soc: qcom: rpmh-rsc: Remove the pm_lock

It has been postulated that the pm_lock is bad for performance because
a CPU currently running rpmh_flush() could block other CPUs from
coming out of idle. Similarly CPUs coming out of / going into idle
all need to contend with each other for the spinlock just to update
the variable tracking who's in PM.

Let's optimize this a bit. Specifically:

- Use a count rather than a bitmask. This is faster to access and
also means we can use the atomic_inc_return() function to really
detect who the last one to enter PM was.
- Accept that it's OK if we race and are doing the flush (because we
think we're last) while another CPU is coming out of idle. As long
as we block that CPU if/when it tries to do an active-only transfer
we're OK.

Signed-off-by: Douglas Anderson <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Link: https://lore.kernel.org/r/20200504104917.v6.5.I295cb72bc5334a2af80313cbe97cb5c9dcb1442c@changeid
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/soc/qcom/rpmh-internal.h

···
  * @num_tcs:            Number of TCSes in this DRV.
  * @rsc_pm:             CPU PM notifier for controller.
  *                      Used when solver mode is not present.
- * @cpus_entered_pm:    CPU mask for cpus in idle power collapse.
+ * @cpus_in_pm:         Number of CPUs not in idle power collapse.
  *                      Used when solver mode is not present.
  * @tcs:                TCS groups.
  * @tcs_in_use:         S/W state of the TCS; only set for ACTIVE_ONLY
···
  *                      it was borrowed for an active_only transfer.  You
  *                      must hold the lock in this struct (AKA drv->lock) in
  *                      order to update this.
- * @lock:               Synchronize state of the controller.
- * @pm_lock:            Synchronize during PM notifications.
- *                      Used when solver mode is not present.
+ * @lock:               Synchronize state of the controller.  If RPMH's cache
+ *                      lock will also be held, the order is: drv->lock then
+ *                      cache_lock.
  * @client:             Handle to the DRV's client.
  */
 struct rsc_drv {
···
 	int id;
 	int num_tcs;
 	struct notifier_block rsc_pm;
-	struct cpumask cpus_entered_pm;
+	atomic_t cpus_in_pm;
 	struct tcs_group tcs[TCS_TYPE_NR];
 	DECLARE_BITMAP(tcs_in_use, MAX_TCS_NR);
 	spinlock_t lock;
-	spinlock_t pm_lock;
 	struct rpmh_ctrlr client;
 };
 

drivers/soc/qcom/rpmh-rsc.c

···
  * SLEEP and WAKE sets. If AMCs are busy, controller can not enter
  * power collapse, so deny from the last cpu's pm notification.
  *
+ * Context: Must be called with the drv->lock held.
+ *
  * Return:
  * * False		- AMCs are idle
  * * True		- AMCs are busy
···
 	 * dedicated TCS for active state use, then re-purposed wake TCSes
 	 * should be checked for not busy, because we used wake TCSes for
 	 * active requests in this case.
-	 *
-	 * Since this is called from the last cpu, need not take drv->lock
-	 * before checking tcs_is_free().
 	 */
 	if (!tcs->num_tcs)
 		tcs = &drv->tcs[WAKE_TCS];
···
 {
 	struct rsc_drv *drv = container_of(nfb, struct rsc_drv, rsc_pm);
 	int ret = NOTIFY_OK;
-
-	spin_lock(&drv->pm_lock);
+	int cpus_in_pm;
 
 	switch (action) {
 	case CPU_PM_ENTER:
-		cpumask_set_cpu(smp_processor_id(), &drv->cpus_entered_pm);
-
-		if (!cpumask_equal(&drv->cpus_entered_pm, cpu_online_mask))
-			goto exit;
+		cpus_in_pm = atomic_inc_return(&drv->cpus_in_pm);
+		/*
+		 * NOTE: comments for num_online_cpus() point out that it's
+		 * only a snapshot so we need to be careful. It should be OK
+		 * for us to use, though.  It's important for us not to miss
+		 * if we're the last CPU going down so it would only be a
+		 * problem if a CPU went offline right after we did the check
+		 * AND that CPU was not idle AND that CPU was the last non-idle
+		 * CPU. That can't happen. CPUs would have to come out of idle
+		 * before the CPU could go offline.
+		 */
+		if (cpus_in_pm < num_online_cpus())
+			return NOTIFY_OK;
 		break;
 	case CPU_PM_ENTER_FAILED:
 	case CPU_PM_EXIT:
-		cpumask_clear_cpu(smp_processor_id(), &drv->cpus_entered_pm);
-		goto exit;
+		atomic_dec(&drv->cpus_in_pm);
+		return NOTIFY_OK;
 	default:
-		ret = NOTIFY_DONE;
-		goto exit;
+		return NOTIFY_DONE;
 	}
 
