[PATCH v27 31/32] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

[Wesley Cheng <quic_wcheng@xxxxxxxxxxx>]

In case of notifying SND platform drivers of connection events, some of
these use cases, such as offloading, require an ASoC USB backend device to
be initialized before the events can be handled.  If the USB backend device
has not yet been probed, this leads to missing initial USB audio device
connection events.

Expose an API that traverses the usb_chip array for connected devices, and
to call the respective connection callback registered to the SND platform
driver.

Signed-off-by: Wesley Cheng <quic_wcheng@xxxxxxxxxxx>
---
 sound/usb/card.c                  | 21 +++++++++++++++++++++
 sound/usb/card.h                  |  2 ++
 sound/usb/qcom/qc_audio_offload.c |  2 ++
 3 files changed, 25 insertions(+)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 326292f37841..1f21d11416f9 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -155,6 +155,27 @@ int snd_usb_unregister_platform_ops(void)
 }
 EXPORT_SYMBOL_GPL(snd_usb_unregister_platform_ops);
 
+/*
+ * in case the platform driver was not ready at the time of USB SND
+ * device connect, expose an API to discover all connected USB devices
+ * so it can populate any dependent resources/structures.
+ */
+void snd_usb_rediscover_devices(void)
+{
+	int i;
+
+	guard(mutex)(&register_mutex);
+
+	if (!platform_ops || !platform_ops->connect_cb)
+		return;
+
+	for (i = 0; i < SNDRV_CARDS; i++) {
+		if (usb_chip[i])
+			platform_ops->connect_cb(usb_chip[i]);
+	}
+}
+EXPORT_SYMBOL_GPL(snd_usb_rediscover_devices);
+
 /*
  * Checks to see if requested audio profile, i.e sample rate, # of
  * channels, etc... is supported by the substream associated to the
diff --git a/sound/usb/card.h b/sound/usb/card.h
index d8b8522e1613..94404c24d240 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -222,4 +222,6 @@ snd_usb_find_suppported_substream(int card_idx, struct snd_pcm_hw_params *params
 
 int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops);
 int snd_usb_unregister_platform_ops(void);
+
+void snd_usb_rediscover_devices(void);
 #endif /* __USBAUDIO_CARD_H */
diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
index 5b9262a116be..2dc651cd3d05 100644
--- a/sound/usb/qcom/qc_audio_offload.c
+++ b/sound/usb/qcom/qc_audio_offload.c
@@ -1929,6 +1929,8 @@ static int __init qc_usb_audio_offload_init(void)
 	if (ret < 0)
 		goto release_qmi;
 
+	snd_usb_rediscover_devices();
+
 	return 0;
 
 release_qmi: