ALSA: usb-audio: Add hw constraint for implicit fb sync

In the current code, there is no check at the stream open time whether
the endpoint is being already used by others.  In the normal
operations, this shouldn't happen, but in the case of the implicit
feedback mode, it's a common problem with the full duplex operation,
because the capture stream is always opened by the playback stream as
an implicit sync source.

Although we recently introduced the check of such a conflict of
parameters at the PCM hw_params time, it doesn't give any hint at the
hw_params itself and just gives the error.  This isn't quite
comfortable, and it caused problems on many applications.

This patch attempts to make the parameter handling easier by
introducing the strict hw constraint matching with the counterpart
stream that is being used.  That said, when an implicit feedback
playback stream is running before a capture stream is opened, the
capture stream carries the PCM hw-constraint to allow only the same
sample rate, format, periods and period frames as the running playback
stream.  If not opened or there is no conflict of endpoints, the
behavior remains as same as before.

Note that this kind of "weak link" should work for most cases, but
this is no concrete solution; e.g. if an application changes the hw
params multiple times while another stream is opened, this would lead
to inconsistencies.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-11-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 9 insertions, 0 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index 898a283576df..1f61be98a31d 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -114,6 +114,14 @@ struct snd_usb_endpoint {
 					   in a stream */
 	bool is_implicit_feedback;      /* This endpoint is used as implicit feedback */
 
+	/* for hw constraints */
+	unsigned int cur_rate;
+	snd_pcm_format_t cur_format;
+	unsigned int cur_channels;
+	unsigned int cur_period_frames;
+	unsigned int cur_period_bytes;
+	unsigned int cur_buffer_periods;
+
 	spinlock_t lock;
 	struct list_head list;
 };
@@ -144,6 +152,7 @@ struct snd_usb_substream {
 	unsigned int stream_offset_adj;	/* Bytes to drop from beginning of stream (for non-compliant devices) */
 
 	unsigned int running: 1;	/* running status */
+	unsigned int fixed_hw:1;	/* fixed hw constraints due to sync EP */
 
 	unsigned int hwptr_done;	/* processed byte position in the buffer */
 	unsigned int transfer_done;		/* processed frames since last period update */