fmc-eeprom: Maintain the reference count of the device.

With nvmem framework, the reference counter of the nvmem device is
increased when we ask the device from the kernel via helper functions
like "nvmem_device_find()", "nvmem_device_get()", etc. If reference
count is zero, the driver can be unloaded (rmmod) during ongoing
operations.

Thus, the reference count should be incremented during add, and
decremented during delete.

Reported-by: Federico Vaga <federico.vaga@cern.ch>
Signed-off-by: Vaibhav Gupta <vaibhav.gupta@cern.ch>

drivers/fmc/fmc-eeprom.c

@@ -115,29 +115,7 @@ ssize_t fmc_slot_eeprom_read(struct fmc_slot *slot,
 {
 	int err = 0;
 #if KERNEL_VERSION(4, 6, 0) <= LINUX_VERSION_CODE
-	size_t len;
-	struct nvmem_device *nvmem;
-	char *nvmem_parent_name;
-
-	len = strlen(dev_name(&slot->eeprom->dev)) + 1;
-	nvmem_parent_name = kzalloc(len, GFP_KERNEL);
-
-	if (nvmem_parent_name) {
-		snprintf(nvmem_parent_name, len, "%s",
-			 dev_name(&slot->eeprom->dev));
-		nvmem = nvmem_device_find(nvmem_parent_name,
-					  fmc_nvmem_device_find_match);
-		if (!IS_ERR_OR_NULL(nvmem)) {
-			err = nvmem_device_read(nvmem, offset, count, buf);
-#if KERNEL_VERSION(5, 1, 0) <= LINUX_VERSION_CODE
-			nvmem_device_put(nvmem);
-#endif
-		}
-		else {
-			err = PTR_ERR(nvmem);
-		}
-		kfree(nvmem_parent_name);
-	}
+	err = nvmem_device_read(slot->nvmem, offset, count, buf);
 #else
 	/*
 	 * TODO if we export this function, do we have to lock it when we
@@ -198,6 +176,20 @@ static int __fmc_slot_eeprom_add(struct fmc_slot *slot,
 		dev_err(&slot->dev, "Failed to create eeprom symlink to %s\n",
 			dev_name(&slot->eeprom->dev));
 
+#if KERNEL_VERSION(4, 6, 0) <= LINUX_VERSION_CODE
+	slot->nvmem_parent_name = kzalloc(strlen(dev_name(&slot->eeprom->dev)) + 1,
+					  GFP_KERNEL);
+	if (!slot->nvmem_parent_name)
+		return -ENOMEM;
+
+	snprintf(slot->nvmem_parent_name, strlen(dev_name(&slot->eeprom->dev)),
+		"%s", dev_name(&slot->eeprom->dev));
+	slot->nvmem = nvmem_device_find(slot->nvmem_parent_name,
+					fmc_nvmem_device_find_match);
+	if (IS_ERR_OR_NULL(slot->nvmem))
+		return PTR_ERR(slot->nvmem);
+#endif
+
 	return 0;
 }
 
@@ -226,6 +218,11 @@ int fmc_slot_eeprom_add(struct fmc_slot *slot)
  */
 void fmc_slot_eeprom_del(struct fmc_slot *slot)
 {
+#if KERNEL_VERSION(5, 1, 0) <= LINUX_VERSION_CODE
+	nvmem_device_put(slot->nvmem);
+#endif
+	kfree(slot->nvmem_parent_name);
+
 	if (!slot || !slot->eeprom)
 		return;
 

include/linux/fmc.h

@@ -89,6 +89,8 @@ struct fmc_slot {
 #if KERNEL_VERSION(4, 6, 0) <= LINUX_VERSION_CODE
 #define AT24_NUM_PROPERTIES 4
 	struct property_entry at24_data[AT24_NUM_PROPERTIES];
+	struct nvmem_device *nvmem;
+	char *nvmem_parent_name;
 #else
 	struct memory_accessor *macc;
 	struct at24_platform_data at24_data;