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>
diff --git a/Documentation/ABI/testing/sysfs-dev b/Documentation/ABI/testing/sysfs-dev
new file mode 100644
index 0000000..a9f2b8b
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-dev
@@ -0,0 +1,20 @@
+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>
diff --git a/Documentation/filesystems/sysfs.txt b/Documentation/filesystems/sysfs.txt
index 7f27b8f..9e9c348 100644
--- a/Documentation/filesystems/sysfs.txt
+++ b/Documentation/filesystems/sysfs.txt
@@ -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/.