USB-audio driver behaves a bit strangely for the playback stream -- namely, it starts sending silent packets at PCM prepare state while the actual data is submitted at first when the trigger START is kicked off. This is a workaround for the behavior where URBs are processed too quickly at the beginning. That is, if we start submitting URBs at trigger START, the first few URBs will be immediately completed, and this would result in the immediate period-elapsed calls right after the start, which may confuse applications.
OTOH, submitting the data after silent URBs would, of course, result in a certain delay of the actual data processing, and this is rather more serious problem on modern systems, in practice.
This patch tries to revert the workaround and lets the URB submission starting at PCM trigger for the playback again. As far as I've tested with various backends (native ALSA, PA, JACK, PW), I haven't seen any problems (famous last words :)
Note that the capture stream handling needs no such workaround, since the capture is driven per received URB.
Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/usb/pcm.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index e26d37365f02..c66831ee15f9 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -613,11 +613,6 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) subs->last_frame_number = 0; runtime->delay = 0;
- /* for playback, submit the URBs now; otherwise, the first hwptr_done - * updates for all URBs would happen at the same time when starting */ - if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK) - ret = start_endpoints(subs); - unlock: snd_usb_unlock_shutdown(chip); return ret; @@ -1430,6 +1425,7 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea int cmd) { struct snd_usb_substream *subs = substream->runtime->private_data; + int err;
switch (cmd) { case SNDRV_PCM_TRIGGER_START: @@ -1440,6 +1436,14 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea prepare_playback_urb, retire_playback_urb, subs); + if (cmd == SNDRV_PCM_TRIGGER_START) { + err = start_endpoints(subs); + if (err < 0) { + snd_usb_endpoint_set_callback(subs->data_endpoint, + NULL, NULL, NULL); + return err; + } + } subs->running = 1; dev_dbg(&subs->dev->dev, "%d:%d Start Playback PCM\n", subs->cur_audiofmt->iface,