RFC: Linux Device Registry
==========================
Summary
-------
The device registry provides a /proc file in text format that contains
a list of all connected physical devices. Each list entry
includes (as far as available) the name and vendor of the device, its
location/topological information, the device files (/dev) that the
driver(s) have registered and a unique DeviceID that makes it possible to
find the device after a reboot.
An experimental patch for 2.4.2 can be found at http://www.tjansen.de/devreg
Reasons for the Device Registry
-------------------------------
Finding the available devices with the current Linux kernel is difficult
and not always possible. For example the only way to get all printers on
Linux is to open all devices in /dev that could represent one, but
this has some limitations. The first problem is that the user-space app must
know which files should be probed. The /dev/lpX files are candidates
and for USB printers also /dev/usb/lpX. Everytime a new bus for printers
is added there is a new range of device files that must be probed, for
example for IEEE1394/FireWire printers user-space apps would have to probe
the /dev/ieee1394/lpX (or whatever their names will be). This makes it
impossible to write forward-compatible user-space applications.
Even if the application knows which devices are actual printers they
must be presented to the user so that he can select one of them,
and for this things like the printers name and vendor are neccessary.
Unfortunately every bus has a different facility to provide these
information. Parallel ports expose them in /proc/sys/dev/parport; USB
shows them in usbdevfs, but it is not possible to figure out which
/dev/usb/lpX file belongs to which physical printer. And as IEEE1394 will
have to implement another method to get information about the printer
device, user-apps today have no chance to get show them when kernel support is
ready.
The next problem is associated with hotplugging. If you have two printers
attached to the USB bus, say an Epson and a HP printer, you dont know
their device files. If you turn on the Epson printer and then the HP printer,
the Epson will be /dev/usb/lp0 and the HP /dev/usb/lp1. When you first
power on the HP on the next reboot (or turn off the Epson before you turn the
HP on) it will be on /dev/usb/lp0.
Sometimes devices have several independent interfaces that belong together.
For example there are USB scanners with buttons that are intended to trigger
actions by user-space applications like starting a scan. The USB driver uses
two separate drivers for them, in the case of a scanner it would use a HID
driver (there is a patch available for non-keyboards/mice/joystick input
devices) and a regular scanner driver. A similar setup is also common for USB
audio devices.
With the current implementation it is not possible to find out which device
files represent which physical device.
Solutions provided by the Device Registry
-----------------------------------------
The Device Registry (DevReg) puts a text list of all physical devices
into /proc/devreg. Each entry consists of:
- the model and vendor name of the device (if available)
- the type of the device (for example hub/usb)
- the bus/location of the device (for example COM2 or USB)
- topology information (to build a tree of all devices)
- the unique DeviceID to find the device after a reboot
- the device files (if devfs is used) and the major/minor number of
the device files and the interface it implements (for example OSS or V4L)
This should be sufficient for the following use cases:
- find all devices that implement a interface (for example
all OSS devices) and present them to the user he can chose one
- get the DeviceID of a device so its device file can be found after a
reboot
- find a device file for a DeviceID
- show all devices in the computer
Finding a device with a DeviceID instead of a device file name is neither
supported by current applications nor by other Unix-like operating
systems. Therefore, to stay compatible with existing software, a user-space
daemon could take care of creating stable names for devices.
For example, if you have the Epson and HP printer from the example above,
the daemon could create two symlinks called /dev/lpEpson and /dev/lpHP that
always point to the correct device file as long as the physical device is
connected.
As the format of the /proc/devreg file is relatively complex a user-space
library that implements the use cases given above will be provided.
DeviceID
--------
Each device get a unique device id. The id is a string in the format
"bus-type/location/model/serialnum". bus-type is "usb" or "pci"
for the currently supported busses. location is
"busnumber:slotnumber" for PCI devices and
"busnumber[:1st-hub-model:1st-hub-vendor[:2nd-hub-model:2nd-hub-vendor..]]"
for USB devices. The format of the model is "vendorID:deviceID" for
both USB and PCI. The serial number is driver and device specific,
ethernet adapters would take their hardware address here. For
those devices that cannot be identified the field should be empty.
When finding devices the app should try to get an exact match first.
If this is not possible, it should try to find a match with
identical model and serial number but different location.
Device Type
-----------
To display the devices for the user an identification of the type (or class)
of the device is needed. The format is "base-type[/sub-type]", for example
"network-adapter/ethernet" or "controller/scsi". The classes should
be managed centrally to avoid duplicate entries.
The device type is intentionally simple and does not reflect
all possible devices. This makes it possible to provide icons for all
device types, but also prevents uncommon devices from being categorized.