-	ret = rpmh_rsc_ctrlr_is_busy(drv);
-	if (ret) {
-		ret = NOTIFY_BAD;
-		goto exit;
+	/*
+	 * It's likely we're on the last CPU. Grab the drv->lock and write
+	 * out the sleep/wake commands to RPMH hardware. Grabbing the lock
+	 * means that if we race with another CPU coming up we are still
+	 * guaranteed to be safe. If another CPU came up just after we checked
+	 * and has grabbed the lock or started an active transfer then we'll
+	 * notice we're busy and abort. If another CPU comes up after we start
+	 * flushing it will be blocked from starting an active transfer until
+	 * we're done flushing. If another CPU starts an active transfer after
+	 * we release the lock we're still OK because we're no longer the last
+	 * CPU.
+	 */
+	if (spin_trylock(&drv->lock)) {
+		if (rpmh_rsc_ctrlr_is_busy(drv) || rpmh_flush(&drv->client))
+			ret = NOTIFY_BAD;
+		spin_unlock(&drv->lock);
+	} else {
+		/* Another CPU must be up */
+		return NOTIFY_OK;
 	}
 
-	ret = rpmh_flush(&drv->client);
-	if (ret)
-		ret = NOTIFY_BAD;
-	else
-		ret = NOTIFY_OK;
+	if (ret == NOTIFY_BAD) {
+		/* Double-check if we're here because someone else is up */
+		if (cpus_in_pm < num_online_cpus())
+			ret = NOTIFY_OK;
+		else
+			/* We won't be called w/ CPU_PM_ENTER_FAILED */
+			atomic_dec(&drv->cpus_in_pm);
+	}
 
-exit:
-	if (ret == NOTIFY_BAD)
-		/* We won't be called w/ CPU_PM_ENTER_FAILED */
-		cpumask_clear_cpu(smp_processor_id(), &drv->cpus_entered_pm);
-
-	spin_unlock(&drv->pm_lock);
 	return ret;
 }
 
···
 	solver_config = solver_config >> DRV_HW_SOLVER_SHIFT;
 	if (!solver_config) {
 		drv->rsc_pm.notifier_call = rpmh_rsc_cpu_pm_callback;
-		spin_lock_init(&drv->pm_lock);
 		cpu_pm_register_notifier(&drv->rsc_pm);
 	}
 

drivers/soc/qcom/rpmh.c

···
  *
  * @ctrlr: Controller making request to flush cached data
  *
- * This function is called from sleep code on the last CPU
- * (thus no spinlock needed).
- *
  * Return:
  * * 0          - Success
  * * Error code - Otherwise
···
 int rpmh_flush(struct rpmh_ctrlr *ctrlr)
 {
 	struct cache_req *p;
-	int ret;
+	int ret = 0;
 
 	lockdep_assert_irqs_disabled();
 
+	/*
+	 * Currently rpmh_flush() is only called when we think we're running
+	 * on the last processor.  If the lock is busy it means another
+	 * processor is up and it's better to abort than spin.
+	 */
+	if (!spin_trylock(&ctrlr->cache_lock))
+		return -EBUSY;
+
 	if (!ctrlr->dirty) {
 		pr_debug("Skipping flush, TCS has latest data.\n");
-		return 0;
+		goto exit;
 	}
 
 	/* Invalidate the TCSes first to avoid stale data */
···
 	/* First flush the cached batch requests */
 	ret = flush_batch(ctrlr);
 	if (ret)
-		return ret;
+		goto exit;
 
 	list_for_each_entry(p, &ctrlr->cache, list) {
 		if (!is_req_valid(p)) {
···
 		ret = send_single(ctrlr, RPMH_SLEEP_STATE, p->addr,
 				  p->sleep_val);
 		if (ret)
-			return ret;
+			goto exit;
 		ret = send_single(ctrlr, RPMH_WAKE_ONLY_STATE, p->addr,
 				  p->wake_val);
 		if (ret)
-			return ret;
+			goto exit;
 	}
 
 	ctrlr->dirty = false;
 
-	return 0;
+exit:
+	spin_unlock(&ctrlr->cache_lock);
+	return ret;
 }
 
 /**