need help

hello,
    I am writing the firmware for an ARM-based development board
(CX82100 from Conexant System Inc.), and it has a usb slave port.
I can't tell which chip it uses since lacking of info, and I only
know it uses the chip from Sand Microelectronic Inc., which seems
disappered. My host system is linux 2.4.9, with builtin usb support.
    My problem is, after initing the USB device controller on the
board, I can receive the linux's request, get-device-descriptor, which
is the first time request, and is 8 bytes length, i.e. Linux wants
to know my device's max packet size for the control endpoint, and
then, as the response, I send the device info(8 bytes also) to linux
(by filling the dma buffer), but the device always tells me it's
pending on sending the packet, at the same time, at the linux side,
printing debug info "usb_control/bulk_msg: timeout" for five times,
and then "USB device not responding, giving up (error=-110)". Linux
will retry the above process again, and gives up finally. The fact
is my device can receive all the ten(5 * 2) 8-byte-length requests
without any problem, but the transporting function suspended after
the first response. As the board data sheet says, it seems the device
is waiting for host's ACK. But linux seems doesn't recieve the response
from the device, and won't send ACK but timeout again and again.
What's the problem?
    Since my device only hands on the limited setup requests(e.g.
GetDescriptor, GetConfiguration, SetConfiguration)to me, and the
others are dealed with by the hardware such as SetAddress. I can't tell
which step I was on, but I check the linux usb source(usb.c),
and I notice the function usb_new_device(), seems the SetAddress has
completed properly, but then failed at

2151: err = usb_get_descriptor(dev, USB_DT_DEVICE, 0, &dev->descriptor, 8);

    Any kind of help or advice is welcome, thanks!