On 2011.01.20 15:21, Alan Stern wrote:
> Although this duplicates the Windows API, it seems that a more general
> approach would be better. What good does it do to know the port number
> on the parent hub, if you don't also know the port number on the hub's
> parent?
That's what the parent_dev from the topology struct is for.
issue a libusb_get_device_topology(parent_dev, ...) and you get the port
number from the HUB's parent.
> I suggest passing an array of libusb_device_topology structures, where
> each member of the array contains:
>
> libusb_device *dev; /* Pointer to the device at this level */
> unsigned bus; /* Bus number */
> unsigned depth; /* Distance from the root hub */
> unsigned port; /* Port number for the upstream connection */
The thing that bothers me about this is that it probably returns more
information than a user needs (but I'd like to see how topology is
actually used to see if that assertion is true). I think the requirement
to have the whole branch topology up to the HCD is only part of what the
get_topology call will be used for, and as such, I think that users who
want to build a whole branch (or tree) topology should issue multiple
calls instead.
For now, the cautious approach would be to keep it simple, see how it's
used in real life, and then reassert. My guess is that, for most of our
users, the primary use they'll have for topology is read the port number
on the hub or just compare the topology values return from different
devices using the same VID:PID, and little else. I don't think obtaining
the whole branch/tree structure in one go would be the prime use. But
again, I'd like to find out if that's true. The fact that it's being
introduced in an experimental branch means we should be able to find out
what's needed, before introducing it into official.
How do you see the topology call being used in real life, outside of the
one use of displaying a diagram of the USB tree?
Regards,
/Pete
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