sysfs: add /sys/dev/{char,block} to lookup sysfs path by major:minor

Why?: There are occasions where userspace would like to access sysfs attributes for a device but it may not know how sysfs has named the device or the path. For example what is the sysfs path for /dev/disk/by-id/ata-ST3160827AS_5MT004CK? With this change a call to stat(2) returns the major:minor then userspace can see that /sys/dev/block/8:32 links to /sys/block/sdc. What are the alternatives?: 1/ Add an ioctl to return the path: Doable, but sysfs is meant to reduce the need to proliferate ioctl interfaces into the kernel, so this seems counter productive. 2/ Use udev to create these symlinks: Also doable, but it adds a udev dependency to utilities that might be running in a limited environment like an initramfs. 3/ Do a full-tree search of sysfs. [kay.sievers@vrfy.org: fix duplicate registrations] [kay.sievers@vrfy.org: cleanup suggestions] Cc: Neil Brown <neilb@suse.de> Cc: Tejun Heo <htejun@gmail.com> Acked-by: Kay Sievers <kay.sievers@vrfy.org> Reviewed-by: SL Baur <steve@xemacs.org> Acked-by: Kay Sievers <kay.sievers@vrfy.org> Acked-by: Mark Lord <lkml@rtr.ca> Acked-by: H. Peter Anvin <hpa@zytor.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

sysfs: add /sys/dev/{char,block} to lookup sysfs path by major:minor
Why?: There are occasions where userspace would like to access sysfs attributes for a device but it may not know how sysfs has named the device or the path. For example what is the sysfs path for /dev/disk/by-id/ata-ST3160827AS_5MT004CK? With this change a call to stat(2) returns the major:minor then userspace can see that /sys/dev/block/8:32 links to /sys/block/sdc. What are the alternatives?: 1/ Add an ioctl to return the path: Doable, but sysfs is meant to reduce the need to proliferate ioctl interfaces into the kernel, so this seems counter productive. 2/ Use udev to create these symlinks: Also doable, but it adds a udev dependency to utilities that might be running in a limited environment like an initramfs. 3/ Do a full-tree search of sysfs. [kay.sievers@vrfy.org: fix duplicate registrations] [kay.sievers@vrfy.org: cleanup suggestions] Cc: Neil Brown <neilb@suse.de> Cc: Tejun Heo <htejun@gmail.com> Acked-by: Kay Sievers <kay.sievers@vrfy.org> Reviewed-by: SL Baur <steve@xemacs.org> Acked-by: Kay Sievers <kay.sievers@vrfy.org> Acked-by: Mark Lord <lkml@rtr.ca> Acked-by: H. Peter Anvin <hpa@zytor.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Dan Williams · Greg Kroah-Hartman
1 parent 93ded9b8fd
Showing 7 changed files with 124 additions and 2 deletions Side-by-side Diff
Documentation/ABI/testing/sysfs-dev
Documentation/filesystems/sysfs.txt
block/genhd.c
drivers/base/class.c
drivers/base/core.c
drivers/usb/core/devio.c
include/linux/device.h
+What:		/sys/dev
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Dan Williams <dan.j.williams@intel.com>
+Description:	The /sys/dev tree provides a method to look up the sysfs
+		path for a device using the information returned from
+		stat(2).  There are two directories, 'block' and 'char',
+		beneath /sys/dev containing symbolic links with names of
+		the form "<major>:<minor>".  These links point to the
+		corresponding sysfs path for the given device.
+
+		Example:
+		$ readlink /sys/dev/block/8:32
+		../../block/sdc
+
+		Entries in /sys/dev/char and /sys/dev/block will be
+		dynamically created and destroyed as devices enter and
+		leave the system.
+
+Users:		mdadm <linux-raid@vger.kernel.org>
@@ -248,6 +248,7 @@
 block/
 bus/
 class/
+dev/
 devices/
 firmware/
 net/
@@ -274,6 +275,11 @@
 filesystem wanting to export attributes must create its own hierarchy
 below fs/ (see ./fuse.txt for an example).
  
