device-dax: Add support for a dax override driver

Introduce the 'new_id' concept for enabling a custom device-driver attach
policy for dax-bus drivers. The intended use is to have a mechanism for
hot-plugging device-dax ranges into the page allocator on-demand. With
this in place the default policy of using device-dax for performance
differentiated memory can be overridden by user-space policy that can
arrange for the memory range to be managed as 'System RAM' with
user-defined NUMA and other performance attributes.

Signed-off-by: Dan Williams <[e-mail]>

drivers/dax/bus.c

···
 /* Copyright(c) 2017-2018 Intel Corporation. All rights reserved. */
 #include <linux/memremap.h>
 #include <linux/device.h>
+#include <linux/mutex.h>
+#include <linux/list.h>
 #include <linux/slab.h>
 #include <linux/dax.h>
 #include "dax-private.h"
 #include "bus.h"
+
+static DEFINE_MUTEX(dax_bus_lock);
+
+#define DAX_NAME_LEN 30
+struct dax_id {
+	struct list_head list;
+	char dev_name[DAX_NAME_LEN];
+};
 
 static int dax_bus_uevent(struct device *dev, struct kobj_uevent_env *env)
 {
···
 	return add_uevent_var(env, "MODALIAS=" DAX_DEVICE_MODALIAS_FMT, 0);
 }
 
+static struct dax_device_driver *to_dax_drv(struct device_driver *drv)
+{
+	return container_of(drv, struct dax_device_driver, drv);
+}
+
+static struct dax_id *__dax_match_id(struct dax_device_driver *dax_drv,
+		const char *dev_name)
+{
+	struct dax_id *dax_id;
+
+	lockdep_assert_held(&dax_bus_lock);
+
+	list_for_each_entry(dax_id, &dax_drv->ids, list)
+		if (sysfs_streq(dax_id->dev_name, dev_name))
+			return dax_id;
+	return NULL;
+}
+
+static int dax_match_id(struct dax_device_driver *dax_drv, struct device *dev)
+{
+	int match;
+
+	mutex_lock(&dax_bus_lock);
+	match = !!__dax_match_id(dax_drv, dev_name(dev));
+	mutex_unlock(&dax_bus_lock);
+
+	return match;
+}
+
+static ssize_t do_id_store(struct device_driver *drv, const char *buf,
+		size_t count, bool add)
+{
+	struct dax_device_driver *dax_drv = to_dax_drv(drv);
+	unsigned int region_id, id;
+	char devname[DAX_NAME_LEN];
+	struct dax_id *dax_id;
+	ssize_t rc = count;
+	int fields;
+
+	fields = sscanf(buf, "dax%d.%d", &region_id, &id);
+	if (fields != 2)
+		return -EINVAL;
+	sprintf(devname, "dax%d.%d", region_id, id);
+	if (!sysfs_streq(buf, devname))
+		return -EINVAL;
+
+	mutex_lock(&dax_bus_lock);
+	dax_id = __dax_match_id(dax_drv, buf);
+	if (!dax_id) {
+		if (add) {
+			dax_id = kzalloc(sizeof(*dax_id), GFP_KERNEL);
+			if (dax_id) {
+				strncpy(dax_id->dev_name, buf, DAX_NAME_LEN);
+				list_add(&dax_id->list, &dax_drv->ids);
+			} else
+				rc = -ENOMEM;
+		} else
+			/* nothing to remove */;
+	} else if (!add) {
+		list_del(&dax_id->list);
+		kfree(dax_id);
+	} else
+		/* dax_id already added */;
+	mutex_unlock(&dax_bus_lock);
+	return rc;
+}
+
+static ssize_t new_id_store(struct device_driver *drv, const char *buf,
+		size_t count)
+{
+	return do_id_store(drv, buf, count, true);
+}
+static DRIVER_ATTR_WO(new_id);
+
+static ssize_t remove_id_store(struct device_driver *drv, const char *buf,
+		size_t count)
+{
+	return do_id_store(drv, buf, count, false);
+}
+static DRIVER_ATTR_WO(remove_id);
+
+static struct attribute *dax_drv_attrs[] = {
+	&driver_attr_new_id.attr,
+	&driver_attr_remove_id.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(dax_drv);
+
 static int dax_bus_match(struct device *dev, struct device_driver *drv);
 
