usb: gadget: function: f_uac1: implement get_alt()

After commit 7e4da3fcf7c9 ("usb: gadget: composite:
Test get_alt() presence instead of set_alt()") f_uac1
function became broken because it doesn't have
get_alt() callback implementation and composite
framework never set altsetting 1 for audiostreaming
interface. On host site it looks like:

[424339.017711] 21:1:1: usb_set_interface failed (-32)

Since host can't set altsetting 1, it can't start
playing audio.

In order to fix it implemented get_alt along with
minor improvements (error conditions checking)
similar to what existing f_uac2 has.

Cc: Krzysztof Opasiak <[e-mail]>
Signed-off-by: Ruslan Bilovol <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/f_uac1.c

···
 struct f_audio {
 	struct gaudio			card;
 
+	u8 ac_intf, ac_alt;
+	u8 as_intf, as_alt;
+
 	/* endpoints handle full and/or high speeds */
 	struct usb_ep			*out_ep;
 
···
 	req_count = opts->req_count;
 	audio_buf_size = opts->audio_buf_size;
 
-	if (intf == 1) {
+	/* No i/f has more than 2 alt settings */
+	if (alt > 1) {
+		ERROR(cdev, "%s:%d Error!\n", __func__, __LINE__);
+		return -EINVAL;
+	}
+
+	if (intf == audio->ac_intf) {
+		/* Control I/f has only 1 AltSetting - 0 */
+		if (alt) {
+			ERROR(cdev, "%s:%d Error!\n", __func__, __LINE__);
+			return -EINVAL;
+		}
+		return 0;
+	} else if (intf == audio->as_intf) {
 		if (alt == 1) {
 			err = config_ep_by_speed(cdev->gadget, f, out_ep);
 			if (err)
···
 				schedule_work(&audio->playback_work);
 			}
 		}
+		audio->as_alt = alt;
 	}
 
 	return err;
+}
+
+static int f_audio_get_alt(struct usb_function *f, unsigned intf)
+{
+	struct f_audio		*audio = func_to_audio(f);
+	struct usb_composite_dev *cdev = f->config->cdev;
+
+	if (intf == audio->ac_intf)
+		return audio->ac_alt;
+	else if (intf == audio->as_intf)
+		return audio->as_alt;
+	else
+		ERROR(cdev, "%s:%d Invalid Interface %d!\n",
+		      __func__, __LINE__, intf);
+
+	return -EINVAL;
 }
 
 static void f_audio_disable(struct usb_function *f)
···
 	if (status < 0)
 		goto fail;
 	ac_interface_desc.bInterfaceNumber = status;
+	audio->ac_intf = status;
+	audio->ac_alt = 0;
 
 	status = usb_interface_id(c, f);
 	if (status < 0)
 		goto fail;
 	as_interface_alt_0_desc.bInterfaceNumber = status;
 	as_interface_alt_1_desc.bInterfaceNumber = status;
+	audio->as_intf = status;
+	audio->as_alt = 0;
 
 	status = -ENODEV;
 
···
 	audio->card.func.bind = f_audio_bind;
 	audio->card.func.unbind = f_audio_unbind;
 	audio->card.func.set_alt = f_audio_set_alt;
+	audio->card.func.get_alt = f_audio_get_alt;
 	audio->card.func.setup = f_audio_setup;
 	audio->card.func.disable = f_audio_disable;
 	audio->card.func.free_func = f_audio_free;