[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: Clemens Ladisch <clemens@ladisch.de>

1 files changed, 21 insertions, 5 deletions

diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index a1bd8040dea4..95729730bcdc 100644
--- a/sound/usb/usbaudio.c
+++ b/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;
 }