[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>
diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index a1bd804..9572973 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -478,22 +478,38 @@
/*
* 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;
}
