[Buildroot] read-only NFS devtmpfs mount fails

[Massimo Callegari]

Hello,
I'm running buildroot 2012.05 on a MIPSel device, custom kernel 3.3.0. I'm testing over NFS, building a .TAR target.
When devices are created statically and fs is mounted in RW, all is OK.
When I try to use devtmpfs (with or without udev) and read-only filesystem over NFS, I am not able to obtain a dynamic /dev creation.
I thought the kernel was supposed to do the mounting automagically, but since I'm mounting a NFS tree my init script is busybox, so I added this instruction at the beginning of /etc/inittab:

::sysinit:/bin/mount -t devtmpfs devtmpfs /dev


Although I get this error:
mount: mounting devtmpfs on /dev failed: No such device


I've read a lot of forums and docs and checked everything I could. 
On my kernel folder: "grep TMPFS .config" says:
CONFIG_DEVTMPFS=y
CONFIG_DEVTMPFS_MOUNT=y
CONFIG_TMPFS=y

I also had to create some devices manually to avoid several errors, especially for /dev/null (isn't buildroot supposed to create a minimum set of devices to avoid this ?) so I created these:

crw-rw-rw-  1 root root  5,  1 2012-10-01 17:14 console
crw-rw-rw-  1 root root  1,  3 2012-10-01 17:14 null

crw-rw-rw-  1 root root  4,  0 2012-10-01 17:14 tty0

crw-rw-rw-  1 root root  4, 64 2012-10-01 17:16 ttyS0
crw-rw-rw-  1 root root  1,  5 2012-10-01 17:38 zero

If I try the mount command manually I get the same error. If I strace it seems the C mount instruction itself fails:

stat64(0x7fb8beb9, 0x7fb8b658)          = -1 ENOENT (No such file or directory)
mount("devtmpfs", "/dev", "devtmpfs", 0x8000, 0) = -1 ENODEV (No such device)
brk(0x4bd000)                           = 0x4bd000
brk(0x4be000)                           = 0x4be000
write(2, "mount: mounting devtmpfs on /dev"..., 56mount: mounting devtmpfs on /dev failed: No such device
) = 56
SYS_4246( <unfinished ... exit status 255>


So I'm wondering if the mount command of busybox (I'm using version 1.20.1) is missing the devtmpfs support for some reason.

Is there anything else I can do to understand this issue ?

Thanks in advance,
Massimo
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