Commit e105b8bf authored by Dan Williams's avatar Dan Williams Committed by Greg Kroah-Hartman
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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: default avatarKay Sievers <kay.sievers@vrfy.org>
Reviewed-by: default avatarSL Baur <steve@xemacs.org>
Acked-by: default avatarKay Sievers <kay.sievers@vrfy.org>
Acked-by: default avatarMark Lord <lkml@rtr.ca>
Acked-by: default avatarH. Peter Anvin <hpa@zytor.com>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent 93ded9b8

Documentation/ABI/testing/sysfs-dev

0 → 100644
+20 −0
Original line number Diff line number Diff line 
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>

Documentation/filesystems/sysfs.txt

+6 −0
Original line number Diff line number Diff line
@@ -248,6 +248,7 @@ The top level sysfs directory looks like: 
block/

bus/

class/

dev/

devices/

firmware/

net/
@@ -274,6 +275,11 @@ fs/ contains a directory for some filesystems. Currently each 
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/.

block/genhd.c

+4 −1
Original line number Diff line number Diff line
@@ -370,7 +370,10 @@ static struct kobject *base_probe(dev_t devt, int *part, void *data) 


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);

drivers/base/class.c

+4 −0
Original line number Diff line number Diff line
@@ -148,6 +148,10 @@ int class_register(struct class *cls) 
	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)

drivers/base/core.c

+82 −1
Original line number Diff line number Diff line
@@ -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)
@@ -775,6 +778,54 @@ int dev_set_name(struct device *dev, const char *fmt, ...) 
}

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 @@ int device_add(struct device *dev) 
		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);
@@ -872,6 +927,9 @@ int device_add(struct device *dev) 
 AttrsError:

	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:
@@ -948,8 +1006,10 @@ void device_del(struct device *dev) 
	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 @@ int __init devices_init(void) 
	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);
@@ -1447,4 +1525,7 @@ void device_shutdown(void) 
			dev->driver->shutdown(dev);

		}

	}

	kobject_put(sysfs_dev_char_kobj);

	kobject_put(sysfs_dev_block_kobj);

	kobject_put(dev_kobj);

}

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