+dev/ contains two directories char/ and block/. Inside these two
+directories there are symlinks named <major>:<minor>.  These symlinks
+point to the sysfs directory for the given device.  /sys/dev provides a
+quick way to lookup the sysfs interface for a device from the result of
+a stat(2) operation.
  
 More information can driver-model specific features can be found in
 Documentation/driver-model/. 
@@ -370,7 +370,10 @@
  
 static int __init genhd_device_init(void)
 {
-	int error = class_register(&block_class);
+	int error;
+
+	block_class.dev_kobj = sysfs_dev_block_kobj;
+	error = class_register(&block_class);
 	if (unlikely(error))
 		return error;
 	bdev_map = kobj_map_init(base_probe, &block_class_lock);
@@ -148,6 +148,10 @@
 	if (error)
 		return error;
  
+	/* set the default /sys/dev directory for devices of this class */
+	if (!cls->dev_kobj)
+		cls->dev_kobj = sysfs_dev_char_kobj;
+
 #if defined(CONFIG_SYSFS_DEPRECATED) && defined(CONFIG_BLOCK)
 	/* let the block class directory show up in the root of sysfs */
 	if (cls != &block_class)
@@ -27,6 +27,9 @@
  
 int (*platform_notify)(struct device *dev) = NULL;
 int (*platform_notify_remove)(struct device *dev) = NULL;
+static struct kobject *dev_kobj;
+struct kobject *sysfs_dev_char_kobj;
+struct kobject *sysfs_dev_block_kobj;
  
 #ifdef CONFIG_BLOCK
 static inline int device_is_not_partition(struct device *dev)
@@ -776,6 +779,54 @@
 EXPORT_SYMBOL_GPL(dev_set_name);
  
 /**
+ * device_to_dev_kobj - select a /sys/dev/ directory for the device
+ * @dev: device
+ *
+ * By default we select char/ for new entries.  Setting class->dev_obj
+ * to NULL prevents an entry from being created.  class->dev_kobj must
+ * be set (or cleared) before any devices are registered to the class
+ * otherwise device_create_sys_dev_entry() and
+ * device_remove_sys_dev_entry() will disagree about the the presence
+ * of the link.
+ */
+static struct kobject *device_to_dev_kobj(struct device *dev)
+{
+	struct kobject *kobj;
+
+	if (dev->class)
+		kobj = dev->class->dev_kobj;
+	else
+		kobj = sysfs_dev_char_kobj;
+
+	return kobj;
+}
+
+static int device_create_sys_dev_entry(struct device *dev)
+{
+	struct kobject *kobj = device_to_dev_kobj(dev);
+	int error = 0;
+	char devt_str[15];
+
+	if (kobj) {
+		format_dev_t(devt_str, dev->devt);
+		error = sysfs_create_link(kobj, &dev->kobj, devt_str);
+	}
+
+	return error;
+}
+
+static void device_remove_sys_dev_entry(struct device *dev)
+{
+	struct kobject *kobj = device_to_dev_kobj(dev);
+	char devt_str[15];
+
+	if (kobj) {
+		format_dev_t(devt_str, dev->devt);
+		sysfs_remove_link(kobj, devt_str);
+	}
+}
+
+/**
  * device_add - add device to device hierarchy.
  * @dev: device.
  *
@@ -829,6 +880,10 @@
 		error = device_create_file(dev, &devt_attr);
 		if (error)
 			goto ueventattrError;
+
+		error = device_create_sys_dev_entry(dev);
+		if (error)
+			goto devtattrError;
 	}
  
 	error = device_add_class_symlinks(dev);
@@ -873,6 +928,9 @@
 	device_remove_class_symlinks(dev);
  SymlinkError:
 	if (MAJOR(dev->devt))
+		device_remove_sys_dev_entry(dev);
+ devtattrError:
+	if (MAJOR(dev->devt))
 		device_remove_file(dev, &devt_attr);
  ueventattrError:
 	device_remove_file(dev, &uevent_attr);
  
