[PATCH v2 1/5] PM: domains: Add GENPD_FLAG_RT_SAFE for PREEMPT_RT

[Krzysztof Kozlowski]

Realtime kernels with PREEMPT_RT must use raw_spinlock_t for domains
which are invoked from CPU idle (thus from atomic section).  Example is
cpuidle PSCI domain driver which itself is PREEMPT_RT safe, but is being
called as part of cpuidle.

Add a flag allowing a power domain provider to indicate it is RT safe.
The flag is supposed to be used with existing GENPD_FLAG_IRQ_SAFE.

Cc: Adrien Thierry <athierry@xxxxxxxxxx>
Cc: Brian Masney <bmasney@xxxxxxxxxx>
Cc: linux-rt-users@xxxxxxxxxxxxxxx
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>

---

Independently from Adrien, I encountered the same problem around genpd
when using PREEMPT_RT kernel.

Previous patch by Adrien:
https://lore.kernel.org/all/20220615203605.1068453-1-athierry@xxxxxxxxxx/
---
 drivers/base/power/domain.c | 59 +++++++++++++++++++++++++++++++++++--
 include/linux/pm_domain.h   | 13 ++++++++
 2 files changed, 70 insertions(+), 2 deletions(-)

diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 967bcf9d415e..4dfce1d476f4 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c
@@ -119,6 +119,48 @@ static const struct genpd_lock_ops genpd_spin_ops = {
 	.unlock = genpd_unlock_spin,
 };
 
+static void genpd_lock_rawspin(struct generic_pm_domain *genpd)
+	__acquires(&genpd->rslock)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&genpd->rslock, flags);
+	genpd->rlock_flags = flags;
+}
+
+static void genpd_lock_nested_rawspin(struct generic_pm_domain *genpd,
+					int depth)
+	__acquires(&genpd->rslock)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave_nested(&genpd->rslock, flags, depth);
+	genpd->rlock_flags = flags;
+}
+
+static int genpd_lock_interruptible_rawspin(struct generic_pm_domain *genpd)
+	__acquires(&genpd->rslock)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&genpd->rslock, flags);
+	genpd->rlock_flags = flags;
+	return 0;
+}
+
+static void genpd_unlock_rawspin(struct generic_pm_domain *genpd)
+	__releases(&genpd->rslock)
+{
+	raw_spin_unlock_irqrestore(&genpd->rslock, genpd->rlock_flags);
+}
+
+static const struct genpd_lock_ops genpd_rawspin_ops = {
+	.lock = genpd_lock_rawspin,
+	.lock_nested = genpd_lock_nested_rawspin,
+	.lock_interruptible = genpd_lock_interruptible_rawspin,
+	.unlock = genpd_unlock_rawspin,
+};
+
 #define genpd_lock(p)			p->lock_ops->lock(p)
 #define genpd_lock_nested(p, d)		p->lock_ops->lock_nested(p, d)
 #define genpd_lock_interruptible(p)	p->lock_ops->lock_interruptible(p)
@@ -126,6 +168,8 @@ static const struct genpd_lock_ops genpd_spin_ops = {
 
 #define genpd_status_on(genpd)		(genpd->status == GENPD_STATE_ON)
 #define genpd_is_irq_safe(genpd)	(genpd->flags & GENPD_FLAG_IRQ_SAFE)
+#define genpd_is_rt_safe(genpd)		(genpd_is_irq_safe(genpd) && \
+					 (genpd->flags & GENPD_FLAG_RT_SAFE))
 #define genpd_is_always_on(genpd)	(genpd->flags & GENPD_FLAG_ALWAYS_ON)
 #define genpd_is_active_wakeup(genpd)	(genpd->flags & GENPD_FLAG_ACTIVE_WAKEUP)
 #define genpd_is_cpu_domain(genpd)	(genpd->flags & GENPD_FLAG_CPU_DOMAIN)
@@ -1838,6 +1882,12 @@ static int genpd_add_subdomain(struct generic_pm_domain *genpd,
 		return -EINVAL;
 	}
 
+	if (!genpd_is_rt_safe(genpd) && genpd_is_rt_safe(subdomain)) {
+		WARN(1, "Parent %s of subdomain %s must be RT safe\n",
+		     genpd->name, subdomain->name);
+		return -EINVAL;
+	}
+
 	link = kzalloc(sizeof(*link), GFP_KERNEL);
 	if (!link)
 		return -ENOMEM;
@@ -2008,8 +2058,13 @@ static void genpd_free_data(struct generic_pm_domain *genpd)
 static void genpd_lock_init(struct generic_pm_domain *genpd)
 {
 	if (genpd->flags & GENPD_FLAG_IRQ_SAFE) {
-		spin_lock_init(&genpd->slock);
-		genpd->lock_ops = &genpd_spin_ops;
+		if (genpd->flags & GENPD_FLAG_RT_SAFE) {
+			raw_spin_lock_init(&genpd->rslock);
+			genpd->lock_ops = &genpd_rawspin_ops;
+		} else {
+			spin_lock_init(&genpd->slock);
+			genpd->lock_ops = &genpd_spin_ops;
+		}
 	} else {
 		mutex_init(&genpd->mlock);
 		genpd->lock_ops = &genpd_mtx_ops;
diff --git a/include/linux/pm_domain.h b/include/linux/pm_domain.h
index 1cd41bdf73cf..0a1600244963 100644
--- a/include/linux/pm_domain.h
+++ b/include/linux/pm_domain.h
@@ -61,6 +61,14 @@
  * GENPD_FLAG_MIN_RESIDENCY:	Enable the genpd governor to consider its
  *				components' next wakeup when determining the
  *				optimal idle state.
+ *
+ * GENPD_FLAG_RT_SAFE:		When used with GENPD_FLAG_IRQ_SAFE, this informs
+ *				genpd that its backend callbacks, ->power_on|off(),
+ *				do not use other spinlocks. They might use
+ *				raw_spinlocks or other pre-emption-disable
+ *				methods, all of which are PREEMPT_RT safe. Note
+ *				that, a genpd having this flag set, requires its
+ *				masterdomains to also have it set.
  */
 #define GENPD_FLAG_PM_CLK	 (1U << 0)
 #define GENPD_FLAG_IRQ_SAFE	 (1U << 1)
@@ -69,6 +77,7 @@
 #define GENPD_FLAG_CPU_DOMAIN	 (1U << 4)
 #define GENPD_FLAG_RPM_ALWAYS_ON (1U << 5)
 #define GENPD_FLAG_MIN_RESIDENCY (1U << 6)
+#define GENPD_FLAG_RT_SAFE	 (1U << 7)
 
 enum gpd_status {
 	GENPD_STATE_ON = 0,	/* PM domain is on */
@@ -164,6 +173,10 @@ struct generic_pm_domain {
 			spinlock_t slock;
 			unsigned long lock_flags;
 		};
+		struct {
+			raw_spinlock_t rslock;
+			unsigned long rlock_flags;
+		};
 	};
 
 };
-- 
2.34.1