Re: When does udev info (/sys/...) become available?

[Greg KH <greg@kroah.com>]

On Thu, Nov 01, 2007 at 12:53:28PM -0600, clemens@dwf.com wrote:
> > On 11/1/07, clemens@dwf.com <clemens@dwf.com> wrote:
> > > When does udev populate the /sys file system?
> > > I tried using some info that is there later (when the user has control) during
> > > an init.d startup scripts and the info is not there yet
> > > (and Im not sure how to write the script w/o the information).
> > > I tried a delay (sleep) in the script but that didnt help...
> > 
> > Whenever you mount sysfs to /sys. You need to do this before udev runs
> > since it relies heavily on the information in /sys. You may need to
> > use 'mount -n' if root is read-only at that point and record the mount
> > information later. For example, see the LFS bootscripts:
> > 
> > http://downloads.linuxfromscratch.org/lfs-bootscripts-20070813.tar.bz2
> > 
> > First, /sys and /proc are mounted. Then the /dev tmpfs is mounted and
> > udev* is run. Then /etc/mtab is rewritten to record the /, /proc and
> > /sys mount points and the rest of the mounts are done. The three
> > scripts are mountkernfs, udev and mountfs.
> > 
> > That's not the only way to do it, but it should get you pointed in
> > right direction.
> > 
> OK, let me be more specific.
> I am using the standard Linux boot.
> During the execution of a script in the init.d directory, I try to cat
> information from a file in the /sys directory.  
> I get error messages.
> Here is what I see:
>     ls /sys/class/pps		[ this file exists and I get the ls, which is empty ]

So your class is created now.

>     ls /sys/class/pps/pps0		[ this fails, tho the dir is there at 'user' time ]
>     cat /sys/class/pps/pps0/path	[ this obviously also fails ]

you need to wait until your device shows up.

So don't try to do this in an init script, but do it in a script that
runs when udev sees this device show up.  That's the only way you are
going to properly syncronize something like this.

thanks,

greg k-h

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