[PATCH 6/6] ALSA: usb-audio: Guard FCP protocol transfers

[Will Porter]

FCP meter and hwdep operations issue control transfers without
preventing runtime suspend or disconnect. Protect the central
request-and-response transport. One reference then covers the command,
acknowledgment wait, and response.

The initial step-zero request bypasses that transport. Hold an outer
reference across the complete initialization sequence so the device stays
active through step zero, notification-URB setup, and both initialization
commands. The central transport keeps its guard for calls outside
initialization; the existing active counter balances the nested calls.

FCP has no private resume callback. Its suspend callback only removes the
notification URB. Taking the initialization and transport guards under the
existing protocol mutex causes no resume-side lock inversion.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Will Porter <mrwillporter@xxxxxxxxx>
---
 sound/usb/fcp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sound/usb/fcp.c b/sound/usb/fcp.c
index 5fc2131b4561..68bb7eabf107 100644
--- a/sound/usb/fcp.c
+++ b/sound/usb/fcp.c
@@ -191,6 +191,10 @@ static int fcp_usb(struct usb_mixer_interface *mixer, u32 opcode,
 	const int max_retries = 5;
 	int err;
 
+	CLASS(snd_usb_lock, pm)(mixer->chip);
+	if (pm.err < 0)
+		return -EIO;
+
 	if (!private->urb)
 		return -ENODEV;
 
@@ -1026,6 +1030,10 @@ static int fcp_init(struct usb_mixer_interface *mixer,
 	struct usb_device *dev = mixer->chip->dev;
 	int err;
 
+	CLASS(snd_usb_lock, pm)(mixer->chip);
+	if (pm.err < 0)
+		return -EIO;
+
 	err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0),
 		FCP_USB_REQ_STEP0,
 		USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
-- 
2.47.3