[PATCH v3 1/4] genirq/irqdomain: Allow partial trimming of irq_data hierarchy

[Marc Zyngier]

It appears that some HW is ugly enough that not all the interrupts
connected to a particular interrupt controller end up with the same
hierarchy depth (some of them are terminated early). This leaves
the irqchip hacker with only two choices, both equally bad:

- create discrete domain chains, one for each "hierarchy depth",
  which is very hard to maintain

- create fake hierarchy levels for the shallow paths, leading
  to all kind of problems (what are the safe hwirq values for these
  fake levels?)

Implement the ability to cut short a single interrupt hierarchy
from the first level that doesn't have a corresponding irqchip.
As this is never a valid option (we have the no_irq_chip chip
for the "do nothing" case), the hierarchy can be trimmed from
that level.

Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
 kernel/irq/irqdomain.c | 58 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 52 insertions(+), 6 deletions(-)

diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
index 76cd7ebd1178..375eb2b79fe5 100644
--- a/kernel/irq/irqdomain.c
+++ b/kernel/irq/irqdomain.c
@@ -1136,6 +1136,17 @@ static struct irq_data *irq_domain_insert_irq_data(struct irq_domain *domain,
 	return irq_data;
 }
 
+static void __irq_domain_free_hierarchy(struct irq_data *irq_data)
+{
+	struct irq_data *tmp;
+
+	while (irq_data) {
+		tmp = irq_data;
+		irq_data = irq_data->parent_data;
+		kfree(tmp);
+	}
+}
+
 static void irq_domain_free_irq_data(unsigned int virq, unsigned int nr_irqs)
 {
 	struct irq_data *irq_data, *tmp;
@@ -1147,14 +1158,47 @@ static void irq_domain_free_irq_data(unsigned int virq, unsigned int nr_irqs)
 		irq_data->parent_data = NULL;
 		irq_data->domain = NULL;
 
-		while (tmp) {
-			irq_data = tmp;
-			tmp = tmp->parent_data;
-			kfree(irq_data);
-		}
+		__irq_domain_free_hierarchy(tmp);
 	}
 }
 
+/**
+ * irq_domain_trim_hierarchy - Trim the uninitialized part of a irq hierarchy
+ * @virq:	IRQ number to trim where the hierarchy is to be trimmed
+ *
+ * Drop the partial irq_data hierarchy from the level where the
+ * irq_data->chip is NULL.
+ *
+ * Its only use is to be able to trim levels of hierarchy that do not
+ * have any real meaning for this interrupt, and that the driver leaves
+ * uninitialized in its .alloc() callback.
+ */
+static void irq_domain_trim_hierarchy(unsigned int virq)
+{
+	struct irq_data *tail, *irq_data = irq_get_irq_data(virq);
+
+	/* It really needs to be a hierarchy, and not a single entry */
+	if (!irq_data->parent_data)
+		return;
+
+	/* Skip until we find a parent irq_data without a populated chip */
+	while (irq_data->parent_data && irq_data->parent_data->chip)
+		irq_data = irq_data->parent_data;
+
+	/* All levels populated */
+	if (!irq_data->parent_data)
+		return;
+
+	pr_info("IRQ%d: trimming hierarchy from %s\n",
+		virq, irq_data->parent_data->domain->name);
+
+	/* Sever the inner part of the hierarchy...  */
+	tail = irq_data->parent_data;
+	irq_data->parent_data = NULL;
+	__irq_domain_free_hierarchy(tail);
+}
+
+
 static int irq_domain_alloc_irq_data(struct irq_domain *domain,
 				     unsigned int virq, unsigned int nr_irqs)
 {
@@ -1362,8 +1406,10 @@ int __irq_domain_alloc_irqs(struct irq_domain *domain, int irq_base,
 		mutex_unlock(&irq_domain_mutex);
 		goto out_free_irq_data;
 	}
-	for (i = 0; i < nr_irqs; i++)
+	for (i = 0; i < nr_irqs; i++) {
+		irq_domain_trim_hierarchy(virq + i);
 		irq_domain_insert_irq(virq + i);
+	}
 	mutex_unlock(&irq_domain_mutex);
 
 	return virq;
-- 
2.28.0