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103 lines
4.4 KiB
Plaintext
103 lines
4.4 KiB
Plaintext
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This is a proposal from the early days of udev. A lot of the things
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mentioned here are implemented differently and most of the features
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implemented in udev are not part of that document.
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This document is only kept in the tree for sentimental reasons.
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-------------------------------------------------------------------------
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We've got a couple of goals for udev:
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1) dynamic replacement for /dev
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2) device naming
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3) API to access info about current system devices
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Splitting these goals into separate subsystems:
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1) udev - dynamic replacement for /dev
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2) namedev - device naming
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3) libsysfs - a standard library for accessing device information on the
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system.
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Udev
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------
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Udev will be responsible for responding to /sbin/hotplug on device
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events. It will receive the device class information along with
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device's sysfs directory. Udev will call the name_device function from
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the naming device subsystem with that information and receive a unique
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device name in return. Udev will then query sysfs through the libsysfs
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for specific device information required for creating the /dev node like
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major and minor number. Once it has the important information, udev
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will create a /dev entry for the device, add the device to the in memory
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table of current devices, and send notification of the successful event
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through a D-BUS message. On a remove call, udev will remove the /dev
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entry, remove the device from the in memory table, and send
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notification.
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Udev will consist of a command udev - to be called from /sbin/hotplug.
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It will require the in memory dynamic database/table for keeping track
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of current system devices, and a library of routines for accessing that
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database/table. Udev will not care about "how" devices are named, that
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will be separated into the device naming subsystem. It's presented a
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common device naming API by the device naming subsystem to use for
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naming devices.
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namedev
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----------
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From comments people have made, the device naming part of udev has been
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pushed into its own "subsystem". The reason is to make this as flexible
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and pluggable as possible. The device naming subsystem, or namedev, will
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present a standard interface for udev to call for naming a particular
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device. Under that interface, system administrators can plug in their
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own methods for device naming.
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We would provide a default naming scheme. The first prototype
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implementation could simply take the sysfs directory passed in with the
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device name function, query sysfs for the major and minor numbers, and
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then look up in a static device name mapping file the name of the
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device. The static device naming file could look just like devices.txt
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in the Linux kernel's Documentation directory. Obviously, this isn't a
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great implementation because eventually we'd like major an minor numbers
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to be dynamic.
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The default naming scheme in the future would have a set of policies to
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go through in order to determine the name of the device. The device
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naming subsystem would get the sysfs directory of the to be named device
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and would use the following information in order to map the device's
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name:
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1) Label info - like SCSI's UUID
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2) Bus Device Number
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3) Topology on Bus
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4) Kernel Name - DEFAULT
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System administrators could use the default naming system or enterprise
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computing environments could plug in their Universal Unique Identifier
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(UUID) policies. The idea is to make the device naming as flexible and
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pluggable as possible.
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The device naming subsystem would require accessing sysfs for device
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information. It will receive the device's sysfs directory in the call
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from udev and use it to get more information to determine naming. The
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namedev subsystem will include a standard naming API for udev to use.
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The default naming scheme will include a set of functions and a static
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device naming file, which will reside in /etc or /var.
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libsysfs
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--------
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There is a need for a common API to access device information in sysfs.
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The device naming subsystem and the udev subsystem need to take the
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sysfs directory path and query device information. Instead of copying
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code so each one will have to readdir, etc., splitting this logic of
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sysfs calls into a separate library that will sit atop sysfs makes more
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sense. Sysfs callbacks aren't standard across devices, so this is
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another reason for creating a common and standard library interface for
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querying device information.
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