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Commit 4b284928 authored by Clemens Ladisch's avatar Clemens Ladisch Committed by Jaroslav Kysela
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[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: default avatarClemens Ladisch <clemens@ladisch.de>
parent 54c63cfc

sound/usb/usbaudio.c

+21 −5
Original line number Diff line number Diff line
@@ -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->transfer_buffer_length = 0;

		urb->iso_frame_desc[i].offset = offs * stride;

		urb->iso_frame_desc[i].length = counts * stride;

		offs += counts;

	}

	urb->transfer_buffer_length = offs * stride;

	memset(urb->transfer_buffer,

	       subs->cur_audiofmt->format == SNDRV_PCM_FORMAT_U8 ? 0x80 : 0,

	       offs * stride);

	return 0;

}