firmware: raspberrypi: Introduce rpi_firmware_put()

When unbinding the firmware device we need to make sure it has no
consumers left. Otherwise we'd leave them with a firmware handle
pointing at freed memory.

Keep a reference count of all consumers and make sure they all finished
unbinding before we do.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

drivers/firmware/raspberrypi.c

@@ -11,7 +11,9 @@
 #include <linux/module.h>
 #include <linux/of_platform.h>
 #include <linux/platform_device.h>
+#include <linux/refcount.h>
 #include <linux/slab.h>
+#include <linux/wait.h>
 #include <soc/bcm2835/raspberrypi-firmware.h>
 
 #define MBOX_MSG(chan, data28)		(((data28) & ~0xf) | ((chan) & 0xf))
@@ -27,6 +29,9 @@ struct rpi_firmware {
 	struct mbox_chan *chan; /* The property channel. */
 	struct completion c;
 	u32 enabled;
+
+	refcount_t consumers;
+	wait_queue_head_t wait;
 };
 
 static DEFINE_MUTEX(transaction_lock);
@@ -247,6 +252,8 @@ static int rpi_firmware_probe(struct platform_device *pdev)
 	}
 
 	init_completion(&fw->c);
+	refcount_set(&fw->consumers, 1);
+	init_waitqueue_head(&fw->wait);
 
 	platform_set_drvdata(pdev, fw);
 
@@ -275,6 +282,8 @@ static int rpi_firmware_remove(struct platform_device *pdev)
 	rpi_hwmon = NULL;
 	platform_device_unregister(rpi_clk);
 	rpi_clk = NULL;
+
+	wait_event(fw->wait, refcount_dec_if_one(&fw->consumers));
 	mbox_free_channel(fw->chan);
 
 	return 0;
@@ -289,14 +298,33 @@ static int rpi_firmware_remove(struct platform_device *pdev)
 struct rpi_firmware *rpi_firmware_get(struct device_node *firmware_node)
 {
 	struct platform_device *pdev = of_find_device_by_node(firmware_node);
+	struct rpi_firmware *fw;
 
 	if (!pdev)
 		return NULL;
 
-	return platform_get_drvdata(pdev);
+	fw = platform_get_drvdata(pdev);
+	if (!fw)
+		return NULL;
+
+	if (!refcount_inc_not_zero(&fw->consumers))
+		return NULL;
+
+	return fw;
 }
 EXPORT_SYMBOL_GPL(rpi_firmware_get);
 
+/**
+ * rpi_firmware_put - Put pointer to rpi_firmware structure.
+ * @rpi_firmware:    Pointer to struct rpi_firmware
+ */
+void rpi_firmware_put(struct rpi_firmware *fw)
+{
+	refcount_dec(&fw->consumers);
+	wake_up(&fw->wait);
+}
+EXPORT_SYMBOL_GPL(rpi_firmware_put);
+
 static const struct of_device_id rpi_firmware_of_match[] = {
 	{ .compatible = "raspberrypi,bcm2835-firmware", },
 	{},

include/soc/bcm2835/raspberrypi-firmware.h

@@ -141,6 +141,7 @@ int rpi_firmware_property(struct rpi_firmware *fw,
 int rpi_firmware_property_list(struct rpi_firmware *fw,
 			       void *data, size_t tag_size);
 struct rpi_firmware *rpi_firmware_get(struct device_node *firmware_node);
+void rpi_firmware_put(struct rpi_firmware *fw);
 #else
 static inline int rpi_firmware_property(struct rpi_firmware *fw, u32 tag,
 					void *data, size_t len)
@@ -158,6 +159,8 @@ static inline struct rpi_firmware *rpi_firmware_get(struct device_node *firmware
 {
 	return NULL;
 }
+
+void rpi_firmware_put(struct rpi_firmware *fw) { }
 #endif
 
 #endif /* __SOC_RASPBERRY_FIRMWARE_H__ */