Re: SCSI Patches - mostly on/off-line stuff

[Douglas Gilbert <dougg@torque.net>]

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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