@@ -948,8 +1006,10 @@
 	device_pm_remove(dev);
 	if (parent)
 		klist_del(&dev->knode_parent);
-	if (MAJOR(dev->devt))
+	if (MAJOR(dev->devt)) {
+		device_remove_sys_dev_entry(dev);
 		device_remove_file(dev, &devt_attr);
+	}
 	if (dev->class) {
 		device_remove_class_symlinks(dev);
  
  
@@ -1074,7 +1134,25 @@
 	devices_kset = kset_create_and_add("devices", &device_uevent_ops, NULL);
 	if (!devices_kset)
 		return -ENOMEM;
+	dev_kobj = kobject_create_and_add("dev", NULL);
+	if (!dev_kobj)
+		goto dev_kobj_err;
+	sysfs_dev_block_kobj = kobject_create_and_add("block", dev_kobj);
+	if (!sysfs_dev_block_kobj)
+		goto block_kobj_err;
+	sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
+	if (!sysfs_dev_char_kobj)
+		goto char_kobj_err;
+
 	return 0;
+
+ char_kobj_err:
+	kobject_put(sysfs_dev_block_kobj);
+ block_kobj_err:
+	kobject_put(dev_kobj);
+ dev_kobj_err:
+	kset_unregister(devices_kset);
+	return -ENOMEM;
 }
  
 EXPORT_SYMBOL_GPL(device_for_each_child);
@@ -1447,5 +1525,8 @@
 			dev->driver->shutdown(dev);
 		}
 	}
+	kobject_put(sysfs_dev_char_kobj);
+	kobject_put(sysfs_dev_block_kobj);
+	kobject_put(dev_kobj);
 }
@@ -1792,6 +1792,11 @@
 		usb_classdev_class = NULL;
 		goto out;
 	}
+	/* devices of this class shadow the major:minor of their parent
+	 * device, so clear ->dev_kobj to prevent adding duplicate entries
+	 * to /sys/dev
+	 */
+	usb_classdev_class->dev_kobj = NULL;
  
 	usb_register_notify(&usbdev_nb);
 #endif
@@ -193,6 +193,7 @@
 	struct semaphore	sem; /* locks children, devices, interfaces */
 	struct class_attribute		*class_attrs;
 	struct device_attribute		*dev_attrs;
+	struct kobject			*dev_kobj;
  
 	int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);
  
@@ -205,6 +206,8 @@
 	struct pm_ops *pm;
 };
  
