ALSA: firewire-lib: taskletize the snd_pcm_period_elapsed() call

The following patch might introduce this call chain:
  PCM .pointer callback
  + fw_iso_context_flush_completions
    + packet callback
      + snd_pcm_period_elapsed
        + PCM .pointer callback
Recursive calls to the pointer callback are not possible due to the PCM
group locking, so avoid this by moving the period notification into
a separate tasklet.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/amdtp.c

@@ -31,6 +31,8 @@
 #define INTERRUPT_INTERVAL	16
 #define QUEUE_LENGTH		48
 
+static void pcm_period_tasklet(unsigned long data);
+
 /**
  * amdtp_out_stream_init - initialize an AMDTP output stream structure
  * @s: the AMDTP output stream to initialize
@@ -47,6 +49,7 @@ int amdtp_out_stream_init(struct amdtp_out_stream *s, struct fw_unit *unit,
 	s->flags = flags;
 	s->context = ERR_PTR(-1);
 	mutex_init(&s->mutex);
+	tasklet_init(&s->period_tasklet, pcm_period_tasklet, (unsigned long)s);
 	s->packet_index = 0;
 
 	return 0;
@@ -164,6 +167,20 @@ void amdtp_out_stream_set_pcm_format(struct amdtp_out_stream *s,
 }
 EXPORT_SYMBOL(amdtp_out_stream_set_pcm_format);
 
+/**
+ * amdtp_out_stream_pcm_prepare - prepare PCM device for running
+ * @s: the AMDTP output stream
+ *
+ * This function should be called from the PCM device's .prepare callback.
+ */
+void amdtp_out_stream_pcm_prepare(struct amdtp_out_stream *s)
+{
+	tasklet_kill(&s->period_tasklet);
+	s->pcm_buffer_pointer = 0;
+	s->pcm_period_pointer = 0;
+}
+EXPORT_SYMBOL(amdtp_out_stream_pcm_prepare);
+
 static unsigned int calculate_data_blocks(struct amdtp_out_stream *s)
 {
 	unsigned int phase, data_blocks;
@@ -376,9 +393,18 @@ static void queue_out_packet(struct amdtp_out_stream *s, unsigned int cycle)
 		s->pcm_period_pointer += data_blocks;
 		if (s->pcm_period_pointer >= pcm->runtime->period_size) {
 			s->pcm_period_pointer -= pcm->runtime->period_size;
+			tasklet_hi_schedule(&s->period_tasklet);
+		}
+	}
+}
+
+static void pcm_period_tasklet(unsigned long data)
+{
+	struct amdtp_out_stream *s = (void *)data;
+	struct snd_pcm_substream *pcm = ACCESS_ONCE(s->pcm);
+
+	if (pcm)
 		snd_pcm_period_elapsed(pcm);
-		}
-	}
 }
 
 static void out_packet_callback(struct fw_iso_context *context, u32 cycle,
@@ -532,6 +558,7 @@ void amdtp_out_stream_stop(struct amdtp_out_stream *s)
 		return;
 	}
 
+	tasklet_kill(&s->period_tasklet);
 	fw_iso_context_stop(s->context);
 	fw_iso_context_destroy(s->context);
 	s->context = ERR_PTR(-1);

sound/firewire/amdtp.h

@@ -1,6 +1,7 @@
 #ifndef SOUND_FIREWIRE_AMDTP_H_INCLUDED
 #define SOUND_FIREWIRE_AMDTP_H_INCLUDED
 
+#include <linux/interrupt.h>
 #include <linux/mutex.h>
 #include <linux/spinlock.h>
 #include "packets-buffer.h"
@@ -55,6 +56,7 @@ struct amdtp_out_stream {
 	struct iso_packets_buffer buffer;
 
 	struct snd_pcm_substream *pcm;
+	struct tasklet_struct period_tasklet;
 
 	int packet_index;
 	unsigned int data_block_counter;
@@ -81,6 +83,7 @@ void amdtp_out_stream_stop(struct amdtp_out_stream *s);
 
 void amdtp_out_stream_set_pcm_format(struct amdtp_out_stream *s,
 				     snd_pcm_format_t format);
+void amdtp_out_stream_pcm_prepare(struct amdtp_out_stream *s);
 void amdtp_out_stream_pcm_abort(struct amdtp_out_stream *s);
 
 /**
@@ -122,18 +125,6 @@ static inline bool amdtp_out_streaming_error(struct amdtp_out_stream *s)
 	return s->packet_index < 0;
 }
 
-/**
- * amdtp_out_stream_pcm_prepare - prepare PCM device for running
- * @s: the AMDTP output stream
- *
- * This function should be called from the PCM device's .prepare callback.
- */
-static inline void amdtp_out_stream_pcm_prepare(struct amdtp_out_stream *s)
-{
-	s->pcm_buffer_pointer = 0;
-	s->pcm_period_pointer = 0;
-}
-
 /**
  * amdtp_out_stream_pcm_trigger - start/stop playback from a PCM device
  * @s: the AMDTP output stream