Unable to mount root fs
 

I get kernel panic from my freshly compiled linux kernel booting from USB key (like here Unable to mount root fs)

1. error : unable to mount root fs on unknown-block(0,0)
2. kernel version : 5.5-rc5
3. bootloader : none (EFI stub)
4. built-in kernel command line : root=PARTUUID=xx1-xx1-xx1-1xx
5. kernel supports root's filesystem : f2fs
6. kernel is moduleless

Solutions tried/tested

1. checked PARTUUID of the root device 3 times
2. kernel do load my sata ssd with proper PARTUUID
3. kernel doesn't mount other USB drive
4. BLK_DEV=y, BLK_DEV_SD=y, USB_STORAGE=y, ...
5. tried with UUID and /dev/sda2 same result

It seems to be a USB problem something wrong in kernel configuration ?

Presumably CONFIG_F2FS_FS=y.

On the kernel command line, add "ignore_loglevel" to print all log messages to the console from early in boot.

that was number 1. in solutions (1. kernel supports root's filesystem)
and rechecked by 4. in solutions (cause my SSD is also f2fs)

Ok it turns to be a slow USB subsystem.
rootwait added to the kernel command line fixed that

all the credit goes to NeddySeagoon from Gentoo Forum
link to the post Unable to mount root fs