[RFC][PATCH 2/3] ALSA: firewire-lib: queue event of period elapse in process context

[Takashi Sakamoto]

All of drivers in ALSA firewire stack processes two chances to process
isochronous packets in any isochronous context; in software IRQ context
for 1394 OHCI, and in process context of ALSA PCM application.

In the process context, callbacks of .pointer and .ack are utilized. The
callbacks are done by ALSA PCM core under acquiring lock of PCM substream,

In design of ALSA PCM core, call of snd_pcm_period_elapsed() is used for
drivers to awaken user processes from waiting for available frames. The
function voluntarily acquires lock of PCM substream, therefore it is not
called in the process context since it causes dead lock.

As a workaround to avoid the dead lock, all of drivers in ALSA firewire
stack uses workqueue to delegate the call. A variant of
snd_pcm_period_elapsed() without lock acquisition can obsolete the
workqueue.

An extra care is needed for the callback of .pointer since it's called
from snd_pcm_period_elapsed(). The isochronous context in Linux FireWire
subsystem is safe mostly for nested call except in software IRQ context.
---
 sound/firewire/amdtp-stream.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index 150ee0b9e707..b39328eab67e 100644
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -613,8 +613,16 @@ static void update_pcm_pointers(struct amdtp_stream *s,
 		// The program in user process should periodically check the status of intermediate
 		// buffer associated to PCM substream to process PCM frames in the buffer, instead
 		// of receiving notification of period elapsed by poll wait.
-		if (!pcm->runtime->no_period_wakeup)
-			queue_work(system_highpri_wq, &s->period_work);
+		if (!pcm->runtime->no_period_wakeup) {
+			if (in_interrupt()) {
+				// In software IRQ context for 1394 OHCI.
+				snd_pcm_period_elapsed(pcm);
+			} else {
+				// In process context of ALSA PCM application under acquiring lock
+				// of PCM substream.
+				snd_pcm_period_elapsed_without_lock(pcm);
+			}
+		}
 	}
 }
 
@@ -1740,22 +1748,11 @@ unsigned long amdtp_domain_stream_pcm_pointer(struct amdtp_domain *d,
 {
 	struct amdtp_stream *irq_target = d->irq_target;
 
+	// Process isochronous packets queued till recent isochronous cycle to handle PCM frames.
 	if (irq_target && amdtp_stream_running(irq_target)) {
-		// This function is called in software IRQ context of
-		// period_work or process context.
-		//
-		// When the software IRQ context was scheduled by software IRQ
-		// context of IT contexts, queued packets were already handled.
-		// Therefore, no need to flush the queue in buffer furthermore.
-		//
-		// When the process context reach here, some packets will be
-		// already queued in the buffer. These packets should be handled
-		// immediately to keep better granularity of PCM pointer.
-		//
-		// Later, the process context will sometimes schedules software
-		// IRQ context of the period_work. Then, no need to flush the
-		// queue by the same reason as described in the above
-		if (current_work() != &s->period_work)
+		// In software IRQ context, the call causes dead-lock to disable the tasklet
+		// synchronously.
+		if (!in_interrupt())
 			fw_iso_context_flush_completions(irq_target->context);
 	}
 
-- 
2.27.0