Hi Folks,
I've got a few issues with a MIDI driver that I'm developing, and so
far, my research has turned up nothing. The MIDI driver is based on
the SampleUSBMIDI driver project, and although it is working, but it
has some strange behaviors that may be causing other problems with
the device.
The first issue we are having is in the USBDevice class during the
OpenAndConfigure() method. For some reason, the call to USBDeviceOpen
() always returns kIOReturnExclusiveAccess from the device. As such,
I've had to change this class to allow it to continue, as it normally
should return upon receiving anything other than kIOReturnSuccess.
Is there any particular reason that this call would return
kIOReturnExclusiveAccess, even when it is not being used by any other
application, and also after "fresh" reboots? Is it possible that
there could be an issue with our firmware or hardware which would be
causing this? The device also has a corresponding audio driver... is
there any chance that this could cause this error?
While the device runs normally (despite the above return errors), I
suspect that some of these "soft" errors could be causing more
serious problems for some of our users. Also, I believe that this
behavior is responsible for a related call to USBDeviceClose()
failing during the driver destructor with a code of
kIOReturnNotOpen. I think that this occurs due to SetConfiguration()
not being called in USBDevice::OpenAndConfigure(), since the error
code from USBDeviceOpen() is not IOReturnSuccess. Calling
SetConfiguration() with anything other than kIOReturnSuccess crashes
the driver, so I have left the error checking mechanisms in, and the
call is never getting executed. Is there something in the
SetConfiguration() method that "marks" the device as being open for
the system so it could be cleanly closed later on?
Finally, the last question I have is in regards to semaphores being
used in MIDI drivers. Presently, our driver has three mutexes that
are used to protect read/write overlaps. While I think that these
mutexes are correctly implemented and stable, I am still a bit uneasy
about their presence in the driver. Generally speaking, is it safe
to use semaphores in MIDI drivers? I have read that some hosts, such
as Pro Tools, have many problems dealing with plugins using
semaphores. Could a plugin accessing such a MIDI driver with
semaphores also cause such problems? Also, is there a preferred
threading library that we should be using? At present, we are just
using pthreads, but I've read that IOKit provides some of this
functionality itself.
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