Subject: sysfs: add /sys/dev/{char,block} to lookup sysfs path by major:minor
From: Dan Williams <dan.j.williams@xxxxxxxxx>
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.
Cc: NeilBrown <neilb@xxxxxxx>
Cc: Tejun Heo <htejun@xxxxxxxxx>
Cc: Kay Sievers <kay.sievers@xxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxx>
Acked-by: Mark Lord <lkml@xxxxxx>
Acked-by: H. Peter Anvin <hpa@xxxxxxxxx>
Reviewed-by: SL Baur <steve@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Andrew here is an updated patch with some presumptive acked-by's from Mark and hpa.
* fixed up ENOMEM handling in devices_init()
* added a short blurb in Documentation/filesystems/sysfs.txt
* dropped the size of the buffer passed to format_dev_t a bit
Documentation/filesystems/sysfs.txt | 6 +++++
drivers/base/core.c | 46 ++++++++++++++++++++++++++++++++++-
2 files changed, 51 insertions(+), 1 deletions(-)
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 @@ 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/.
diff --git a/drivers/base/core.c b/drivers/base/core.c
index 24198ad..ba21118 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -27,6 +27,9 @@
int (*platform_notify)(struct device *dev) = NULL;
int (*platform_notify_remove)(struct device *dev) = NULL;
+static struct kobject *dev_kobj;
+static struct kobject *char_kobj;
+static struct kobject *block_kobj;
#ifdef CONFIG_BLOCK
static inline int device_is_not_partition(struct device *dev)
@@ -759,6 +762,11 @@ static void device_remove_class_symlinks(struct device *dev)
sysfs_remove_link(&dev->kobj, "subsystem");
}
+static struct kobject *device_to_dev_kobj(struct device *dev)
+{
+ return dev->class == &block_class ? block_kobj : char_kobj;
+}
+
/**
* device_add - add device to device hierarchy.
* @dev: device.
@@ -775,6 +783,7 @@ int device_add(struct device *dev)
struct device *parent = NULL;
struct class_interface *class_intf;
int error;
+ char devt_str[15];
dev = get_device(dev);
if (!dev || !strlen(dev->bus_id)) {
@@ -806,9 +815,16 @@ int device_add(struct device *dev)
goto attrError;
if (MAJOR(dev->devt)) {
+ struct kobject *kobj = device_to_dev_kobj(dev);
+
error = device_create_file(dev, &devt_attr);
if (error)
goto ueventattrError;
+
+ format_dev_t(devt_str, dev->devt);
+ error = sysfs_create_link(kobj, &dev->kobj, devt_str);
+ if (error)
+ goto devtattrError;
}
error = device_add_class_symlinks(dev);
@@ -854,6 +870,9 @@ int device_add(struct device *dev)
device_remove_class_symlinks(dev);
SymlinkError:
if (MAJOR(dev->devt))
+ sysfs_remove_link(device_to_dev_kobj(dev), devt_str);
+ devtattrError:
+ if (MAJOR(dev->devt))
device_remove_file(dev, &devt_attr);
ueventattrError:
device_remove_file(dev, &uevent_attr);
@@ -925,12 +944,16 @@ void device_del(struct device *dev)
{
struct device *parent = dev->parent;
struct class_interface *class_intf;
+ char devt_str[15];
device_pm_remove(dev);
if (parent)
klist_del(&dev->knode_parent);
- if (MAJOR(dev->devt))
+ if (MAJOR(dev->devt)) {
+ format_dev_t(devt_str, dev->devt);
+ sysfs_remove_link(device_to_dev_kobj(dev), devt_str);
device_remove_file(dev, &devt_attr);
+ }
if (dev->class) {
device_remove_class_symlinks(dev);
@@ -1055,7 +1078,25 @@ 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;
+ block_kobj = kobject_create_and_add("block", dev_kobj);
+ if (!block_kobj)
+ goto block_kobj_err;
+ char_kobj = kobject_create_and_add("char", dev_kobj);
+ if (!char_kobj)
+ goto char_kobj_err;
+
return 0;
+
+ char_kobj_err:
+ kobject_put(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);
@@ -1380,4 +1421,7 @@ void device_shutdown(void)
dev->driver->shutdown(dev);
}
}
+ kobject_put(char_kobj);
+ kobject_put(block_kobj);
+ kobject_put(dev_kobj);
}
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