Re: some questions about ehci period scheduling

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hi Alan:

2013/9/7 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> On Sat, 7 Sep 2013, vichy wrote:
>
>> hi all:
>> when I trace linux ehci driver source code.
>> I have some questions
>> 1. in itd_slot_ok, why we have to
>>      uframe %= period;
>>       and use this uframe to calculate the bandwidth?
>>
>> suppose start passed to itd_slot_ok is 537, period, usb->interval, is 8.
>> that mean we want to know whether bandwidth of microframe 537 is enough.
>> Not want to know whether 537%8 =1
>> the bandwidth of microframe 1 is enough.
>
> In fact you need to know both.  If an isochronous endpoint has interval
> 8 and it uses microframe 537, then it's also going to use microframes
> 545, 553, 561, ..., 8177, 8185, 1, 9, ..., 529.
So at first time stream is scheduled, we have to make sure all
possible microframe bandwidth is enough, right?


>
>> 2. Below is part of message excerpted from usb mon log:
>> db1e2000 81051026 S Zi:1:002:6 -115:8:5327 1 -18:0:132 132 <
>> db1e2000 81058043 C Zi:1:002:6 0:8:5391:0 1 0:0:4 132 = 2efd82fd
>> 0cfeffff 84feb3ff 98fec4fb 08ffdaf8 060059fb 8b01d2ff db02de01
>>
>> why these 2 ep->start_frame are different, 5327 in "-115:8:5327" and
>> 5391 in "0:8:5391:0"?
>>
>> The log is captured with the same urb, for submitting and completing.
>> That mean  ep->start_frame=urb->start_frame in submit and complete
>> should be the same, right?
>
> No.  The value in urb->start_frame during submission is meaningless.
> Usually it is left over from the last time urb was used.  After the URB
> is submitted, the host controller driver stores the correct value in
> urb->start_frame, and usbmon shows the correct value during completion.
I got it.

BTW, I have another question about iso_stream_schedule.
When if (likely (!list_empty (&stream->td_list)))  happen,
why don't we just take last scheduled microframe,  stream->next_uframe
as start point directly?

appreciate your kind help,
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