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

[Sergey]

Fri 09 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.

>>> 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()?
>
> A bit better, but not really.  You can give a comment to each function
> to explain what they do.  Then you might find better names while
> explaining functionality to yourself.

pcm_poll_block() function blocks pcm polling if pcm is not ready to send
data (capture pcm) or receive data (playback pcm). And pcm_poll_unblock()
unblocks poll() if pcm is ready to send/receive data.
But block_poll_if_pcm_not_ready() looks too cumbersome.
Maybe pcm_poll_block_check() and pcm_poll_unblock_check()?

>> Playback pcm accepts writes since _prepare(), so poll() is unblocked there.
>
> Then do it only for the playback.  For capture, the poll shouldn't
> return at prepare state.

I thought that socket is created empty, so poll() is blocked by default.
But you're right, I forgot about XRUN recovery, when _prepare() can be
called with unblocked poll. It could be blocked by the next _poll_revents(),
but it's probably better to have it explicitly blocked in _prepare().
I modified _block function and added its call to _prepare().

>> 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.
>
> You can declare a function.

Done.

Patch updated.

---
diff --git a/jack/pcm_jack.c b/jack/pcm_jack.c
index 7a8b24d..36fbdc9 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 +51,41 @@ typedef struct {
 	jack_client_t *client;
 } snd_pcm_jack_t;
 
+static int snd_pcm_jack_stop(snd_pcm_ioplug_t *io);
+
+static int pcm_poll_block_check(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 ||
+	    (io->state == SND_PCM_STATE_PREPARED && io->stream == SND_PCM_STREAM_CAPTURE)) {
+		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_check(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 +117,10 @@ 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_check(io))
 		*revents |= (io->stream == SND_PCM_STREAM_PLAYBACK) ? POLLOUT : POLLIN;
 	return 0;
 }
@@ -105,7 +137,6 @@ 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++) {
@@ -145,7 +176,7 @@ 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_check(io); /* unblock socket for polling if needed */
 
 	return 0;
 }
@@ -154,9 +185,26 @@ 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);
+
+	if (io->stream == SND_PCM_STREAM_PLAYBACK)
+		pcm_poll_unblock_check(io); /* playback pcm initially accepts writes */
+	else
+		pcm_poll_block_check(io); /* block capture pcm if that's XRUN recovery */
+
 	if (jack->ports)
 		return 0;
 
--