pi@raspberrypi ~ $ sudo mount -t debugfs none /sys/kernel/debug/
pi@raspberrypi ~ $ mount
/dev/root on / type ext4 (rw,noatime,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=61972k,nr_inodes=15493,mode=755)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=12408k,mode=755)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /run/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=24800k)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620)
/dev/mmcblk0p1 on /boot type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=cp437,iocharset=ascii,shortname=mixed,errors=remount-ro)
none on /sys/kernel/debug type debugfs (rw,relatime)
pi@raspberrypi ~ $ ls -l /sys/kernel/debug/
total 0
drwxr-xr-x 28 root root 0 Jan  1  1970 bdi
-r--r--r--  1 root root 0 Jan  1  1970 gpio
drwxr-xr-x  2 root root 0 Jan  1  1970 hid
drwxr-xr-x  2 root root 0 Jan  1  1970 kprobes
drwxr-xr-x  2 root root 0 Jan  1  1970 memblock
drwxr-xr-x  3 root root 0 Jan  1  1970 mmc0
-rw-r--r--  1 root root 0 Jan  1  1970 sched_features
-r--r--r--  1 root root 0 Jan  1  1970 suspend_stats
drwxr-xr-x  2 root root 0 Jan  1  1970 usb
-r--r--r--  1 root root 0 Jan  1  1970 wakeup_sources