Re: [PATCH] alsa-plugins: fix polling and recovering in alsa-jack plugin

[Sergey <sergemp@mail.ru>]

At Mon 05 May 2014 Takashi Iwai wrote:

>> This patch fixes polling in alsa-to-jack plugin.
>> It makes poll()+_poll_revents() return correct values
>> whenever there's something to read/write.
>>
>> Additionally jack_deactivate() code makes it survive snd_pcm_recover().
>>
>> It works for the code example I posted before, aplay, arecord, mozilla, etc.
>>
>> Comments and suggestions are welcome.
>
> Thanks for the patch.  The basic idea looks good, but some suggestions
> below.

Thank you for your response and comments.

> You don't need inline in general.  Compilers should be smart enough
> nowadays.

Uninlined.

> Please follow the coding style in that file.  The brace is placed in
> the same line as if ().

Missed that, sorry. Fixed.

> So, it's intended to clear all pending writes, right?
> If so, do reads in loop.  Then you don't have to use such a big
> buffer.  The size of 32 or such should be enough.

Done.

> The function names are a bit too ambiguous and hard to understand what
> they are really doing only from the names.

How about pcm_poll_block() and pcm_poll_unblock()?

>> +    poll_unblocked(io); /* unblock socket for initial polling if needed */
>
> Is this really needed?

Playback pcm accepts writes since _prepare(), so poll() is unblocked there.
Without it playback pcm gets poll() blocked by default and a code like:
  _open();
  _set_params();
  while (still_playing) {
    if (poll() > 0 && _revents() == 0 && revents&POLLOUT)
      _write();
  }
never starts writing.

>> +
>> +    if (jack->activated) {
>> +            jack_deactivate(jack->client);
>> +            jack->activated = 0;
>> +    }
>
> This is same as just calling snd_pcm_jack_stop().

Right. To put snd_pcm_jack_stop() call in snd_pcm_jack_prepare()
I have to move snd_pcm_jack_prepare() below snd_pcm_jack_stop(),
that makes patch larger.

Patch updated and ready for more comments.

---
diff --git a/jack/pcm_jack.c b/jack/pcm_jack.c
index 7a8b24d..837e15c 100644
--- a/jack/pcm_jack.c
+++ b/jack/pcm_jack.c
@@ -42,6 +42,7 @@ typedef struct {
 	unsigned int num_ports;
 	unsigned int hw_ptr;
 	unsigned int sample_bits;
+	snd_pcm_uframes_t min_avail;
 
 	unsigned int channels;
 	snd_pcm_channel_area_t *areas;
@@ -50,6 +51,38 @@ typedef struct {
 	jack_client_t *client;
 } snd_pcm_jack_t;
 
+static int pcm_poll_block(snd_pcm_ioplug_t *io)
+{
+	static char buf[32];
+	snd_pcm_sframes_t avail;
+	snd_pcm_jack_t *jack = io->private_data;
+
+	if (io->state == SND_PCM_STATE_RUNNING) {
+		avail = snd_pcm_avail_update(io->pcm);
+		if (avail >= 0 && avail < jack->min_avail) {
+			while (read(io->poll_fd, &buf, sizeof(buf)) == sizeof(buf));
+			return 1;
+		}
+	}
+
+	return 0;
+}
+
+static int pcm_poll_unblock(snd_pcm_ioplug_t *io)
+{
+	static char buf[1];
+	snd_pcm_sframes_t avail;
+	snd_pcm_jack_t *jack = io->private_data;
+
+	avail = snd_pcm_avail_update(io->pcm);
+	if (avail < 0 || avail >= jack->min_avail) {
+		write(jack->fd, &buf, 1);
+		return 1;
+	}
+
+	return 0;
+}
+
 static void snd_pcm_jack_free(snd_pcm_jack_t *jack)
 {
 	if (jack) {
@@ -81,14 +114,10 @@ static int snd_pcm_jack_poll_revents(snd_pcm_ioplug_t *io,
 				     struct pollfd *pfds, unsigned int nfds,
 				     unsigned short *revents)
 {
-	static char buf[1];
-	
 	assert(pfds && nfds == 1 && revents);
 
