[PATCH v5 1/3] firmware: arm_scmi: support only one single SystemPower device

[Cristian Marussi <cristian.marussi@arm.com>]

In order to minimize SCMI platform fw-side complexity, only one platform
should be in charge of SCMI SystemPower protocol communications with the
OSPM: enforce the existence of one single unique device associated with
SystemPower protocol across any possible number of SCMI platforms, and
warn if a system tries to register different SystemPower devices from
multiple platforms.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 drivers/firmware/arm_scmi/bus.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/firmware/arm_scmi/bus.c b/drivers/firmware/arm_scmi/bus.c
index 88149a46e6d9..e8542a7e8862 100644
--- a/drivers/firmware/arm_scmi/bus.c
+++ b/drivers/firmware/arm_scmi/bus.c
@@ -19,6 +19,9 @@ static DEFINE_IDA(scmi_bus_id);
 static DEFINE_IDR(scmi_available_protocols);
 static DEFINE_SPINLOCK(protocol_lock);
 
+/* Track globally the creation of SCMI SystemPower related devices */
+static bool scmi_syspower_registered;
+
 static const struct scmi_device_id *
 scmi_dev_match_id(struct scmi_device *scmi_dev, struct scmi_driver *scmi_drv)
 {
@@ -175,6 +178,23 @@ scmi_device_create(struct device_node *np, struct device *parent, int protocol,
 	int id, retval;
 	struct scmi_device *scmi_dev;
 
+	/*
+	 * Check if any SCMI SystemPower device was already created.
+	 *
+	 * Note that, shared global @scmi_syspower_registered is not protected
+	 * by a mutex since:
+	 *  - scmi_device_create() is not concurrently invoked by the SCMI core
+	 *  - scmi_device_destroy() is effectively called only upon SCMI core
+	 *    unloading/unbinding, so no race is either possible between create
+	 *    and destroy.
+	 */
+	if (protocol == SCMI_PROTOCOL_SYSTEM && scmi_syspower_registered) {
+		dev_warn(parent,
+			 "SCMI SystemPower protocol device must be unique !\n");
+		dump_stack();
+		return NULL;
+	}
+
 	scmi_dev = kzalloc(sizeof(*scmi_dev), GFP_KERNEL);
 	if (!scmi_dev)
 		return NULL;
@@ -204,6 +224,9 @@ scmi_device_create(struct device_node *np, struct device *parent, int protocol,
 	if (retval)
 		goto put_dev;
 
+	if (protocol == SCMI_PROTOCOL_SYSTEM)
+		scmi_syspower_registered = true;
+
 	return scmi_dev;
 put_dev:
 	kfree_const(scmi_dev->name);
@@ -218,6 +241,8 @@ void scmi_device_destroy(struct scmi_device *scmi_dev)
 	scmi_handle_put(scmi_dev->handle);
 	ida_simple_remove(&scmi_bus_id, scmi_dev->id);
 	device_unregister(&scmi_dev->dev);
+	if (scmi_dev->protocol_id == SCMI_PROTOCOL_SYSTEM)
+		scmi_syspower_registered = false;
 }
 
 void scmi_set_handle(struct scmi_device *scmi_dev)
-- 
2.17.1