See include/linux/compiler_attributes.h:
/*
* Add the pseudo keyword 'fallthrough' so case statement blocks
* must end with any of these keywords:
* break;
* fallthrough;
* goto <label>;
* return [expression];
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
*/
-if __has_attribute(__fallthrough__)
- define fallthrough __attribute__((__fallthrough__))
-else
- define fallthrough do {} while (0) /* fallthrough */
-endif
The whole warning is quite long but looks like this:
CC [M] seeed-voicecard/ac108.o
In file included from ./include/linux/printk.h:331,
from ./include/linux/kernel.h:15,
from ./include/linux/list.h:9,
from ./include/linux/module.h:12,
from seeed-voicecard/ac108.c:15:
seeed-voicecard/ac108.c: In function ‘ac108_set_fmt’:
./include/linux/dynamic_debug.h:122:52: warning: this statement may fall through [-Wimplicit-fallthrough=]
122 | #define __dynamic_func_call(id, fmt, func, ...) do { \
| ^
./include/linux/dynamic_debug.h:143:2: note: in expansion of macro ‘__dynamic_func_call’
143 | __dynamic_func_call(__UNIQUE_ID(ddebug), fmt, func, ##__VA_ARGS__)
| ^~~~~~~~~~~~~~~~~~~
./include/linux/dynamic_debug.h:157:2: note: in expansion of macro ‘_dynamic_func_call’
157 | _dynamic_func_call(fmt,__dynamic_dev_dbg, \
| ^~~~~~~~~~~~~~~~~~
./include/linux/device.h:1795:2: note: in expansion of macro ‘dynamic_dev_dbg’
1795 | dynamic_dev_dbg(dev, dev_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~~~~~~~~
seeed-voicecard/ac108.c:866:4: note: in expansion of macro ‘dev_dbg’
866 | dev_dbg(dai->dev, "used as slave when AC101 is master\n");
| ^~~~~~~
seeed-voicecard/ac108.c:868:2: note: here
868 | case SND_SOC_DAIFMT_CBS_CFS: /*AC108 Slave*/
| ^~~~
