usb: gadget: f_uac2: Add suspend callback

When USB cable gets disconnected, the undergoing playback/capture
stalls, without any notification to u_audio about the change.
Experiments with a dwc2 gadget revealed that Suspend interrupt is
thrown at cable disconnection, which the gadget framework translates to
calling suspend callback of a function, if it is defined.

Add the suspend callback to f_uac2 function, calling
corresponding method of u_audio in order to stop the respective PCM
streams and to notify subscribed clients at cable disconnection.
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-10-pavel.hofman@ivitera.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_uac2.c

@@ -1433,6 +1433,14 @@ afunc_disable(struct usb_function *fn)
 		usb_ep_disable(uac2->int_ep);
 }
 
+static void
+afunc_suspend(struct usb_function *fn)
+{
+	struct f_uac2 *uac2 = func_to_uac2(fn);
+
+	u_audio_suspend(&uac2->g_audio);
+}
+
 static int
 in_rq_cur(struct usb_function *fn, const struct usb_ctrlrequest *cr)
 {
@@ -2104,6 +2112,7 @@ static struct usb_function *afunc_alloc(struct usb_function_instance *fi)
 	uac2->g_audio.func.set_alt = afunc_set_alt;
 	uac2->g_audio.func.get_alt = afunc_get_alt;
 	uac2->g_audio.func.disable = afunc_disable;
+	uac2->g_audio.func.suspend = afunc_suspend;
 	uac2->g_audio.func.setup = afunc_setup;
 	uac2->g_audio.func.free_func = afunc_free;