Re: DLT_ request

[Scott Deandrea <sdeandrea () apple com>]

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 <guy () alum mit edu> wrote:
>
> On Dec 12, 2016, at 6:11 PM, Scott Deandrea <sdeandrea () 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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