On Fri, 31 Jan 2014, vichy wrote:
> Hi all:
> I have some questions about bandwidth calculation
> 1. why tt time need to include one maxp bus time ?
> qh->tt_usecs = NS_TO_US (think_time +
> usb_calc_bus_time (urb->dev->speed,
> is_input, 0, max_packet (maxp)));
Because tt_usecs is the time required to send a maximum-size packet.
So of course you have to include the maxp bus time.
> 2. in tt_available, below is used to check whether tt time is bigger than 125us
> if (125 < usecs) {
> int ufs = (usecs / 125);
> int i;
> for (i = uframe; i < (uframe + ufs) && i < 8; i++)
> if (0 < tt_usecs[i]) {
> ehci_vdbg(ehci,
> "multi-uframe xfer can't fit "
> "in frame %d uframe %d\n",
> frame, i);
> return 0;
> }
> }
> is it possible tt time bigger than 1 uframe?
Yes. Any isochronous transfer that is longer than 188 bytes will
require more than one uframe.
> 3. below is the fomula to calculate bus time
> Full-speed (Input)
> Non-Isochronous Transfer (Handshake Included)
> = 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
> what is 9107 and 3.167 used for?
> (9102 is not equal to FS bit time * FS protocol overhead)
9107 is the overhead. It includes things like inter-packet delays, the
IN or OUT token packet, the ACK packet, and so on.
The 3.167 is the time required for the DATAx PID byte at the start of
the data packet and the CRC bytes at the end.
Alan Stern
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