Hi Alan:
2014-01-31 2:23 GMT+08:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> 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.
is it the requirement defined in spec?
if so, would you please tell me where I can find it?
>
>> 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.
in spec I only find the formula, but spec didn't tell where this 9107 come from?
take Full buck for example, which is non-iso, the protocol overhead is
only 13Bytes.
appreciate your help,
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