Introduce the helper function amdtp_out_stream_running().  This makes
many checks in amdtp.c clearer and frees the device drivers from having
to track this with a separate variable.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

Ensure that misbehaving or aborted userspace programs do not
accidentally keep the lock.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

Implement the hwdep locking and notification mechanisms.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

When aborting a PCM stream, the xrun is signaled only if the stream is
running.  When disconnecting a PCM stream, calling snd_card_disconnect()
too early would change the stream into a non-running state and thus
prevent the xrun from being noticed by user space.

To prevent this, move the snd_card_disconnect() call after the xrun.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

Allow AMDTP output streams to use blocking mode.

Use it for DICE devices, because the old DICE-II chip will in some cases
not be able to lock to non-blocking streams (erratum E7).
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

The notification bits are not of general interest; log them only when
debugging.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

Instead of forcing a constant 44.1 kHz, read the current sample rate
from the device when opening the PCM device.

Actually changing the sample rate requires some separate controller
application.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

After a bus reset, do not stop the stream completely to avoid having to
reconfigure the device when restarting the stream.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

As a start point for further development, this is an incomplete driver
for DICE devices:
- only playback (so no clock source except the bus clock)
- only 44.1 kHz
- no MIDI
- recovery after bus reset is slow
- hwdep device is created, but not actually implemented

Contains compilation fixes by Stefan Richter.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>