+extern struct kobject *sysfs_dev_block_kobj;
+extern struct kobject *sysfs_dev_char_kobj;
 extern int __must_check class_register(struct class *class);
 extern void class_unregister(struct class *class);
 extern int class_for_each_device(struct class *class, void *data,
	1	+What: /sys/dev
	2	+Date: April 2008
	3	+KernelVersion: 2.6.26
	4	+Contact: Dan Williams <dan.j.williams@intel.com>
	5	+Description: The /sys/dev tree provides a method to look up the sysfs
	6	+ path for a device using the information returned from
	7	+ stat(2). There are two directories, 'block' and 'char',
	8	+ beneath /sys/dev containing symbolic links with names of
	9	+ the form "<major>:<minor>". These links point to the
	10	+ corresponding sysfs path for the given device.
	11	+
	12	+ Example:
	13	+ $ readlink /sys/dev/block/8:32
	14	+ ../../block/sdc
	15	+
	16	+ Entries in /sys/dev/char and /sys/dev/block will be
	17	+ dynamically created and destroyed as devices enter and
	18	+ leave the system.
	19	+
	20	+Users: mdadm <linux-raid@vger.kernel.org>
...	...	@@ -248,6 +248,7 @@
248	248	block/
249	249	bus/
250	250	class/
	251	+dev/
251	252	devices/
252	253	firmware/
253	254	net/
...	...	@@ -274,6 +275,11 @@
274	275	filesystem wanting to export attributes must create its own hierarchy
275	276	below fs/ (see ./fuse.txt for an example).
276	277
	278	+dev/ contains two directories char/ and block/. Inside these two
	279	+directories there are symlinks named <major>:<minor>. These symlinks
	280	+point to the sysfs directory for the given device. /sys/dev provides a
	281	+quick way to lookup the sysfs interface for a device from the result of
	282	+a stat(2) operation.
277	283
278	284	More information can driver-model specific features can be found in
279	285	Documentation/driver-model/.
...	...	@@ -370,7 +370,10 @@
370	370
371	371	static int __init genhd_device_init(void)
372	372	{
373		- int error = class_register(&block_class);
	373	+ int error;
	374	+
	375	+ block_class.dev_kobj = sysfs_dev_block_kobj;
	376	+ error = class_register(&block_class);
374	377	if (unlikely(error))
375	378	return error;
376	379	bdev_map = kobj_map_init(base_probe, &block_class_lock);
...	...	@@ -148,6 +148,10 @@
148	148	if (error)
149	149	return error;
150	150
	151	+ /* set the default /sys/dev directory for devices of this class */
	152	+ if (!cls->dev_kobj)
	153	+ cls->dev_kobj = sysfs_dev_char_kobj;
	154	+
151	155	#if defined(CONFIG_SYSFS_DEPRECATED) && defined(CONFIG_BLOCK)
152	156	/* let the block class directory show up in the root of sysfs */
153	157	if (cls != &block_class)
...	...	@@ -27,6 +27,9 @@
27	27
28	28	int (platform_notify)(struct device dev) = NULL;
29	29	int (platform_notify_remove)(struct device dev) = NULL;
	30	+static struct kobject *dev_kobj;
	31	+struct kobject *sysfs_dev_char_kobj;
	32	+struct kobject *sysfs_dev_block_kobj;
30	33
31	34	#ifdef CONFIG_BLOCK
32	35	static inline int device_is_not_partition(struct device *dev)
...	...	@@ -776,6 +779,54 @@
776	779	EXPORT_SYMBOL_GPL(dev_set_name);
777	780
778	781	/**
	782	+ * device_to_dev_kobj - select a /sys/dev/ directory for the device
	783	+ * @dev: device
	784	+ *
	785	+ * By default we select char/ for new entries. Setting class->dev_obj
	786	+ * to NULL prevents an entry from being created. class->dev_kobj must
	787	+ * be set (or cleared) before any devices are registered to the class
	788	+ * otherwise device_create_sys_dev_entry() and
	789	+ * device_remove_sys_dev_entry() will disagree about the the presence
	790	+ * of the link.
	791	+ */
	792	+static struct kobject device_to_dev_kobj(struct device dev)
	793	+{
	794	+ struct kobject *kobj;
	795	+
	796	+ if (dev->class)
	797	+ kobj = dev->class->dev_kobj;
	798	+ else
	799	+ kobj = sysfs_dev_char_kobj;
	800	+
	801	+ return kobj;
	802	+}
	803	+
	804	+static int device_create_sys_dev_entry(struct device *dev)
