debugfs usage - SD card

1. To check SD speed mode by debugfs. It needs to enable the "Debug Fiflesystem" from menuconfig and then re-build kernel.

$ make linux-menuconfig

Kernel hacking  --->
  [*] Debug Filesystem

2. If the SPI mode is correct, the messages will be shown like below "5 (sd uhs DDR50)".

$ mount -t debugfs debug /mnt

$ cat /mnt/mmc0/ios
clock:          50000000 Hz
vdd:            15 (2.7 ~ 2.8 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      2 (4 bits)
timing spec:    5 (sd uhs DDR50)