David Brownell wrote:
>
> It seemed to me that something was missing in that stack though;
> the layers above (4), specifically filesystems, that would also need
> to know about new devices that were added. When everything is
> working smoothly, users need to see filesystems get mounted as
> a direct consequence of hotplugging such storage units.
>
> One way to look at that issue is to ask what user mode notifications
> will be used to address that part of the hotplug problem. Devfsd
> is what some folk like, but it's not universally accepted. GUI
> driven solutions don't seem right in all cases either.
> [snip]
Devfs has the useful property that only devices that are
present and have their drivers ready appear in the /dev
hierarchy. In the case of SCSI devices, devfs maintains
this dynamically via calls in the _attach() and _detach()
functions within the upper level drivers. The new
directory notification capability in lk 2.4 (see
Documentation/dnotify.txt) could then be used to detect
changes in the /dev directory (or below).
A user mode notification daemon could read /proc/scsi/scsi
periodically to detect the arrival and departure of SCSI
(pseudo) devices. To this end, the output read from
/proc/scsi/sg/devices would be easier to parse.
If a cdrom drive has just be introduced, it would relatively
easy to work out which /dev/sr<n> device name it was. I guess
existing cdrom applications would then be smart enough to
work out whether the drive was empty, had a music cd, or a
data cdrom (which fs type, etc).
Currently there is a boolean flag: 'emulated' that sg can
yield via an ioctl and is available in /proc/scsi/sg/hosts .
I assume it was introduced for the first pseudo scsi driver:
ide-scsi. Seems to me that this flag could be expanded to
be the major device number of the primary protocol stack
that a pseudo SCSI device belongs to (e.g. IDE, USB,
IEEE1394 etc). This would allow a user mode notification
daemon to cross reference the primary protocol stack.
Doug Gilbert
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