what is the precise udev event that handles miscdevices?

what is the precise udev event that handles miscdevices?

[Robert P. J. Day]

  i can trivially create misc character devices, and i know special
device files for them will automatically show up under /dev, but what
is the precise udev event/rule that comes into play for this?

  if i start to monitor udevadm with:

$ udevadm monitor --kernel --udev --property

and insert my misc char module, i see:

KERNEL[5342.740294] add      /module/mymisc (module)
ACTION=add
DEVPATH=/module/mymisc
SEQNUM=3007
SUBSYSTEM=module

KERNEL[5342.740528] add      /devices/virtual/misc/mymisc (misc)
ACTION=add
DEVNAME=/dev/mymisc
DEVPATH=/devices/virtual/misc/mymisc
MAJOR=10
MINOR=55
SEQNUM=3008
SUBSYSTEM=misc

UDEV  [5342.741071] add      /module/mymisc (module)
ACTION=add
DEVPATH=/module/mymisc
SEQNUM=3007
SUBSYSTEM=module
USEC_INITIALIZED=740419

UDEV  [5342.741921] add      /devices/virtual/misc/mymisc (misc)
ACTION=add
DEVNAME=/dev/mymisc
DEVPATH=/devices/virtual/misc/mymisc
MAJOR=10
MINOR=55
SEQNUM=3008
SUBSYSTEM=misc
USEC_INITIALIZED=740750

so, yes, i can see the line "DEVNAME=/dev/mymisc", but is there
something i can point at in the collection of udev rules that deals
with that? or is that simply done internally?

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

what is the precise udev event that handles miscdevices?

[Greg KH]

On Wed, Apr 01, 2015 at 07:43:05AM -0400, Robert P. J. Day wrote:
> 
>   i can trivially create misc character devices, and i know special
> device files for them will automatically show up under /dev, but what
> is the precise udev event/rule that comes into play for this?

There isn't one.

Gee, this is going to be easy  :)

what is the precise udev event that handles miscdevices?

[Greg KH]

On Wed, Apr 01, 2015 at 07:43:05AM -0400, Robert P. J. Day wrote:
> 
>   i can trivially create misc character devices, and i know special
> device files for them will automatically show up under /dev, but what
> is the precise udev event/rule that comes into play for this?
> 
>   if i start to monitor udevadm with:
> 
> $ udevadm monitor --kernel --udev --property
> 
> and insert my misc char module, i see:
> 
> KERNEL[5342.740294] add      /module/mymisc (module)
> ACTION=add
> DEVPATH=/module/mymisc
> SEQNUM=3007
> SUBSYSTEM=module
> 
> KERNEL[5342.740528] add      /devices/virtual/misc/mymisc (misc)
> ACTION=add
> DEVNAME=/dev/mymisc
> DEVPATH=/devices/virtual/misc/mymisc
> MAJOR=10
> MINOR=55
> SEQNUM=3008
> SUBSYSTEM=misc
> 
> UDEV  [5342.741071] add      /module/mymisc (module)
> ACTION=add
> DEVPATH=/module/mymisc
> SEQNUM=3007
> SUBSYSTEM=module
> USEC_INITIALIZED=740419
> 
> UDEV  [5342.741921] add      /devices/virtual/misc/mymisc (misc)
> ACTION=add
> DEVNAME=/dev/mymisc
> DEVPATH=/devices/virtual/misc/mymisc
> MAJOR=10
> MINOR=55
> SEQNUM=3008
> SUBSYSTEM=misc
> USEC_INITIALIZED=740750
> 
> so, yes, i can see the line "DEVNAME=/dev/mymisc", but is there
> something i can point at in the collection of udev rules that deals
> with that? or is that simply done internally?

Ok, I'll be nice here, udev doesn't create device nodes anymore, and
hasn't for years, the kernel does that directly with devtmpfs.

Yes, if you want to write your own version of udev, you can listen to
the uevent messages, and pick out the right one to trigger off of to
create a device node, but as the kernel does this all for you with
devtmpfs, why would you want to?

In fact, no one does this anymore, and people run systems without udev
and have dynamic device nodes just fine.  And have been doing so for
years, udev isn't needed for device nodes anymore, if that's all you
care about.

Hint, you are correct, it's the DEVNAME event, but watch out here, you
are seeing events twice, once from the kernel, the second from UDEV,
which is exposing how udev works internally, but will quickly get
confusing if you aren't careful.  I'd recommend just filtering on the
KERNEL events to keep things "sane" for now.

good luck,

greg k-h

what is the precise udev event that handles miscdevices?

[Robert P. J. Day]

On Wed, 1 Apr 2015, Greg KH wrote:

> On Wed, Apr 01, 2015 at 07:43:05AM -0400, Robert P. J. Day wrote:
> >
> >   i can trivially create misc character devices, and i know special
> > device files for them will automatically show up under /dev, but what
> > is the precise udev event/rule that comes into play for this?
>
> There isn't one.
>
> Gee, this is going to be easy  :)

  don't get smug. :-) i had sort of concluded there was no actual udev
rule i could point at, so it's just the case that udev processes those
events internally? is this written down anywhere? everyone happily
explains how misc devices get their /dev file automatically, but no
one i've seen goes that extra step to explain how that happens.

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

what is the precise udev event that handles miscdevices?

