Re: DLT_ request

[Scott Deandrea <sdeandrea () apple com>]

The deviceLocation is an Apple specific property that describes the bus topology to which the device is connected.  The 
most significant byte contains the bus number; a unique value given to a host controller and also used when generating 
the interface numbers.  The following nibbles correspond to the port number(s) through which the device is connected.  
For example:

FaceTime HD Camera (Built-in)@1a110000  <class IOUSBHostDevice, id 0x10000f263, registered, matched, active, busy 0 (9 
ms), retain 24>
The bus number is 0x1a and the camera is connected to port 1 of the hub that is connected to root port 1 on the host 
controller.

The ioFrameCount field, though not currently supported, is intended to be used for isochronous endpoints as their I/O 
model is slightly different in that you pass down a vector describing I/O for multiple USB frames.  On completion you 
would receive the header followed by and ioFrameCount array of isochronous header followed by the packet data.  The 
isochronous header is currently defined as follows:
struct
{
    // Control information
    uint32_t frameHeaderLength;
    
    // Frame information
    uint32_t frameLength;
    uint64_t frameNumber;
    uint64_t ioTimestamp;
} __attribute__((packed, aligned (sizeof(uint32_t))));

—scott


> On Dec 10, 2016, at 7:00 PM, Guy Harris <guy () alum mit edu> wrote:
>
> On Dec 9, 2016, at 1:37 PM, Scott Deandrea <sdeandrea () apple com> wrote:
>
> >   uint32_t deviceLocation;    // locationID of the device
>
> So is a locationID something defined by a USB specification, by Apple in a fashion specific to Darwin, or something 
> else?  What is the format of a locationID?
>
> >   uint32_t ioFrameCount;      // number of isoch frames following
>
> So would that value affect the interpretation of the payload and, if so, how?

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