Re: Need persistent node to USB port naming

[Greg KH <greg@kroah.com>]

On Fri, May 13, 2005 at 11:55:05AM -0500, Stephen Morgan wrote:
> On Fri, 2005-05-03 at 2:50 ocomber wrote:
> > On Thu, 2005-05-12 at 14:58 -0500, Stephen Morgan wrote:
> > > Hello all,
> > >
> > > I'm converting a driver for a USB device from Linux 2.4 to 2.6.
> >  The device
> > > is embedded and the host is an embedded PC.  There may be as many as 6
> > > identical devices attached.  The only way they are
> > differentiated is by the
> > > USB port they're plugged into - the cabling is fixed.  So, when the user
> > > asks for 'foo_dev0' for instance, he wants the first USB device
> > - 'foo_dev1'
> > > should give him the 2nd device, etc.
> > >
> > > Under 2.4, I created six nodes, assigning them minors 192 thru
> > 197.  In the
> > > driver's probe(), I used usb_make_path to create an index to the device.
> > > For file ops, I used (MINOR(inode_p->i_rdev) - 192) to get an
> > index from the
> > > node.
> > >
> > > So far, under 2.6, the best approach I can see is to create a
> > udev .rules
> > > file to make the nodes, like so: BUS="usb", PLACE="1-1",
> > NAME="usb/foo_dev0"
> > >
> > > Is this the best way to go, or have I overlooked something simpler?
> > >
> > > Thanks
> > >
> > > Stephen Morgan
> >
> > If your driver is aware of which usb port it is plugged into, it could
> > turn this into a sysfs attribute which can be picked up by udev:
> >
> > BUS="usb", KERNEL="foo", NAME="%k", SYMLINK="%s{USBPORT}"
> >
> > It may be that the USB port already exists as a sysfs attribute without
> > you having to explicitly code this - I can't check this at the mo.
> 
> It does - it's called PLACE :-) (or, more recently, ID) and it works - I
> just want to make sure this is the best way to get the nodes I need.

Yes it is.  PLACE is just for what you are looking for.

> One problem I've run into is trying to create a dummy node that allows me to
> communicate with the driver thru ioctl's even when no device is plugged in.
> In 2.4, I just created a node with a minor I knew would never be used, and
> in the driver, I filtered for that minor.
> 
> Do I <need> to use udev?

Not if you don't want to, no.

> It's obviously working, but can I turn it off and make nodes the way I
> did in 2.4?  If so, how would I turn it off?  What kinds of havoc
> would that wreak in 2.6?

Depends on your distro, and how it is set up.  Lots of people use 2.6
without using udev.

thanks,

greg k-h


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