mt76: add utility functions for deferring work to a kernel thread

In order to avoid keeping work like tx scheduling pinned to the CPU it was
scheduled from, it makes sense to switch from tasklets to kernel threads.

Unlike a workqueue, this one only allows one fixed worker function to be
executed by the worker thread. Because of that, there is less locking
and less code for scheduling involved.
This is important because the tx worker is scheduled often in a hot path
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/util.c

@@ -110,4 +110,32 @@ int mt76_get_min_avg_rssi(struct mt76_dev *dev, bool ext_phy)
 }
 EXPORT_SYMBOL_GPL(mt76_get_min_avg_rssi);
 
+int __mt76_worker_fn(void *ptr)
+{
+	struct mt76_worker *w = ptr;
+
+	while (!kthread_should_stop()) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
+		if (kthread_should_park()) {
+			kthread_parkme();
+			continue;
+		}
+
+		if (!test_and_clear_bit(MT76_WORKER_SCHEDULED, &w->state)) {
+			schedule();
+			continue;
+		}
+
+		set_bit(MT76_WORKER_RUNNING, &w->state);
+		set_current_state(TASK_RUNNING);
+		w->fn(w);
+		cond_resched();
+		clear_bit(MT76_WORKER_RUNNING, &w->state);
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__mt76_worker_fn);
+
 MODULE_LICENSE("Dual BSD/GPL");

drivers/net/wireless/mediatek/mt76/util.h

@@ -10,6 +10,19 @@
 #include <linux/skbuff.h>
 #include <linux/bitops.h>
 #include <linux/bitfield.h>
+#include <net/mac80211.h>
+
+struct mt76_worker
+{
+	struct task_struct *task;
+	void (*fn)(struct mt76_worker *);
+	unsigned long state;
+};
+
+enum {
+	MT76_WORKER_SCHEDULED,
+	MT76_WORKER_RUNNING,
+};
 
 #define MT76_INCR(_var, _size) \
 	(_var = (((_var) + 1) % (_size)))
@@ -45,4 +58,67 @@ mt76_skb_set_moredata(struct sk_buff *skb, bool enable)
 		hdr->frame_control &= ~cpu_to_le16(IEEE80211_FCTL_MOREDATA);
 }
 
+int __mt76_worker_fn(void *ptr);
+
+static inline int
+mt76_worker_setup(struct ieee80211_hw *hw, struct mt76_worker *w,
+		  void (*fn)(struct mt76_worker *),
+		  const char *name)
+{
+	const char *dev_name = wiphy_name(hw->wiphy);
+	int ret;
+
+	if (fn)
+		w->fn = fn;
+	w->task = kthread_create(__mt76_worker_fn, w, "mt76-%s %s",
+				 name, dev_name);
+
+	ret = PTR_ERR_OR_ZERO(w->task);
+	if (ret) {
+		w->task = NULL;
+		return ret;
+	}
+
+	wake_up_process(w->task);
+
+	return 0;
+}
+
+static inline void mt76_worker_schedule(struct mt76_worker *w)
+{
+	if (!w->task)
+		return;
+
+	if (!test_and_set_bit(MT76_WORKER_SCHEDULED, &w->state) &&
+	    !test_bit(MT76_WORKER_RUNNING, &w->state))
+		wake_up_process(w->task);
+}
+
+static inline void mt76_worker_disable(struct mt76_worker *w)
+{
+	if (!w->task)
+		return;
+
+	kthread_park(w->task);
+	WRITE_ONCE(w->state, 0);
+}
+
+static inline void mt76_worker_enable(struct mt76_worker *w)
+{
+	if (!w->task)
+		return;
+
+	kthread_unpark(w->task);
+	mt76_worker_schedule(w);
+}
+
+static inline void mt76_worker_teardown(struct mt76_worker *w)
+{
+	if (!w->task)
+		return;
+
+	kthread_stop(w->task);
+	w->task = NULL;
+}
+
 #endif