Read DMA4 status register....

I use DMA4 to read external device on NET Arm 50. And it is successful. But if I add a while-loop to check DMA4 status register (just to block until dma transaction is completed) after I set DMA4 Control register to start DMA transation, then the while-loop will never ends. I’ve tried the same source code on Net Arm 40, and it worked perfect. I really don’t know why. Does any body knows why? Thanks very much.