Is life without udev possible?

gearheadgeek · 12-17-2014, 08:48 AM

I'm not really a luddite - it is just that udev does nothing useful in my application and instead causes problems. Pat implies it it possible "with some tweaks". Does anyone know what those tweaks might be?

genss · 12-17-2014, 08:51 AM

get a ps2 keyboard and try it out

gearheadgeek · 12-17-2014, 09:37 AM

Quote:

Originally Posted by genss

get a ps2 keyboard and try it out

Huh? I don't follow. My application does not use a keyboard.

genss · 12-17-2014, 09:58 AM

the kernel itself makes most of the needed nodes in /dev

of the things that it doesn't here's what i got so far
what you look for depends on what your program needs

#/dev/X0R -> null
ln -s /proc/kcore /dev/core
ln -s /proc/self/fd /dev/fd
ln -s /input/mice /dev/mouse
#/dev/rtc -> rtc0
ln -s /proc/self/fd/2 /dev/stderr
ln -s /proc/self/fd/0 /dev/stdin
ln -s /proc/self/fd/1 /dev/stdout
(the two hashed i haven't looked at yet)

then there are modules for devices, something like

Code:

for i in /sys/block/*/device/modalias /sys/bus/*/devices/*/device/modalias /sys/bus/*/devices/*/device/modalias /sys/class/*/*/device/modalias
  do
    if [ -f $i ]
    then
      #echo "modprobe `cat $i`"
      modprobe `cat $i`
    fi
  done

to make nodes for block devices

Code:

for i in /sys/block/*/dev
  do
    if [ -f $i ]
    then
      MAJOR_MINOR=`sed 's/:*:/\ /' < $i`
      DEVNAME=`echo $i | sed -e 's@/dev@@' -e 's@.*/@@'`
      #echo `echo $i | cut -d \/ -f 4`
      #echo "mknod /dev/$DEVNAME b $MAJOR_MINOR"
      mknod /dev/$DEVNAME b $MAJOR_MINOR
    fi
  done
  
for i in /sys/block/*/*/dev
  do
    if [ -f $i ]
    then
      MAJOR_MINOR=`sed 's/:*:/\ /' < $i`
      DEVNAME=`echo $i | sed -e 's@/dev@@' -e 's@.*/@@'`
      #echo `echo $i | cut -d \/ -f 4`
      #echo "mknod /dev/$DEVNAME b $MAJOR_MINOR"
      mknod /dev/$DEVNAME b $MAJOR_MINOR
    fi
  done

maybe some more things, idk

PS i didn't put char devices as some have special directories in /dev

gearheadgeek · 12-17-2014, 11:16 AM

What is wrong with the /dev that you have to install to boot with before tmpsysfs overlays it? My application is static and uses no modules. I need to add to /dev for a touch screen, which, of course, doesn't work. Maybe I could learn udev rules but since the touch stuff lives outside the kernel, I don't think udev can help. In any case, not using udev is easier than learning how to make rules. I did an install without udev but tmpsysfs is still laid on top of /dev.

genss · 12-17-2014, 11:29 AM

/dev is a special kind of virtual fs, devfs
the kernel populates most of it automatically

what rules do you need ?
what are you doing anyway ?
do you use usb ?
do you know how to use modprobe and mknod ?

gearheadgeek · 12-17-2014, 12:38 PM

Quote:

Originally Posted by genss

/dev is a special kind of virtual fs, devfs
the kernel populates most of it automatically

I assumed as much, but the question is, what is wrong with the static version that gets hidden by devfs?

Quote:

what rules do you need ?

If I don't have udev, I don't need rules. What I need is a /dev entry for a touch screen that the kernel does know know about or understand.

Quote:

what are you doing anyway ?

Embedded systems.

Quote:

do you use usb ?

Yes, but that does not require udev.

Quote:

do you know how to use modprobe and mknod ?

Yes, but I don't use modprobe to because I compile all necessary pieces into the kernel and using mknod on tmpdevfs does not work.