[Robert P. J. Day]

On Wed, 1 Apr 2015, Greg KH wrote:

> On Wed, Apr 01, 2015 at 07:43:05AM -0400, Robert P. J. Day wrote:
> >
> >   i can trivially create misc character devices, and i know special
> > device files for them will automatically show up under /dev, but what
> > is the precise udev event/rule that comes into play for this?
> >
> >   if i start to monitor udevadm with:
> >
> > $ udevadm monitor --kernel --udev --property
> >
> > and insert my misc char module, i see:
> >
> > KERNEL[5342.740294] add      /module/mymisc (module)
> > ACTION=add
> > DEVPATH=/module/mymisc
> > SEQNUM=3007
> > SUBSYSTEM=module
> >
> > KERNEL[5342.740528] add      /devices/virtual/misc/mymisc (misc)
> > ACTION=add
> > DEVNAME=/dev/mymisc
> > DEVPATH=/devices/virtual/misc/mymisc
> > MAJOR=10
> > MINOR=55
> > SEQNUM=3008
> > SUBSYSTEM=misc
> >
> > UDEV  [5342.741071] add      /module/mymisc (module)
> > ACTION=add
> > DEVPATH=/module/mymisc
> > SEQNUM=3007
> > SUBSYSTEM=module
> > USEC_INITIALIZED=740419
> >
> > UDEV  [5342.741921] add      /devices/virtual/misc/mymisc (misc)
> > ACTION=add
> > DEVNAME=/dev/mymisc
> > DEVPATH=/devices/virtual/misc/mymisc
> > MAJOR=10
> > MINOR=55
> > SEQNUM=3008
> > SUBSYSTEM=misc
> > USEC_INITIALIZED=740750
> >
> > so, yes, i can see the line "DEVNAME=/dev/mymisc", but is there
> > something i can point at in the collection of udev rules that deals
> > with that? or is that simply done internally?
>
> Ok, I'll be nice here, udev doesn't create device nodes anymore, and
> hasn't for years, the kernel does that directly with devtmpfs.

  and, suddenly, so many things make much more sense.

> Yes, if you want to write your own version of udev, you can listen
> to the uevent messages, and pick out the right one to trigger off of
> to create a device node, but as the kernel does this all for you
> with devtmpfs, why would you want to?

  at this point, of course i wouldn't.

> In fact, no one does this anymore, and people run systems without
> udev and have dynamic device nodes just fine.  And have been doing
> so for years, udev isn't needed for device nodes anymore, if that's
> all you care about.

  yup, i can see that now. although i would *think* that you could
still use udev to go above and beyond just creating device files, to
do things like create symlinks, run arbitrary programs and so on. but,
yes, for just straight device file manipulation, devtmpfs is
sufficient.

> Hint, you are correct, it's the DEVNAME event, but watch out here,
> you are seeing events twice, once from the kernel, the second from
> UDEV, which is exposing how udev works internally, but will quickly
> get confusing if you aren't careful.  I'd recommend just filtering
> on the KERNEL events to keep things "sane" for now.

  i figured as much. now i know what to go read to keep me busy for
the rest of the day. thanks muchly.

rday

p.s. one more potentially silly question -- is there a devtmpfs
monitoring utility, or is it adequate to just use udevadm to monitor
kernel events to get the same effect?

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

what is the precise udev event that handles miscdevices?

[Sudip Mukherjee]

>
>   don't get smug. :-) i had sort of concluded there was no actual udev
> rule i could point at, so it's just the case that udev processes those
> events internally? is this written down anywhere? everyone happily
> explains how misc devices get their /dev file automatically, but no
> one i've seen goes that extra step to explain how that happens.

misc_register() is ultimately calling device_add() which is again
calling device_create_file() , and this one is actually creating the
node in  /dev . (if i am not wrong).

regards
sudip

what is the precise udev event that handles miscdevices?

[Robert P. J. Day]

On Wed, 1 Apr 2015, Sudip Mukherjee wrote:

> >   don't get smug. :-) i had sort of concluded there was no actual
> > udev rule i could point at, so it's just the case that udev
> > processes those events internally? is this written down anywhere?
> > everyone happily explains how misc devices get their /dev file
> > automatically, but no one i've seen goes that extra step to
> > explain how that happens.
>
> misc_register() is ultimately calling device_add() which is again
> calling device_create_file() , and this one is actually creating the
> node in /dev . (if i am not wrong).

  close, and now i can come across as an expert since i checked the
source file. :-) from drivers/base/core.c, in device_add():

        if (MAJOR(dev->devt)) {
                error = device_create_file(dev, &dev_attr_dev);
                if (error)
                        goto DevAttrError;

                error = device_create_sys_dev_entry(dev);
                if (error)
                        goto SysEntryError;

                devtmpfs_create_node(dev);
        }

so device_add() creates device_create_file(), then calls
devtmpfs_create_node() which, based on whether you selected that
devtmpfs CONFIG option, either creates the /dev file or does nothing.
it's all clear to me now ...

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

what is the precise udev event that handles miscdevices?

[Greg KH]

On Wed, Apr 01, 2015 at 08:18:13AM -0400, Robert P. J. Day wrote:
> On Wed, 1 Apr 2015, Greg KH wrote:
> > In fact, no one does this anymore, and people run systems without
> > udev and have dynamic device nodes just fine.  And have been doing
> > so for years, udev isn't needed for device nodes anymore, if that's
> > all you care about.
> 
>   yup, i can see that now. although i would *think* that you could
> still use udev to go above and beyond just creating device files, to
> do things like create symlinks, run arbitrary programs and so on. but,
> yes, for just straight device file manipulation, devtmpfs is
> sufficient.

Yes, your persistent device names (the whole reason udev was created in
the first name) are still created by udev, as that is best left up to
userspace.  Those are symlinks to the device nodes devtmpfs created.

Also, sometimes you want different group/user permissions on device
nodes from what the kernel provides (the kernel doesn't know the group
file), so udev will modify the mode and owners of some nodes as well.

good luck!

greg k-h