[PATCH 1/6] ALSA: usb-audio: Add PM guard to Studio 1810c controls

[Will Porter]

The Studio 1810c control callbacks issue vendor transfers without
preventing runtime suspend or disconnect. A transfer attempted after
runtime suspend can fail because the USB device cannot accept
submissions.

Take snd_usb_lock before the driver data and USB mutexes in both
callbacks. This resumes the device before either the state read or
control write. It also prevents disconnect cleanup from racing the
complete operation.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Will Porter <mrwillporter@xxxxxxxxx>
---
 sound/usb/mixer_s1810c.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sound/usb/mixer_s1810c.c b/sound/usb/mixer_s1810c.c
index 2e5a8d37ec57..bdb5e3aaff3b 100644
--- a/sound/usb/mixer_s1810c.c
+++ b/sound/usb/mixer_s1810c.c
@@ -474,6 +474,10 @@ snd_s1810c_switch_get(struct snd_kcontrol *kctl,
 	u32 state = 0;
 	int ret;
 
+	CLASS(snd_usb_lock, pm)(mixer->chip);
+	if (pm.err < 0)
+		return -EIO;
+
 	guard(mutex)(&private->data_mutex);
 	ret = snd_s1810c_get_switch_state(mixer, kctl, &state);
 	if (ret < 0)
@@ -504,6 +508,10 @@ snd_s1810c_switch_set(struct snd_kcontrol *kctl,
 	u32 newval = 0;
 	int ret = 0;
 
+	CLASS(snd_usb_lock, pm)(mixer->chip);
+	if (pm.err < 0)
+		return -EIO;
+
 	guard(mutex)(&private->data_mutex);
 	ret = snd_s1810c_get_switch_state(mixer, kctl, &curval);
 	if (ret < 0)
-- 
2.47.3