Using udev to collect device information

Using udev to collect device information

[Eric Munson]

I am working on a project that maintains a list of active hardware on a
 machine and we want to make it hotplug "aware".  Making udev call our
process for adding or removing an entry to or from our list each time
hardware is added or removed seems easy enough, but I have several
questions:

1.  Our system will be thread safe, but if we are calling the hotplug
updater each time a device is added or removed with the rules:
ACTION="add" RUN+="/usr/sbin/program"
and
ACTION="remove" RUN+="/usr/sbin/program"
Are we going to cause serious slow downs for the rest of the system if
we are flooded with adds/removes?  The program that we will be running
won't be terribly slow but it will include some disk IO.

2.  How can I pass information about the event, things like what device
is being added/removed into the program being called?  Does udev provide
any of this information during rule execution (I assume that udev has
it, I just need to know where to get it)?

Sorry if this is a gross abuse of udev, but we are looking to plug into
a "hotplug" like system without requiring our users to install the
linux-hotplug system so if there is a better way to accomplish the same
thing please let me know.

Thanks for your help!
-- 
Eric Munson
ebmunson@us.ibm.com
503.578.3104 T/L 775.3104

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Re: Using udev to collect device information

[Greg KH]

On Mon, Jun 26, 2006 at 04:10:41PM -0700, Eric Munson wrote:
> I am working on a project that maintains a list of active hardware on a
>  machine and we want to make it hotplug "aware".  Making udev call our
> process for adding or removing an entry to or from our list each time
> hardware is added or removed seems easy enough, but I have several
> questions:
> 
> 1.  Our system will be thread safe, but if we are calling the hotplug
> updater each time a device is added or removed with the rules:
> ACTION="add" RUN+="/usr/sbin/program"
> and
> ACTION="remove" RUN+="/usr/sbin/program"
> Are we going to cause serious slow downs for the rest of the system if
> we are flooded with adds/removes?  The program that we will be running
> won't be terribly slow but it will include some disk IO.

It all depends on what your /usr/sbin/program does, we can't answer that
for you.

> 2.  How can I pass information about the event, things like what device
> is being added/removed into the program being called?  Does udev provide
> any of this information during rule execution (I assume that udev has
> it, I just need to know where to get it)?
> 
> Sorry if this is a gross abuse of udev, but we are looking to plug into
> a "hotplug" like system without requiring our users to install the
> linux-hotplug system so if there is a better way to accomplish the same
> thing please let me know.

No, this is a fine way.  But why not just hook into HAL?  It already
does all of this for you, and you can get it to send you DBUS messages
for any new device in the system.

Or, if you don't like how HAL works, you should at least look into how
it ties into udev, as that sounds like the same exact way you want to do
things.  But don't reinvent the wheel if you don't have to.

thanks,

greg k-h

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Re: Using udev to collect device information

[Kay Sievers]

On Mon, 2006-06-26 at 17:01 -0700, Greg KH wrote:
> On Mon, Jun 26, 2006 at 04:10:41PM -0700, Eric Munson wrote:
> > I am working on a project that maintains a list of active hardware on a
> >  machine and we want to make it hotplug "aware".  Making udev call our
> > process for adding or removing an entry to or from our list each time
> > hardware is added or removed seems easy enough, but I have several
> > questions:
> > 
> > 1.  Our system will be thread safe, but if we are calling the hotplug
> > updater each time a device is added or removed with the rules:
> > ACTION="add" RUN+="/usr/sbin/program"
> > and
> > ACTION="remove" RUN+="/usr/sbin/program"
> > Are we going to cause serious slow downs for the rest of the system if
> > we are flooded with adds/removes?  The program that we will be running
> > won't be terribly slow but it will include some disk IO.
> 
> It all depends on what your /usr/sbin/program does, we can't answer that
> for you.
> 
> > 2.  How can I pass information about the event, things like what device
> > is being added/removed into the program being called?  Does udev provide
> > any of this information during rule execution (I assume that udev has
> > it, I just need to know where to get it)?
> > 
> > Sorry if this is a gross abuse of udev, but we are looking to plug into
> > a "hotplug" like system without requiring our users to install the
> > linux-hotplug system so if there is a better way to accomplish the same
> > thing please let me know.
> 
> No, this is a fine way.  But why not just hook into HAL?  It already
> does all of this for you, and you can get it to send you DBUS messages
> for any new device in the system.

Right, HAL already keeps a nice list for most of the interesting things.
It also tells you for a lot of devices what you can do with the device.
You may want to look at it, if it matches your requirements. Just run
"lshal" on a recent system.

> Or, if you don't like how HAL works, you should at least look into how
> it ties into udev, as that sounds like the same exact way you want to do
> things.  But don't reinvent the wheel if you don't have to.

HAL tells udev to pass the complete events to an abstract namespace
socket (man 7 unix):
  RUN+="socket:/org/freedesktop/hal/udev_event"

On the socket, you get a message like this: <action>@<devpath>\0[<key>=<value>\0, ...]
  recv(3, "add@/class/input/input6/mouse2\0UDEV_LOG=3\0ACTION­d\0
  DEVPATH=/class/input/input6/mouse2\0SUBSYSTEM=input\0
  SEQNUM\x1518\0PHYSDEVPATH=/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0\0


Kay


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