and circular, newer data preempts old one.
Additional information how to enable Dynamic Debug.
First of all, be sure you have CONFIG_DYNAMIC_DEBUG=y in the kernel configuration.
Assume we would like to enable all debug prints in the built-in module with name 8250. To achieve that we simple add to the kernel command line the following 8250.dyndbg=+p.
If the same driver is compiled as loadable module we may either add options 8250 dyndbg to the modprobe configuration or to the shell command line when do it manually, like modprobe 8250 dyndbg.
More details are described in the Dynamic Debug documentation.
The "How certain debug prints are automatically enabled in linux kernel?" raises the question why some debug prints are automatically enabled and how DEBUG affects that when CONFIG_DYNAMIC_DEBUG=y. The answer is lying in the dynamic_debug.h and since it's used during compilation the _DPRINTK_FLAGS_DEFAULT defines the certain message appearence.
#if defined DEBUG
#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
#else
#define _DPRINTK_FLAGS_DEFAULT 0
#endif

Answer 2

you can find dev_err(...) in kernel messages. As the name implies, dev_err(...) messages are error messages, so they will definitely be printed if the execution comes to that point. dev_dbg(...) are debug messages which are more generously used in the kernel driver code and they are not printed by default. So everything you have read about dynamic_debugging comes into play with dev_dbg(...).
There are several pre-conditions to have dynamic debugging working, below 1. and 2. are general preconditions for dynamic debugging. 3. and later are for your particular driver/module/subsystem and can be .
Dynamic debugging support has to be in your kernel config CONFIG_DYNAMIC_DEBUG=y. You may check if it is the case zgrep DYNAMIC_DEBUG /proc/config.gz
debugfs has to be mounted. You can check with sudo mount | grep debugfs and if not existing, you can mount with sudo mount -t debugfs /sys/kernel/debug
refer to dynamic_debugging and enable the particular file/function/line you are interested