[RFC PATCH v2 01/15] of: add support for retrieving cpu node for a given logical cpu index

[Sudeep.KarkadaNagesha@arm.com]

From: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>

Currently different drivers requiring to access cpu device node are
parsing the device tree themselves. Since the ordering in the DT need
not match the logical cpu ordering, the parsing logic needs to consider
that. However, this has resulted in lots of code duplication and in some
cases even incorrect logic.

It's better to consolidate them by adding support for getting cpu
device node for a given logical cpu index in DT core library. However
logical to physical index mapping can be architecture specific.

This patch adds of_get_cpu_node to retrieve a cpu device node for a
given logical cpu index. The default matching of the physical id to the
logical cpu index can be overridden by architecture specific code.

It is recommended to use these helper function only in pre-SMP/early
initialisation stages to retrieve CPU device node pointers in logical
ordering. Once the cpu devices are registered, it can be retrieved easily
from cpu device of_node which avoids unnecessary parsing and matching.

Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
---
 drivers/of/base.c  | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/of.h |  5 +++++
 2 files changed, 71 insertions(+)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 5c54279..363b8f9 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -230,6 +230,72 @@ const void *of_get_property(const struct device_node *np, const char *name,
 }
 EXPORT_SYMBOL(of_get_property);
 
+/*
+ * arch_match_cpu_phys_id - Match the given logical CPU and physical id
+ *
+ * @cpu: logical index of a cpu
+ * @phys_id: physical identifier of a cpu
+ *
+ * CPU logical to physical index mapping is architecure specific.
+ * However this __weak function provides a default match of physical
+ * id to logical cpu index.
+ *
+ * Returns 1 if the physical identifier and the logical index correspond
+ * to the same cpu, 0 otherwise.
+ */
+int __weak arch_match_cpu_phys_id(int cpu, u64 phys_id)
+{
+	return (u32)phys_id == cpu;
+}
+
+/**
+ * of_get_cpu_node - Get device node associated with the given logical CPU
+ *
+ * @cpu: CPU number(logical index) for which device node is required
+ *
+ * The main purpose of this function is to retrieve the device node for the
+ * given logical CPU index. It should be used to intialise the of_node in
+ * cpu device. Once of_node in cpu device is populated, all the further
+ * references can use that instead.
+ *
+ * CPU logical to physical index mapping is architecure specific and is built
+ * before booting secondary cores. This function uses arch_match_cpu_phys_id
+ * which can be overridden by architecture specific implementation.
+ *
+ * Returns a node pointer for the logical cpu if found, else NULL.
+ */
+struct device_node *of_get_cpu_node(int cpu)
+{
+	struct device_node *cpun, *cpus;
+	const u32 *cell;
+	u64 hwid;
+	int ac, prop_len;
+
+	cpus = of_find_node_by_path("/cpus");
+	if (WARN(!cpus, "Missing cpus node, bailing out\n"))
+		return NULL;
+
+	if (WARN_ON(of_property_read_u32(cpus, "#address-cells", &ac)))
+		ac = of_n_addr_cells(cpus);
+
+	for_each_child_of_node(cpus, cpun) {
+		if (of_node_cmp(cpun->type, "cpu"))
+			continue;
+		cell = of_get_property(cpun, "reg", &prop_len);
+		if (WARN(!cell, "%s: missing reg property\n", cpun->full_name))
+			continue;
+
+		while (prop_len) {
+			hwid = of_read_number(cell, ac);
+			prop_len -= ac;
+			if (arch_match_cpu_phys_id(cpu, hwid))
+				return cpun;
+		}
+	}
+
+	return NULL;
+}
+
 /** Checks if the given "compat" string matches one of the strings in
  * the device's "compatible" property
  */
diff --git a/include/linux/of.h b/include/linux/of.h
index 1fd08ca..150b48e 100644
--- a/include/linux/of.h
+++ b/include/linux/of.h
@@ -266,6 +266,7 @@ extern int of_device_is_available(const struct device_node *device);
 extern const void *of_get_property(const struct device_node *node,
 				const char *name,
 				int *lenp);
+extern struct device_node *of_get_cpu_node(int cpu);
 #define for_each_property_of_node(dn, pp) \
 	for (pp = dn->properties; pp != NULL; pp = pp->next)
 
@@ -458,6 +459,10 @@ static inline const void *of_get_property(const struct device_node *node,
 {
 	return NULL;
 }
+static inline struct device_node *of_get_cpu_node(int cpu)
+{
+	return NULL;
+}
 
 static inline int of_property_read_u64(const struct device_node *np,
 				       const char *propname, u64 *out_value)
-- 
1.8.1.2