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When a client opens a PCM device whose substreams are all occupied
without the non-blocking flag, the open function would be blocked in
the kernel until the previous opened ones are closed. This would
cause deadlock if they try to hold the same lock. Most of the ALSA
PCM drivers on embedded systems are implemented with the ALSA SOC
framework. Each PCM device has only one substream. This problem would
happen frequently. To force pcm_open to open PCM devices with
non-blocking flag is beneficial to resolve this problem. It returns
the control to clients to try again later.
The reason why we don't call pcm_open with PCM_NONBLOCK is that the
PCM_NONBLOCK also affects the read and write behaviors.
I also add a test case to test whether the pcm_open would be blocked.
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