A mtd timeout issue on ppc

The size of initrd.boot is about 8.5M, then

dd if=initrd.boot of=/dev/mtd2, cause

  MTD do_write_buffer(): software timeout

  dd: /dev/mtd2: Input/output error

 

When I change the size of initrd.boot to about 2M, it's OK. 

But this 8.5M file can transit by U-boot.

Why ?? Is kernel issue or MTD utility issue ?

 

Thanks,

Johnson Cheng