usb可以识别，但是/dev下没有sda

使用3531a提供的软件包，编译内核和文件系统后
插上usb2.0的U盘，系统应该识别了，但是/dev下没有sda
~ # usb 1-1: new high-speed USB device number 4 using hiusb-ehci
usb-storage 1-1:1.0: USB Mass Storage device detected
scsi6 : usb-storage 1-1:1.0
scsi 6:0:0:0: Direct-Access     Kingston DataTraveler 2.0 PMAP PQ: 0 ANSI: 6
sd 6:0:0:0: [sda] 15249408 512-byte logical blocks: (7.80 GB/7.27 GiB)
sd 6:0:0:0: [sda] Write Protect is off
sd 6:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
sda: sda1
sd 6:0:0:0: [sda] Attached SCSI removable disk

fdisk什么也没有
~ # fdisk -l
~ #
~ #

/proc/scsi # cd usb-storage/
/proc/scsi/usb-storage # ls
6
/proc/scsi/usb-storage # cat 6
   Host scsi6: usb-storage
       Vendor: Kingston
      Product: DataTraveler 2.0
Serial Number: C860008863F7B0903A0A5D62
     Protocol: Transparent SCSI
    Transport: Bulk
       Quirks: SANE_SENSE

请问可能是什么原因造成的？
按照haisi提供的文档，
USB_OHCI_HCD_PLATFORM、USB_EHCI_HCD_PLATFORM和USB_XHCI_HISILICON都编译进内核了。