Yes, if there are fewer tiers of hubs, the unused nibbles will be zero.
Correct, the padding is only added if frameLength isn’t a multiple of 4. This
should be considered the final version. The timestamps are in Mach Absolute
Time Units (https://developer.apple.com/library/content/qa/qa1398/_index.html
<https://developer.apple.com/library/content/qa/qa1398/_index.html>).
—scott
> On Dec 12, 2016, at 9:49 PM, Guy Harris <g...@alum.mit.edu> wrote:
>
> On Dec 12, 2016, at 6:11 PM, Scott Deandrea <sdeand...@apple.com> wrote:
>
>> The bus number is 0 based and the port numbers are 1 based.
>
> So if there are fewer than 6 ports in the route, the unused nibbles will be
> zero.
>
>> I decided to implement isochronous transfers today and changed the structure
>> slightly:
>> struct
>> {
>> // Control information
>> uint32_t frameHeaderLength; // 28
>>
>> // Frame information
>> uint32_t frameLength; // Amount of data sent/received this frame
>> uint32_t frameStatus; // IOReturn result of the I/O this frame
>> uint64_t frameNumber; // Frame number on which this was scheduled/executed
>> by the controller
>> uint64_t ioTimestamp; // Time in which the frame completed
>> } __attribute__((packed, aligned (sizeof(uint32_t))));
>>
>> Therefore, the isochronous format for a request with type
>> kAppleUSBHostPacketFilterRequestComplete is as follows:
>> Link Header
>> padding, if required to force 4-byte alignment
>> Isochronous Frame[0] Header (frameHeaderLength bytes in length)
>> Isochronous Frame[0] Data (frameLength bytes)
>> …
>> padding, if required to force 4-byte alignment
>> Isochronous Frame[linkHeader.ioFrameCount - 1] Header (aligned to 4 bytes,
>> frameHeaderLength bytes in length)
>> Isochronous Frame[linkHeader.ioFrameCount - 1] Data (frameLength bytes)
>
> So the headers and data are interleaved; the padding would presumably be
> needed only if frameLength isn't a multiple of 4.
>
> Is this the final version, or should the format of a payload of a packet for
> an isochronous endpoint be left unspecified for now until it's finalized?
>
> Also, what are the formats of the time stamps? What are the units, and is
> there a specified origin or are they just relative to an unspecified base
> time?
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