-	read(pfds[0].fd, buf, 1);
-
 	*revents = pfds[0].revents & ~(POLLIN | POLLOUT);
-	if (pfds[0].revents & POLLIN)
+	if (pfds[0].revents & POLLIN && !pcm_poll_block(io))
 		*revents |= (io->stream == SND_PCM_STREAM_PLAYBACK) ? POLLOUT : POLLIN;
 	return 0;
 }
@@ -105,7 +134,6 @@ snd_pcm_jack_process_cb(jack_nframes_t nframes, snd_pcm_ioplug_t *io)
 	snd_pcm_jack_t *jack = io->private_data;
 	const snd_pcm_channel_area_t *areas;
 	snd_pcm_uframes_t xfer = 0;
-	static char buf[1];
 	unsigned int channel;
 	
 	for (channel = 0; channel < io->channels; channel++) {
@@ -145,44 +173,11 @@ snd_pcm_jack_process_cb(jack_nframes_t nframes, snd_pcm_ioplug_t *io)
 		xfer += frames;
 	}
 
-	write(jack->fd, buf, 1); /* for polling */
+	pcm_poll_unblock(io); /* unblock socket for polling if needed */
 
 	return 0;
 }
 
-static int snd_pcm_jack_prepare(snd_pcm_ioplug_t *io)
-{
-	snd_pcm_jack_t *jack = io->private_data;
-	unsigned int i;
-
-	jack->hw_ptr = 0;
-
-	if (jack->ports)
-		return 0;
-
-	jack->ports = calloc(io->channels, sizeof(jack_port_t*));
-
-	for (i = 0; i < io->channels; i++) {
-		char port_name[32];
-		if (io->stream == SND_PCM_STREAM_PLAYBACK) {
-
-			sprintf(port_name, "out_%03d", i);
-			jack->ports[i] = jack_port_register(jack->client, port_name,
-							    JACK_DEFAULT_AUDIO_TYPE,
-							    JackPortIsOutput, 0);
-		} else {
-			sprintf(port_name, "in_%03d", i);
-			jack->ports[i] = jack_port_register(jack->client, port_name,
-							    JACK_DEFAULT_AUDIO_TYPE,
-							    JackPortIsInput, 0);
-		}
-	}
-
-	jack_set_process_callback(jack->client,
-				  (JackProcessCallback)snd_pcm_jack_process_cb, io);
-	return 0;
-}
-
 static int snd_pcm_jack_start(snd_pcm_ioplug_t *io)
 {
 	snd_pcm_jack_t *jack = io->private_data;
@@ -233,6 +228,54 @@ static int snd_pcm_jack_stop(snd_pcm_ioplug_t *io)
 	return 0;
 }
 
+static int snd_pcm_jack_prepare(snd_pcm_ioplug_t *io)
+{
+	snd_pcm_jack_t *jack = io->private_data;
+	unsigned int i;
+	snd_pcm_sw_params_t *swparams;
+	int err;
+
+	jack->hw_ptr = 0;
+
+	jack->min_avail = io->period_size;
+	snd_pcm_sw_params_alloca(&swparams);
+	err = snd_pcm_sw_params_current(io->pcm, swparams);
+	if (err == 0) {
+		snd_pcm_sw_params_get_avail_min(swparams, &jack->min_avail);
+	}
+
+	/* deactivate jack connections if this is XRUN recovery */
+	snd_pcm_jack_stop(io);
+
+	/* playback pcm initially accepts writes, poll must be unblocked */
+	pcm_poll_unblock(io);
+
+	if (jack->ports)
+		return 0;
+
+	jack->ports = calloc(io->channels, sizeof(jack_port_t*));
+
+	for (i = 0; i < io->channels; i++) {
+		char port_name[32];
+		if (io->stream == SND_PCM_STREAM_PLAYBACK) {
+
+			sprintf(port_name, "out_%03d", i);
+			jack->ports[i] = jack_port_register(jack->client, port_name,
+							    JACK_DEFAULT_AUDIO_TYPE,
+							    JackPortIsOutput, 0);
+		} else {
+			sprintf(port_name, "in_%03d", i);
+			jack->ports[i] = jack_port_register(jack->client, port_name,
+							    JACK_DEFAULT_AUDIO_TYPE,
+							    JackPortIsInput, 0);
+		}
+	}
+
+	jack_set_process_callback(jack->client,
+				  (JackProcessCallback)snd_pcm_jack_process_cb, io);
+	return 0;
+}
+
 static snd_pcm_ioplug_callback_t jack_pcm_callback = {
 	.close = snd_pcm_jack_close,
 	.start = snd_pcm_jack_start,
--