Josh, you're correct in that this is a long standing issue. Although I would
not call it architectural, it is simply an API issue (the user should be able
to specify the altsetting he wants the max-packet-size for).
As for this blocking this patch, no the patch can go in as is, but I was hoping
you could help to do a new proper API for this.
As for you only fixing things you need for your project. If you're doing
something with isoc endpoints and not using multiple alt-settings you're doing
it wrong. A device is not allowed to have active isoc endpoints in altsetting
0, iow in altsetting 0 it must either advertise maxpacketsize 0, or simply not
advertise the isoc endpoints at all.
The reason for this is that otherwise the OS must reserve the isoc bandwidth
even if the device is not active. IOW devices start in altsetting 0, without
reserved bandwidth, and once the OS/driver wants to use them it changes their
altsetting, thus reserving the bandwidth (note in some OS-es / with some host
drivers the bandwidth is not reserved until the isoc stream is actually
started).
So you should have multiple altsettings in your device, and thus you should
need a new API to ensure you're getting the maxpacketsize for the right
altsetting.
While talking about this, note that many isoc dusing evices have multiple
altsettings, since they don't always use the same bandwidth. This is seen often
with webcams where the used altsetting (== reserved bandwidth) depends on the
resolution and framerate the user wants the webcam to stream in.
---
Reply to this email directly or view it on GitHub:
https://github.com/libusbx/libusbx/pull/163#issuecomment-30300154
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