[ALSA] usb-audio: don't use empty packets at start of playback

Modules: USB generic driver

Some widespread USB interface chips with adaptive iso endpoints hang
after receiving a series of empty packets when they expect data (ALSA
bug#1585).  To avoid this, we have to send packets containing silence
(zero samples) instead.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

sound/usb/usbaudio.c

@@ -478,22 +478,38 @@ static int retire_playback_sync_urb_hs(struct snd_usb_substream *subs,
 /*
  * Prepare urb for streaming before playback starts.
  *
- * We don't care about (or have) any data, so we just send a transfer delimiter.
+ * We don't yet have data, so we send a frame of silence.
  */
 static int prepare_startup_playback_urb(struct snd_usb_substream *subs,
 					struct snd_pcm_runtime *runtime,
 					struct urb *urb)
 {
-	unsigned int i;
+	unsigned int i, offs, counts;
 	struct snd_urb_ctx *ctx = urb->context;
+	int stride = runtime->frame_bits >> 3;
 
+	offs = 0;
 	urb->dev = ctx->subs->dev;
 	urb->number_of_packets = subs->packs_per_ms;
 	for (i = 0; i < subs->packs_per_ms; ++i) {
-		urb->iso_frame_desc[i].offset = 0;
-		urb->iso_frame_desc[i].length = 0;
+		/* calculate the size of a packet */
+		if (subs->fill_max)
+			counts = subs->maxframesize; /* fixed */
+		else {
+			subs->phase = (subs->phase & 0xffff)
+				+ (subs->freqm << subs->datainterval);
+			counts = subs->phase >> 16;
+			if (counts > subs->maxframesize)
+				counts = subs->maxframesize;
+		}
+		urb->iso_frame_desc[i].offset = offs * stride;
+		urb->iso_frame_desc[i].length = counts * stride;
+		offs += counts;
 	}
-	urb->transfer_buffer_length = 0;
+	urb->transfer_buffer_length = offs * stride;
+	memset(urb->transfer_buffer,
+	       subs->cur_audiofmt->format == SNDRV_PCM_FORMAT_U8 ? 0x80 : 0,
+	       offs * stride);
 	return 0;
 }