Day 2: Conference: DevFS

Richard Gooch, being on his home turf came to tell us about DevFS, after it was finally included in the mainstream kernel.

[picture]

Linux so far has had major and minor numbers (8 bits each). Each driver can register one or more major number and use minor numbers for subdevices (like disks and partitions for a disk).

The problems are as follow:

Limits on both minor and major numbers, which can affect the number of disks or partitions for instance. The current scheme already prevents access to thousands of SCSI disks.
Numbering is done on the fly as hardware is detected, which has the unfortunate side effect of not providing a stable name for devices like SCSI disks. As a result, hot pluggability is an issue too.
Other problems are the number of entries that end up in /dev (2500+ entries on recent distributions) or you take the risk of not having the device node for hardware you need.
There is also a major table that's maintained by one person that you need to contact if you need a major number allocated to your driver.
Yet another problem is that it's difficult at best to enumerate devices of a certain kind (for instance all CD-Rom writers)

What you get with DevFS

it removes the device number indirection
it removes limits on number of devices
hot pluggability comes in for free
you can easily scan devices of a certain type
devices can show up in several places (USB and CD-Rom for a USB CD-Rom)
it works at boot time, which is handy for your boot drive
it works for read-only filesystems, even on a non Unix root filesystem
it preserves the "everything is a file" philosophy.
it's lightweight (72 bytes per entry) and provides a uniform API for drivers

Future work:

Volume name space
Physical name space
Hot plug user space device management
Overhaul block and SCSI layers
Use Dcache for devfs internal database

Common questions/comments from people:

Why not just pass device creation/removal events to a daemon?
It doesn't work before the kernel is fully booted
How about a better scsidev?
It doesn't help for anything else then SCSI devices or the root filesystem
Put /dev on a ramdisk
You need initrd, which makes the whole thing more complex, and you still need to register hardware to find out what nodes to create
Do nothing, there is no problem

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2001/01/27 (19:09): Version 1.0