Indeed, I may have mixed up terminology. Sorry about that. What I am
doing (or trying to do) is very simple:
There is a single physical USB device. I have a single device node
representing it. This device can be opened for reading, concurrently,
any number of times. Everyone who open()s the device can read() it at
their own pace. I implemented this by maintaining an individual queue of
incoming data for each open() call. This queue resides in cdevpriv.
So open() instantiates a queue and adds it to the driver's global list
of queues. Whenever a packet arrives from the device, it is placed in
all the queues (I have a linked list of all queues for that purpose).
When the open() is eventually followed by a close(), the cdevpriv
destructor removes the queue from the global list and frees its memory.
In addition to this, I need to start the USB transfer when the first
open() occurs and stop it again when the last close() occurs. I am doing
this by checking the length of the global list. When the list is
zero-length on open(), I start the transfer. When the list i zero-length
in the cdevpriv destructor, I stop the transfer.
I cannot see how else to achieve this behavior (other than device
cloning which I was using before but which is more complicated and
probably more error-prone). If I am doing something wrong and there is a
more correct way to do it, I would love to hear about it.
- Bartosz
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