The frameNumber is a USB spec defined value that correlates with the
start-of-frame packets frame number as defined in section 8.4.3 Start-of-Frame
Packets.
The ioTimestamp is in the same Mach Absolute Time.
Yes, the way you have described the padding is correct.
—scott
> On Jan 5, 2017, at 8:08 PM, Guy Harris <g...@alum.mit.edu> wrote:
>
> On Dec 12, 2016, at 6:11 PM, Scott Deandrea <sdeand...@apple.com> wrote:
>
>> 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
>
> Is that a value specified by the USB standard or is it something specified by
> the Darwin USB stack?
>
>> uint64_t ioTimestamp; // Time in which the frame completed
>
> "In which" sounds like a relative time; is it a relative time or an absolute
> time?
>
>> Therefore, the isochronous format for a request with type
>> kAppleUSBHostPacketFilterRequestComplete is as follows:
>> Link Header
>> padding, if required to force 4-byte alignment
>
> Presumably that padding will not be required with the current link header
> format, as that header's length is a multiple of 4 bytes. (If a future
> version isn't a multiple of 4 bytes in length, it would be required.)
>
>> 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)
>>
>> The isochronous format for a request with type
>> kAppleUSBHostPacketFilterRequestSubmit is similar (no data following the
>> isochronous frame header):
>> Link Header
>> padding, if required to force 4-byte alignment
>> Isochronous Frame[0] Header (frameHeaderLength bytes in length)
>> …
>> padding, if required to force 4-byte alignment
>> Isochronous Frame[linkHeader.ioFrameCount - 1] Header (aligned to 4 bytes,
>> frameHeaderLength bytes in length)
>
> So no padding after the last frame, as it wouldn't be necessary (there's
> nothing to align on a 4-byte boundary).
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