Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 Rules on how to access information in the Linux kernel sysfs 2 3 The kernel-exported sysfs exports internal kernel implementation details 4 and depends on internal kernel structures and layout. It is agreed upon 5 by the kernel developers that the Linux kernel does not provide a stable 6 internal API. As sysfs is a direct export of kernel internal 7 structures, the sysfs interface cannot provide a stable interface either; 8 it may always change along with internal kernel changes. 9 10 To minimize the risk of breaking users of sysfs, which are in most cases 11 low-level userspace applications, with a new kernel release, the users 12 of sysfs must follow some rules to use an as-abstract-as-possible way to 13 access this filesystem. The current udev and HAL programs already 14 implement this and users are encouraged to plug, if possible, into the 15 abstractions these programs provide instead of accessing sysfs directly. 16 17 But if you really do want or need to access sysfs directly, please follow 18 the following rules and then your programs should work with future 19 versions of the sysfs interface. 20 21 - Do not use libsysfs 22 It makes assumptions about sysfs which are not true. Its API does not 23 offer any abstraction, it exposes all the kernel driver-core 24 implementation details in its own API. Therefore it is not better than 25 reading directories and opening the files yourself. 26 Also, it is not actively maintained, in the sense of reflecting the 27 current kernel development. The goal of providing a stable interface 28 to sysfs has failed; it causes more problems than it solves. It 29 violates many of the rules in this document. 30 31 - sysfs is always at /sys 32 Parsing /proc/mounts is a waste of time. Other mount points are a 33 system configuration bug you should not try to solve. For test cases, 34 possibly support a SYSFS_PATH environment variable to overwrite the 35 application's behavior, but never try to search for sysfs. Never try 36 to mount it, if you are not an early boot script. 37 38 - devices are only "devices" 39 There is no such thing like class-, bus-, physical devices, 40 interfaces, and such that you can rely on in userspace. Everything is 41 just simply a "device". Class-, bus-, physical, ... types are just 42 kernel implementation details which should not be expected by 43 applications that look for devices in sysfs. 44 45 The properties of a device are: 46 o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0) 47 - identical to the DEVPATH value in the event sent from the kernel 48 at device creation and removal 49 - the unique key to the device at that point in time 50 - the kernel's path to the device directory without the leading 51 /sys, and always starting with with a slash 52 - all elements of a devpath must be real directories. Symlinks 53 pointing to /sys/devices must always be resolved to their real 54 target and the target path must be used to access the device. 55 That way the devpath to the device matches the devpath of the 56 kernel used at event time. 57 - using or exposing symlink values as elements in a devpath string 58 is a bug in the application 59 60 o kernel name (sda, tty, 0000:00:1f.2, ...) 61 - a directory name, identical to the last element of the devpath 62 - applications need to handle spaces and characters like '!' in 63 the name 64 65 o subsystem (block, tty, pci, ...) 66 - simple string, never a path or a link 67 - retrieved by reading the "subsystem"-link and using only the 68 last element of the target path 69 70 o driver (tg3, ata_piix, uhci_hcd) 71 - a simple string, which may contain spaces, never a path or a 72 link 73 - it is retrieved by reading the "driver"-link and using only the 74 last element of the target path 75 - devices which do not have "driver"-link just do not have a 76 driver; copying the driver value in a child device context is a 77 bug in the application 78 79 o attributes 80 - the files in the device directory or files below subdirectories 81 of the same device directory 82 - accessing attributes reached by a symlink pointing to another device, 83 like the "device"-link, is a bug in the application 84 85 Everything else is just a kernel driver-core implementation detail 86 that should not be assumed to be stable across kernel releases. 87 88 - Properties of parent devices never belong into a child device. 89 Always look at the parent devices themselves for determining device 90 context properties. If the device 'eth0' or 'sda' does not have a 91 "driver"-link, then this device does not have a driver. Its value is empty. 92 Never copy any property of the parent-device into a child-device. Parent 93 device properties may change dynamically without any notice to the 94 child device. 95 96 - Hierarchy in a single device tree 97 There is only one valid place in sysfs where hierarchy can be examined 98 and this is below: /sys/devices. 99 It is planned that all device directories will end up in the tree 100 below this directory. 101 102 - Classification by subsystem 103 There are currently three places for classification of devices: 104 /sys/block, /sys/class and /sys/bus. It is planned that these will 105 not contain any device directories themselves, but only flat lists of 106 symlinks pointing to the unified /sys/devices tree. 107 All three places have completely different rules on how to access 108 device information. It is planned to merge all three 109 classification directories into one place at /sys/subsystem, 110 following the layout of the bus directories. All buses and 111 classes, including the converted block subsystem, will show up 112 there. 113 The devices belonging to a subsystem will create a symlink in the 114 "devices" directory at /sys/subsystem/<name>/devices. 115 116 If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be 117 ignored. If it does not exist, you have always to scan all three 118 places, as the kernel is free to move a subsystem from one place to 119 the other, as long as the devices are still reachable by the same 120 subsystem name. 121 122 Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or 123 /sys/block and /sys/class/block are not interchangeable is a bug in 124 the application. 125 126 - Block 127 The converted block subsystem at /sys/class/block or 128 /sys/subsystem/block will contain the links for disks and partitions 129 at the same level, never in a hierarchy. Assuming the block subsytem to 130 contain only disks and not partition devices in the same flat list is 131 a bug in the application. 132 133 - "device"-link and <subsystem>:<kernel name>-links 134 Never depend on the "device"-link. The "device"-link is a workaround 135 for the old layout, where class devices are not created in 136 /sys/devices/ like the bus devices. If the link-resolving of a 137 device directory does not end in /sys/devices/, you can use the 138 "device"-link to find the parent devices in /sys/devices/. That is the 139 single valid use of the "device"-link; it must never appear in any 140 path as an element. Assuming the existence of the "device"-link for 141 a device in /sys/devices/ is a bug in the application. 142 Accessing /sys/class/net/eth0/device is a bug in the application. 143 144 Never depend on the class-specific links back to the /sys/class 145 directory. These links are also a workaround for the design mistake 146 that class devices are not created in /sys/devices. If a device 147 directory does not contain directories for child devices, these links 148 may be used to find the child devices in /sys/class. That is the single 149 valid use of these links; they must never appear in any path as an 150 element. Assuming the existence of these links for devices which are 151 real child device directories in the /sys/devices tree is a bug in 152 the application. 153 154 It is planned to remove all these links when all class device 155 directories live in /sys/devices. 156 157 - Position of devices along device chain can change. 158 Never depend on a specific parent device position in the devpath, 159 or the chain of parent devices. The kernel is free to insert devices into 160 the chain. You must always request the parent device you are looking for 161 by its subsystem value. You need to walk up the chain until you find 162 the device that matches the expected subsystem. Depending on a specific 163 position of a parent device or exposing relative paths using "../" to 164 access the chain of parents is a bug in the application.
