tmff2,t500rs: route FF_GAIN/FF_AUTOCENTER via base workqueue; drop per-wheel worker; fix "scheduling while atomic"

 The T500RS backend still had its own workqueue for gain and
 autocenter. The maintainer requested to completely remove the per-wheel
 workqueue and use the base driver workqueue instead. While playing a
 game we also hit a WARN ("scheduling while atomic") when FF_GAIN was
 updated: input_ff_event() runs in an RCU read-side section, but the
 T500RS callbacks called usb_interrupt_msg(), which can sleep.

 This change centralizes the deferral in the core (tmff2) and removes the
 extra worker from the wheel backend:

  - tmff2: add pending Gain/Autocenter fields and flags; have
    tmff2_set_gain()/tmff2_set_autocenter() only cache the values under
    the spinlock and schedule the existing delayed_work. The actual USB
    I/O is performed from tmff2_work_handler() in process context after
    the lock is released. The sysfs gain store is routed through the same
    mechanism so everything converges on a single path.

  - t500rs: drop the per-wheel workqueue and any related helpers; use the
    shared send_buffer and let the base driver workqueue perform the USB
    interrupt transfer. Also remove a now-unused helper
    (t500rs_do_set_gain_byte) and minor cleanups.

 Why:
  - Satisfies the maintainer request to remove the extra workqueue and
    reuse the base driver workqueue.
  - Fixes the "scheduling while atomic" warning by ensuring that gain and
    autocenter packets are always sent from process context.
  - Reduces duplication and keeps all FF deferral logic in one place.

 Notes:
  - Effect handlers and the rest of the USB packet layout are unchanged.
  - Logging behaviour is unchanged; follow-ups can add rate limiting if
    needed.

 Build/validate:
  - Builds cleanly (no warnings).
  - Smoke-tested: module reloads, wheel init OK, no WARN/OOPS when a game
    changes FF gain/autocenter.
 MSG

Author: cazzoo

docs/T500RS-async-refactor-plan.md

@@ -83,5 +83,5 @@ Goal: Call wheel callbacks from tmff2_work_handler outside of tmff2->lock so the
 
 ---
 
-Status: minimal change (global high‑pri workqueue) implemented; long‑term core split remains TODO.
+Status: core work handler now performs I/O outside spinlock (commit 5095d47); T500RS now performs synchronous USB I/O directly; the previous per-device op queue/worker has been removed.
 

src/hid-tmff2.c

@@ -258,8 +258,15 @@ static ssize_t gain_store(struct device *dev,
 	}
 
 	gain = value;
-	if (tmff2->set_gain) /* if we can, update gain immediately */
-		tmff2->set_gain(tmff2->data, (GAIN_MAX * gain) / GAIN_MAX);
+	if (tmff2->set_gain) {
+		unsigned long flags;
+		spin_lock_irqsave(&tmff2->lock, flags);
+		tmff2->pending_gain_value = GAIN_MAX;
+		tmff2->gain_pending = 1;
+		spin_unlock_irqrestore(&tmff2->lock, flags);
+		if (!delayed_work_pending(&tmff2->work) && tmff2->allow_scheduling)
+			schedule_delayed_work(&tmff2->work, 0);
+	}
 
 	return count;
 }
@@ -274,6 +281,7 @@ static DEVICE_ATTR_RW(gain);
 static void tmff2_set_gain(struct input_dev *dev, uint16_t value)
 {
 	struct tmff2_device_entry *tmff2 = tmff2_from_input(dev);
+	unsigned long flags;
 
 	if (!tmff2)
 		return;
@@ -283,13 +291,20 @@ static void tmff2_set_gain(struct input_dev *dev, uint16_t value)
 		return;
 	}
 
-	if (tmff2->set_gain(tmff2->data, (value * gain) / GAIN_MAX))
-		hid_warn(tmff2->hdev, "unable to set gain\n");
+	/* Defer to workqueue: store pending gain and schedule */
+	spin_lock_irqsave(&tmff2->lock, flags);
+	tmff2->pending_gain_value = value;
+	tmff2->gain_pending = 1;
+	spin_unlock_irqrestore(&tmff2->lock, flags);
+
+	if (!delayed_work_pending(&tmff2->work) && tmff2->allow_scheduling)
+		schedule_delayed_work(&tmff2->work, 0);
 }
 
 static void tmff2_set_autocenter(struct input_dev *dev, uint16_t value)
 {
 	struct tmff2_device_entry *tmff2 = tmff2_from_input(dev);
+	unsigned long flags;
 
 	if (!tmff2)
 		return;
@@ -299,8 +314,14 @@ static void tmff2_set_autocenter(struct input_dev *dev, uint16_t value)
 		return;
 	}
 
-	if (tmff2->set_autocenter(tmff2->data, value))
-		hid_warn(tmff2->hdev, "unable to set autocenter\n");
+	/* Defer to workqueue: store pending autocenter and schedule */
+	spin_lock_irqsave(&tmff2->lock, flags);
+	tmff2->pending_autocenter_value = value;
+	tmff2->autocenter_pending = 1;
+	spin_unlock_irqrestore(&tmff2->lock, flags);
+
+	if (!delayed_work_pending(&tmff2->work) && tmff2->allow_scheduling)
+		schedule_delayed_work(&tmff2->work, 0);
 }
 
 static void tmff2_work_handler(struct work_struct *w)
@@ -317,6 +338,30 @@ static void tmff2_work_handler(struct work_struct *w)
 	if (!tmff2)
 		return;
 
+	/* Apply pending control changes (gain/autocenter) in process context */
+	{
+		unsigned long f2;
+		uint16_t pg = 0, pac = 0;
+		int do_gain = 0, do_ac = 0;
+		spin_lock_irqsave(&tmff2->lock, f2);
+		if (tmff2->gain_pending) {
+			pg = tmff2->pending_gain_value;
+			tmff2->gain_pending = 0;
+			do_gain = 1;
+		}
+		if (tmff2->autocenter_pending) {
+			pac = tmff2->pending_autocenter_value;
+			tmff2->autocenter_pending = 0;
+			do_ac = 1;
+		}
+		spin_unlock_irqrestore(&tmff2->lock, f2);
+
+		if (do_gain && tmff2->set_gain)
+			tmff2->set_gain(tmff2->data, (pg * gain) / GAIN_MAX);
+		if (do_ac && tmff2->set_autocenter)
+			tmff2->set_autocenter(tmff2->data, pac);
+	}
+
 	for (effect_id = 0; effect_id < tmff2->max_effects; ++effect_id) {
 		unsigned long actions = 0;
 		struct tmff2_effect_state effect;

src/hid-tmff2.h

@@ -64,6 +64,12 @@ struct tmff2_device_entry {
 
 	spinlock_t lock;
 
+	/* Pending control changes to be applied from workqueue context */
+	uint16_t pending_gain_value;
+	uint16_t pending_autocenter_value;
+	int gain_pending;
+	int autocenter_pending;
+
 	int allow_scheduling;
 
 	/* fields relevant to each actual device (T300, T248...) */