[Yaffs] problem mounting yaffs2 : No such device or address

Hi !
I'm using the latest yaffs2 tree (from today 2007.03.20) on an atmel
at91sam9261-ek board.
my kernel is 2.6.20 with the at91 patch
here is what I'm getting :
mount -t yaffs2 /dev/mtdblock0 /mnt/mtd0/
mount: Mounting /dev/mtdblock0 on /mnt/mtd0/ failed: No such device or address

I tried with jffs2 and all is ok on that device.
I tried also with a dummy block device :
mount -t yaffs2 /dev/loop0 /mnt/mtd0/
yaffs: dev is 7340032 name is "loop0"
yaffs_read_super: Using yaffs2
yaffs_read_super: block size 4096
yaffs: Attempting MTD mount on 7.0, "loop0"
mount: Mounting /dev/loop0 on /mnt/mtd0/ failed: Invalid argument
it shows that yaffs is in the kernel...

i don't know if this could be a clue, but in /proc/filesystems, I have :
# cat /proc/filesystems
nodev   sysfs
nodev   rootfs
nodev   bdev
nodev   proc
nodev   sockfs
nodev   usbfs
nodev   pipefs
nodev   futexfs
nodev   tmpfs
nodev   eventpollfs
nodev   devpts
nodev   ramfs
nodev   nfs
nodev   jffs2
        yaffs
        yaffs2
nodev   rpc_pipefs

there's no "nodev" before yaffs*...
does it tell something to anyone ?
regards.
Richard Genoud