question about usb_submit_urb

Hi,
  I don't understand two points in usb_submit_urb(), can anybody
kindly explain it, thank you.

    338         switch (temp) {
    339         case PIPE_ISOCHRONOUS:
    340         case PIPE_INTERRUPT:
    341                 /* too small? */
    342                 if (urb->interval <= 0)
    343                         return -EINVAL;
    344                 /* too big? */
    345                 switch (dev->speed) {
    346                 case USB_SPEED_HIGH:    /* units are microframes */
    347                         // NOTE usb handles 2^15
    348                         if (urb->interval > (1024 * 8))
    349                                 urb->interval = 1024 * 8;
    350                         temp = 1024 * 8;
    351                         break;
    352                 case USB_SPEED_FULL:    /* units are frames/msec */
    353                 case USB_SPEED_LOW:
    354                         if (temp == PIPE_INTERRUPT) {
    355                                 if (urb->interval > 255)
    356                                         return -EINVAL;
    357                                 // NOTE ohci only handles up to 32
    358                                 temp = 128;
    359                         } else {
    360                                 if (urb->interval > 1024)
    361                                         urb->interval = 1024;
    362                                 // NOTE usb and ohci handle up to 2^15
    363                                 temp = 1024;
    364                         }
    365                         break;
    366                 default:
    367                         return -EINVAL;
    368                 }
    369                 /* power of two? */
    370                 while (temp > urb->interval)
    371                         temp >>= 1;
    372                 urb->interval = temp;
    373         }

1. line 348, line 349, for high speed device, if urb->interval is
greater than 1024*8, set it to 1024*8, why?(Consider usb handles 2^15,
but 1024*8 is only 2^13.)
2. line 360, line 361, for full/low speed device, if pipe is
isochronous, and if urb->interval is greater than 1024, set it to
1024, why?

Thanks
Jason

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