For example an integrated keyboard and pointing device could only be
represented as a generic input device (unless they get really popular and a
they new class is created for them). Therefore an application that
tries to find a device should always search for interfaces and not for
the device types.
Subdevices
----------
A device can have subdevices. This is useful for multi-functional devices,
for example an integrated USB and FireWire controller. In this case there
is a list entry for the card itself with the device-type "multi-functional".
The device files are associated with the sub-device, not with the super
device.
Device File Interface
---------------------
To describe the interface of a device file a simple string in the
format "category/apiname/devicename" is used. The devicename field
is empty if the API only uses a single file per device.
For example, the OSS sound API would use the string "sound/oss/dsp"
for its "/dev/dsp" file. ALSA device files start with "sound/alsa".
Text file format
----------------
Each device in the file format consists of several lines in the format
"category/field-name=value". There is one value per line, the devices
are separated by blank lines. User-space apps must ignore those fields
that they don't know.
Fields, one per device in this order:
common/number=<DEVICE-NUMBER>
a number of the device for this list. Required.
common/parent=<PARENT-NUMBER>
the number of the device's topological parent. This is intended
for busses like USB that use hubs (see also common/superdevice).
Required for all devices that are not at the top-level of their bus
or subdevices.
common/superdevice=<SUPERDEVICE-NUMBER>
the number of the devies superdevice. This is intended for
multi-functional devices, for example an integrated FireWire and USB
adapter. Required for subdevices.
common/bus=<BUS-NAME>
the bus of the device, e.g. usb, pci or parport. Required.
common/type=<TYPE-NAME>
the type of the device, e.g. network-adapter/ethernet, hub/usb,
repeater/firewire, controller/usb. See above. Required.
common/deviceid=<DEVICE/ID>
the DeviceID as described above. Required.
common/name=<MODEL-NAME>
the device's name. Can come from the device (USB) or from a
database in the kernel (PCI). Required.
common/vendor=<VENDOR-NAME>
the device's vendor. Can come from the device (USB) or from a
database in the kernel (PCI). Optional.
common/location=<LOCATION>
the location of the device. For example USB, PCI, AGP or COM2.
Required.
common/internal=(true|false)
if true it is an internal device. Optional.
After the common lines there can be fields for other categories.
For example devices on the PCI bus could have fields that contain the
model and vendor id. The name of a field that contains the model id would
be "pci/modelid". This could eventually replace the bus-specific file
systems on /proc/bus.
At the end of device entry follow the lines that list the device files.
For each device files there must be three or four fields in the following
order:
devfile/interface=<DEVICE-FILE-INTERFACE>
The interface that the device file implements, see above. Required.
devfile/name=<DEVICE-FILENAME>
The file name of the device relative to the device directory (in
other word, without the leading '/dev'). Only if devfs is available.
devfile/major=<MAJOR-NUM>
The major number of the device. Required.
devfile/minor=<MINOR-NUM>
The minor number of the device. Required.
Alternative representations
---------------------------
Other representations of the device registry are possible. If desired a binary
format could be implemented using a (R)IFF-like, chunk-based format. However
parsing it would not be significantly easier than the text-based format as it
is not possible to represent the data of a list entry with a simple struct.
Another alternative is to use a file per device in a directory-structure.
With the right structure, for example a directory per interface, this could
improve the scalability of certain user-space operations on machines with a
very large number of devices, but it would also increase complexity
significantly. Optimal performance and scalability could be archived by
implementing ioctl search functions.
Implementation
--------------
The patch can be downloaded from
http://www.tjansen.de/devreg/devreg-2.4.2-0.1.patch.gz . Just apply it,
then select CONFIG_DEVREG (in the general section). CONFIG_PROCFS_FS must
also be enabled, and using CONFIG_DEVFS_FS is recommended but not required.
Be careful with modules, the patch breaks a few interfaces so that some old
modules will not work anymore. Either compile without any (important)
modules, or change the kernel version in the top-level Makefile.
After compilation, reboot and you should see the /proc/devreg file.
The patch currently supports only PCI and USB devices. The following drivers
will register types and/or device files:
hid/USB, hub/USB, emu10k1/PCI.
The following PCI drivers should work, but are untested (all sound cards):
via82cxxx, trident, sonicvibes, maestro3, maestro, i810_audio, esssolo1,
es1371, es1370, cs46xx, cs4281
You can find the kernel API, a preview of the user-space API and some other
stuff on http://www.tjansen.de/devreg
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This archive was generated by hypermail 2b29 : Fri Mar 23 2001 - 21:00:09 EST