	805	+{
	806	+ struct kobject *kobj = device_to_dev_kobj(dev);
	807	+ int error = 0;
	808	+ char devt_str[15];
	809	+
	810	+ if (kobj) {
	811	+ format_dev_t(devt_str, dev->devt);
	812	+ error = sysfs_create_link(kobj, &dev->kobj, devt_str);
	813	+ }
	814	+
	815	+ return error;
	816	+}
	817	+
	818	+static void device_remove_sys_dev_entry(struct device *dev)
	819	+{
	820	+ struct kobject *kobj = device_to_dev_kobj(dev);
	821	+ char devt_str[15];
	822	+
	823	+ if (kobj) {
	824	+ format_dev_t(devt_str, dev->devt);
	825	+ sysfs_remove_link(kobj, devt_str);
	826	+ }
	827	+}
	828	+
	829	+/**
779	830	* device_add - add device to device hierarchy.
780	831	* @dev: device.
781	832	*
...	...	@@ -829,6 +880,10 @@
829	880	error = device_create_file(dev, &devt_attr);
830	881	if (error)
831	882	goto ueventattrError;
	883	+
	884	+ error = device_create_sys_dev_entry(dev);
	885	+ if (error)
	886	+ goto devtattrError;
832	887	}
833	888
834	889	error = device_add_class_symlinks(dev);
...	...	@@ -873,6 +928,9 @@
873	928	device_remove_class_symlinks(dev);
874	929	SymlinkError:
875	930	if (MAJOR(dev->devt))
	931	+ device_remove_sys_dev_entry(dev);
	932	+ devtattrError:
	933	+ if (MAJOR(dev->devt))
876	934	device_remove_file(dev, &devt_attr);
877	935	ueventattrError:
878	936	device_remove_file(dev, &uevent_attr);
879	937
...	...	@@ -948,8 +1006,10 @@
948	1006	device_pm_remove(dev);
949	1007	if (parent)
950	1008	klist_del(&dev->knode_parent);
951		- if (MAJOR(dev->devt))
	1009	+ if (MAJOR(dev->devt)) {
	1010	+ device_remove_sys_dev_entry(dev);
952	1011	device_remove_file(dev, &devt_attr);
	1012	+ }
953	1013	if (dev->class) {
954	1014	device_remove_class_symlinks(dev);
955	1015
956	1016
...	...	@@ -1074,7 +1134,25 @@
1074	1134	devices_kset = kset_create_and_add("devices", &device_uevent_ops, NULL);
1075	1135	if (!devices_kset)
1076	1136	return -ENOMEM;
	1137	+ dev_kobj = kobject_create_and_add("dev", NULL);
	1138	+ if (!dev_kobj)
	1139	+ goto dev_kobj_err;
	1140	+ sysfs_dev_block_kobj = kobject_create_and_add("block", dev_kobj);
	1141	+ if (!sysfs_dev_block_kobj)
	1142	+ goto block_kobj_err;
	1143	+ sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
	1144	+ if (!sysfs_dev_char_kobj)
	1145	+ goto char_kobj_err;
	1146	+
1077	1147	return 0;
	1148	+
	1149	+ char_kobj_err:
	1150	+ kobject_put(sysfs_dev_block_kobj);
	1151	+ block_kobj_err:
	1152	+ kobject_put(dev_kobj);
	1153	+ dev_kobj_err:
	1154	+ kset_unregister(devices_kset);
	1155	+ return -ENOMEM;
1078	1156	}
1079	1157
1080	1158	EXPORT_SYMBOL_GPL(device_for_each_child);
...	...	@@ -1447,5 +1525,8 @@
1447	1525	dev->driver->shutdown(dev);
1448	1526	}
1449	1527	}
	1528	+ kobject_put(sysfs_dev_char_kobj);
	1529	+ kobject_put(sysfs_dev_block_kobj);
	1530	+ kobject_put(dev_kobj);
1450	1531	}
...	...	@@ -1792,6 +1792,11 @@
1792	1792	usb_classdev_class = NULL;
1793	1793	goto out;
1794	1794	}
	1795	+ /* devices of this class shadow the major:minor of their parent
	1796	+ * device, so clear ->dev_kobj to prevent adding duplicate entries
	1797	+ * to /sys/dev
	1798	+ */
	1799	+ usb_classdev_class->dev_kobj = NULL;
1795	1800
1796	1801	usb_register_notify(&usbdev_nb);
1797	1802	#endif
...	...	@@ -193,6 +193,7 @@
193	193	struct semaphore sem; /* locks children, devices, interfaces */
194	194	struct class_attribute *class_attrs;
195	195	struct device_attribute *dev_attrs;
	196	+ struct kobject *dev_kobj;
196	197
197	198	int (dev_uevent)(struct device dev, struct kobj_uevent_env *env);
198	199
...	...	@@ -205,6 +206,8 @@
205	206	struct pm_ops *pm;
206	207	};
207	208
	209	+extern struct kobject *sysfs_dev_block_kobj;
	210	+extern struct kobject *sysfs_dev_char_kobj;
208	211	extern int __must_check class_register(struct class *class);
209	212	extern void class_unregister(struct class *class);
210	213	extern int class_for_each_device(struct class class, void data,