ALSA: usb-audio: Less restriction for low-latency playback mode

The recent support for the improved low-latency playback mode applied
the SNDRV_PCM_INFO_EXPLICIT_SYNC flag for the target streams, but this
was a slight overkill.  The use of the flag above disables effectively
both PCM status and control mmaps, while basically what we want to
track is only about the appl_ptr update.

For less restriction, use a more proper flag,
SNDRV_PCM_INFO_SYNC_APPLPTR instead, which disables only the control
mmap.

Fixes: d5f871f89e21 ("ALSA: usb-audio: Improved lowlatency playback support")
Link: https://lore.kernel.org/r/20211011103650.10182-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 3bb095a..95ec8ee 100644 (file)
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1102,7 +1102,7 @@ static int snd_usb_pcm_open(struct snd_pcm_substream *substream)
        /* need an explicit sync to catch applptr update in low-latency mode */
        if (direction == SNDRV_PCM_STREAM_PLAYBACK &&
            as->chip->lowlatency)
-               runtime->hw.info |= SNDRV_PCM_INFO_EXPLICIT_SYNC;
+               runtime->hw.info |= SNDRV_PCM_INFO_SYNC_APPLPTR;
        runtime->private_data = subs;
        subs->pcm_substream = substream;
        /* runtime PM is also done there */