[Yaffs] [YAFFS2 problem] About using NAND emulator

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I can't mount yaffs2ram file system on linux-2.6.12-mm2?
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I make device like below.?
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mknod /dev/mtd0 c 90 0?
mknod /dev/mtd1 c 90 2?
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but whent I try to mount a yaffs2ram file system. I get an error:?
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[root@localhost 2.6.12]# mount -v -t yaffs2ram /dev/mtd0 /mnt/nand?
mount: /dev/mtd0 is not a block device?
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So I make mtd0 to block device. like this:?
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[root@localhost 2.6.12]# mknod /dev/mtd0 b 90 0?
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and I try to mount again. But I get an error, too:?
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[root@localhost 2.6.12]# mount -v -t yaffs2ram /dev/mtd0 /mnt/nand?
mount: /dev/mtd0 is not a valid block device?
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How can I mount yaffs2ram file system????
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I'm off to debug this, but any pointers as to what to look for would be greatly appreciated.?
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Tae-hoon Lee?