I have been using Slack since 4.0, but these provisions to make desktop users lives easier are beginning to get in the way.

genss · 12-17-2014, 04:11 PM

all devices in /dev are nodes
before they were all made at boot, that lead to problems with some corner cases (not just desktop ones)
GKH decided it should be done by a userspace app (after HAL was hotplug then udev), despite other UNIX-es solving that problems properly

example of a /dev node:

bash-4.3# ls -l /dev/usbmon0
crw------- 1 root root 250, 0 Pro 17 15:14 /dev/usbmon0

c means it's a character device
250 is the major node number
0 is the minor node number

so to make this node i would run
mknod /dev/usbmon0 c 250 0
(system call is almost the same)

to find out that numbers you would look into sysfs's /sys/class/usbmon/usbmon0/dev file (or the uevent file)
bash-4.3# cat /sys/class/usbmon/usbmon0/dev
250:0

from what i know; (the documentation is non existent, as is usually the case with Kay Sievers and gang)
there are no static versions for dynamic devices, just for one of a kind ones
nothing that the kernel puts in devtmpfs gets replaced

so if all you need is the dev node, that should be easy to make at boot
bdw openwrt has it's own udev-like version, and hotplug is good documentation/example

ReaperX7 · 12-17-2014, 04:29 PM

You don't need udev to have a functioning system, but it helps with hardware. Without udev you'll have to use mknod to statically assign device nodes in /dev manually, and rely on the kernel hotplugging system.

veerain · 12-17-2014, 10:16 PM

You can replace udev with:

1) eudev

2) mdev from busybox

3) hotplug from openwrt

Usually Udev gets event from kernel on device installation and udev creates neccessary device nodes in /dev

And devtmpfs doesn't handles it well.

Or you can create static device nodes on /dev directory itself. Only if devices don't get changed in each instance.

Didier Spaier · 12-18-2014, 12:03 AM

For your embedded system, mdev is the way to go. Just in case you didn't come across it yet, see also OpenEmbedded
and of course BusyBox.

gearheadgeek · 12-19-2014, 07:03 AM

Quote:

Originally Posted by genss

all devices in /dev are nodes
GKH decided it should be done by a userspace app (after HAL was hotplug then udev), despite other UNIX-es solving that problems properly

I have too much else to do to keep up with kernel development so I missed that. I have been using 2.6 kernels recently and this is my first go with a 3.x series. I can't find anything anywhere that explains tmpdevfs or how to use it.

Quote:

example of a /dev node:

bash-4.3# ls -l /dev/usbmon0
crw------- 1 root root 250, 0 Pro 17 15:14 /dev/usbmon0

As I mentioned, I have been using Slack in embedded since 4.0 (or maybe it was 3.4) so I actually do understand device nodes know how to make them.

gearheadgeek · 12-19-2014, 07:09 AM

Quote:

Originally Posted by ReaperX7

You don't need udev to have a functioning system, but it helps with hardware.

You are right, it is possible. I worked it out and have it running. Thing is, I don't need (or want) help with hardware. That is the point.

Quote:

Without udev you'll have to use mknod to statically assign device nodes in /dev manually, and rely on the kernel hotplugging system.

I have learned in my 35 years of embedded that Murhpy always wins. Something WILL go wrong. So the approach is to keep things as simple as possible. Adding an unneeded hotplug system just to have device nodes that work just as well statically is tempting Murphy to screw around with you. If I need a device node, I can make it.

gearheadgeek · 12-19-2014, 07:12 AM

Quote:

Originally Posted by veerain

You can replace udev with:

1) eudev

2) mdev from busybox

3) hotplug from openwrt

Usually Udev gets event from kernel on device installation and udev creates neccessary device nodes in /dev

And devtmpfs doesn't handles it well.

Or you can create static device nodes on /dev directory itself. Only if devices don't get changed in each instance.

Thanks, but I have been using Slack in embedded successfully since long before busybox. *nix has worked fine with static device nodes for the whole time. Hot plugging is not useful to me and only represents another potential failure point.

veerain · 12-19-2014, 08:56 AM

Quote:

Originally Posted by gearheadgeek

I have too much else to do to keep up with kernel development so I missed that. I have been using 2.6 kernels recently and this is my first go with a 3.x series. I can't find anything anywhere that explains tmpdevfs or how to use it.

As I mentioned, I have been using Slack in embedded since 4.0 (or maybe it was 3.4) so I actually do understand device nodes know how to make them.

To use devtmpfs all you have to do is:

mount -t devtmpfs devtmpfs /dev

/dev may be empty beforehand if you like.