 static struct bus_type dax_bus_type = {
 	.name = "dax",
 	.uevent = dax_bus_uevent,
 	.match = dax_bus_match,
+	.drv_groups = dax_drv_groups,
 };
 
 static int dax_bus_match(struct device *dev, struct device_driver *drv)
 {
+	struct dax_device_driver *dax_drv = to_dax_drv(drv);
+
 	/*
-	 * The drivers that can register on the 'dax' bus are private to
-	 * drivers/dax/ so any device and driver on the bus always
-	 * match.
+	 * All but the 'device-dax' driver, which has 'match_always'
+	 * set, requires an exact id match.
 	 */
-	return 1;
+	if (dax_drv->match_always)
+		return 1;
+
+	return dax_match_id(dax_drv, dev);
 }
 
 /*
···
 }
 EXPORT_SYMBOL_GPL(devm_create_dev_dax);
 
-int __dax_driver_register(struct device_driver *drv,
+static int match_always_count;
+
+int __dax_driver_register(struct dax_device_driver *dax_drv,
 		struct module *module, const char *mod_name)
 {
+	struct device_driver *drv = &dax_drv->drv;
+	int rc = 0;
+
+	INIT_LIST_HEAD(&dax_drv->ids);
 	drv->owner = module;
 	drv->name = mod_name;
 	drv->mod_name = mod_name;
 	drv->bus = &dax_bus_type;
+
+	/* there can only be one default driver */
+	mutex_lock(&dax_bus_lock);
+	match_always_count += dax_drv->match_always;
+	if (match_always_count > 1) {
+		match_always_count--;
+		WARN_ON(1);
+		rc = -EINVAL;
+	}
+	mutex_unlock(&dax_bus_lock);
+	if (rc)
+		return rc;
 	return driver_register(drv);
 }
 EXPORT_SYMBOL_GPL(__dax_driver_register);
+
+void dax_driver_unregister(struct dax_device_driver *dax_drv)
+{
+	struct dax_id *dax_id, *_id;
+
+	mutex_lock(&dax_bus_lock);
+	match_always_count -= dax_drv->match_always;
+	list_for_each_entry_safe(dax_id, _id, &dax_drv->ids, list) {
+		list_del(&dax_id->list);
+		kfree(dax_id);
+	}
+	mutex_unlock(&dax_bus_lock);
+}
+EXPORT_SYMBOL_GPL(dax_driver_unregister);
 
 int __init dax_bus_init(void)
 {

drivers/dax/bus.h

···
 		struct resource *res, unsigned int align, unsigned long flags);
 struct dev_dax *devm_create_dev_dax(struct dax_region *dax_region, int id,
 		struct dev_pagemap *pgmap);
-int __dax_driver_register(struct device_driver *drv,
+
+struct dax_device_driver {
+	struct device_driver drv;
+	struct list_head ids;
+	int match_always;
+};
+
+int __dax_driver_register(struct dax_device_driver *dax_drv,
 		struct module *module, const char *mod_name);
 #define dax_driver_register(driver) \
 	__dax_driver_register(driver, THIS_MODULE, KBUILD_MODNAME)
+void dax_driver_unregister(struct dax_device_driver *dax_drv);
 void kill_dev_dax(struct dev_dax *dev_dax);
 
 /*

drivers/dax/device.c

···
 	return 0;
 }
 
-static struct device_driver device_dax_driver = {
-	.probe = dev_dax_probe,
-	.remove = dev_dax_remove,
+static struct dax_device_driver device_dax_driver = {
+	.drv = {
+		.probe = dev_dax_probe,
+		.remove = dev_dax_remove,
+	},
+	.match_always = 1,
 };
 
 static int __init dax_init(void)
···
 
 static void __exit dax_exit(void)
 {
-	driver_unregister(&device_dax_driver);
+	dax_driver_unregister(&device_dax_driver);
 }
 
 MODULE_AUTHOR("Intel Corporation");