Re: question about usb_submit_urb

On Wed, 14 Nov 2007, jidong xiao wrote:

> 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?

The host controller drivers don't support intervals longer than 1 
second.  So anything longer than that gets decreased.

Alan Stern


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