Man page 9 : usb_free_coherent

usb_free_coherent - free memory allocated with usb_alloc_coherent

void usb_free_coherent(struct usb_device *  dev , size_t  size , void *  addr , dma_addr_t  dma );

dev
    device the buffer was used with

size
    requested buffer size

addr
    CPU address of buffer

dma
    DMA address of buffer

This reclaims an I/O buffer, letting it be reused. The memory must have been allocated using usb_alloc_coherent, and the parameters must match those provided in that allocation request.