usb: gadget: u_audio: Rate ctl notifies about current srate (0=stopped)

The Playback/Capture ctl currently reports rate value set by USB
control selector UAC2_CS_CONTROL_SAM_FREQ (fixed for UAC1). When the
stops playback/capture, the reported value does not change. The gadget
side has no information whether the host has started/stopped
capture/playback.

This patch sets the value reported by the respective rate ctl to zero
when the host side has stopped playback/capture. Also, it calls
snd_ctl_notify when start/stop occurs, so that a subscribed client can
act appropriately.

Tests have confirmed that USB hosts change UAC2_CS_CONTROL_SAM_FREQ
before switching altsetting to activate playback/capture, resulting in
correct order (params->c/p_srate is set to requested rate before
u_audio_start_capture/playback is called).

The gadget rate notifications are used by user-space audio gadget
controller gaudio_ctl https://github.com/pavhofman/gaudio_ctl.

Signed-off-by: Pavel Hofman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/u_audio.c

···
 
 	struct snd_kcontrol *snd_kctl_rate; /* read-only current rate */
 	int srate; /* selected samplerate */
+	int active; /* playback/capture running */
 
   spinlock_t lock; /* lock for control transfers */
 
···
 		dev_err(uac->card->dev, "%s:%d Error!\n", __func__, __LINE__);
 }
 
+static void set_active(struct uac_rtd_params *prm, bool active)
+{
+	// notifying through the Rate ctrl
+	struct snd_kcontrol *kctl = prm->snd_kctl_rate;
+	unsigned long flags;
+
+	spin_lock_irqsave(&prm->lock, flags);
+	if (prm->active != active) {
+		prm->active = active;
+		snd_ctl_notify(prm->uac->card, SNDRV_CTL_EVENT_MASK_VALUE,
+				&kctl->id);
+	}
+	spin_unlock_irqrestore(&prm->lock, flags);
+}
+
 int u_audio_set_capture_srate(struct g_audio *audio_dev, int srate)
 {
 	struct uac_params *params = &audio_dev->params;
···
 			dev_err(dev, "%s:%d Error!\n", __func__, __LINE__);
 	}
 
+	set_active(&uac->c_prm, true);
+
 	ep_fback = audio_dev->in_ep_fback;
 	if (!ep_fback)
 		return 0;
···
 {
 	struct snd_uac_chip *uac = audio_dev->uac;
 
+	set_active(&uac->c_prm, false);
 	if (audio_dev->in_ep_fback)
 		free_ep_fback(&uac->c_prm, audio_dev->in_ep_fback);
 	free_ep(&uac->c_prm, audio_dev->out_ep);
···
 			dev_err(dev, "%s:%d Error!\n", __func__, __LINE__);
 	}
 
+	set_active(&uac->p_prm, true);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(u_audio_start_playback);
···
 {
 	struct snd_uac_chip *uac = audio_dev->uac;
 
+	set_active(&uac->p_prm, false);
 	free_ep(&uac->p_prm, audio_dev->in_ep);
 }
 EXPORT_SYMBOL_GPL(u_audio_stop_playback);
···
 	unsigned long flags;
 
 	spin_lock_irqsave(&prm->lock, flags);
-	ucontrol->value.integer.value[0] = prm->srate;
+	if (prm->active)
+		ucontrol->value.integer.value[0] = prm->srate;
+	else
+		/* not active: reporting zero rate */
+		ucontrol->value.integer.value[0] = 0;
 	spin_unlock_irqrestore(&prm->lock, flags);
 	return 0;
 }