Three transition killers on drivers that require an explicit drop
(snd-rpi-hifiberry/PCM5122; the desktop's USB DAC tolerated all three,
which is why only the Pi died):
1. stop() called prepare() without drop(): prepare from
RUNNING/DRAINING returns EBUSY there — every stop failed and left
the pcm in limbo. Now drop+prepare (drop EBADFD tolerated).
2. A write racing a still-draining pcm (natural end + late gapless
append) gets EBADFD, and snd_pcm_recover(77) is unsupported on this
stack, so the write died and the writer thread exited. New
recover_write_error(): normal recover first, then drop+prepare for
EBADFD (11 write sites via one helper).
3. snd_pcm_drain can block forever when the device stops clocking —
the writer wedged and no 'engine empty' ever fired. drain() is now
bounded: poll pcm.state() with a 10s deadline; the tail finishing
shows as XRUN (drop+prepare), and a stuck device is cancelled.
km
vicrodh
9fad1dc123