Re: Hardware Abstraction Layer

[David Zeuthen <david@fubar.dk>]

On Wed, 2003-09-24 at 23:08, Greg KH wrote:
> > > >  3. Provide some place where user-made settings are stored and persisted
> > > >     for a device. For instance UI-things like device name, where to
> > > >     mount a storage device and with what options (storage only readable
> > > >     by the user or not etc.). 
> > > 
> > > Some of that will be taken care of in udev for Linux.  But others (like
> > > where to mount the device) would be up to you.
> > > 
> > 
> > Yes. I did install 2.6.0-test5 over the weekend and played around with
> > udev 0.2, /sys and libsysfs + tools. It seems udev can't handle USB
> > storage devices yet? It didn't worked for me at least.
> 
> Should work just fine.  They show up as block devices, just like any
> other disk (look in /sys/block).  My demo at OLS 2003 used my usb camera
> which is a usb storage device.
> 

My bad. I hadn't loaded the kernel module sd_mod. Works like a charm
with this module..

> > Side question: Is it really true that the only way to obtain where the
> > SCSI subsystem mounts the usb-storage is by inspecting /var/log/messages
> > on 2.4? I couldn't find anything on 2.5/2.6 through googling either...
> > It's very frustrating
> 
> Don't remember, sorry.  There are a bunch of different scsi programs for
> doing this.  Search the linux-scsi archives for links to them.
> 

Hmm yes.. I searched for a long time, but found this message

 http://www.dragoninc.on.ca/mail-archives/linux-usb/2002-06/0122.html

where you say this is not possible in 2.4.

> > More on udev: how will it fit with the linux-hotplug project?
> 
> It uses the hotplug-base package, that's all.
> 

Ah, I see.. You symlink udev to /etc/hotplug.d/default/udev.hotplug and
on hotplug ACTION­d, $1=block invocations query the sys/$DEVPATH/dev
file and do a mknod. 

I can see that it's a clever thing to have a (pluggable) naming database
and strategy, especially for those sites you mention with thousands of
disks.

Do you intend udev to send a hotplug message to notify that a new device
file have been created? Or is it only for kernel messages? The thinking
is that one could make a generic script that broadcast each and every
hotplug event. It could be as simple as

  dbus-send --dest=org.freedesktop.DBus.Broadcast org.kernel.Hotplug
  string:$1 string:"`env`"

to broadcast the hotplug event on the dbus system bus. Of course, in a
proper script one would test for /usr/bin/dbus-send and
/var/run/messagebus.pid.

This way, I wouldn't need my hal_hotplug program, but just listen for
org.kernel.Hotplug messages on the dbus system bus and still enjoy the
functionality of udev.

> > Maybe it's already there, but it could be nice to have a set of
> > prioritized hotplug scripts that is invoked a'la /etc/init.d.. So one
> > for loading a kernel module, udev for naming the device and one for
> > sending a D-BUS message, notifying, among others, the HAL. In the chain
> > of invoking these, the environment would grow. 
> 
> Heh, we already have that.  See the latest release of the linux-hotplug
> package :)
> 

OK. One thing though.. When I plug in my USB storage device I get a
number hotplug invocations on 2.6 (instead of only one on 2.4):

 o  usb, DEVPATH=/devices/pci0000:00/0000:00:10.0/0000:02:07.0/usb2/
                 2-1/2-1.3

 o  usb, DEVPATH=/devices/pci0000:00/0000:00:10.0/0000:02:07.0/
                 usb2/2-1/2-1.3/2-1.3:0

 o  scsi_host, DEVPATH=/class/scsi_host/host2

 o  scsi, DEVPATH=/devices/pci0000:00/0000:00:10.0/0000:02:07.0/
                  usb2/2-1/2-1.3/2-1.3:0/host2/2:0:0:0

 o  block, DEVPATH=/block/sda

 o  block, DEVPATH=/block/sda/sda1

 o  scsi_device, DEVPATH=/class/scsi_device/2:0:0:0


These are all handled by stuff loading modules and creating device files
(udev) which is all good. But when do I know when all messages for a
device is sent?

My understanding is that is doesn't make send to talk about hotplug
start and hotplug end messages (though it would be nice), so I guess
when receiving the second usb hotplug message I wait for a sec or two to
received the first block message (since $DEVPATH/device points to the
USB device). Or even better, a message from udev! (otherwise I would
have to create my own device file with mknod?)

Thanks,
David





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