linux-kernel - Re: [PATCH] async scsi scanning, version 12

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Message-ID: <20060928203656.GG5017@parisc-linux.org>
Date:	Thu, 28 Sep 2006 14:36:56 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Douglas Gilbert <dougg@...que.net>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] async scsi scanning, version 12

On Thu, Sep 28, 2006 at 04:15:05PM -0400, Douglas Gilbert wrote:
> > +	scsi_mod.scan=	[SCSI] sync (default) scans SCSI busses as they are
> > +			discovered.  async scans them in kernel threads,
> > +			allowing boot to proceed.  none ignores them, expecting
> > +			user space to do the scan.
> 
> Matthew,
> I like the "none" which is no doubt a place holder at
> the moment.

I wouldn't say it was extensively tested, but no, there's checks for the
value "none" and it does indeed fail to discover any devices ;-)

> For the user space to do discovery, it either needs an out
> of band mechanism (e.g. IP) or the ability to talk to a
> host in the absence of any "devices" (targets or logical units).
> That requires a device node (e.g. /dev/mptctl) or something
> equivalent in sysfs (yuk).

I must confess to having not thought about how userspace probes a
scsi host to find out what devices are behind it.  This was a feature
that James asked for and it was easy to add.

> Your "none" explanation could be slightly extended to say
> that the LLD (and/or its firmware) might do the discovery.

Note that by specifying "none", not even the FC/SAS/etc drivers can
register targets as they find them -- it really is up to userspace to
echo scsi-add-single-device H C T L >/proc/scsi/scsi

I'm a little uncomfortable with that, and I'd be open to adding another
word that means "no scanning, but if the driver's been told about the
device by a switch, add it automatically, don't wait for userspace".
I do think that none needs to mean none though.

> As an
> example the SAS-2 draft now has self-configuring expanders
> (the terms "edge" and "fanout" have been dropped) which
> effectively discover the topology and track changes, configuring
> themselves and dumber expanders as required. Then host discovery
> becomes importing the topology from an external device. However
> not all devices may be visible to self-configuring expanders
> (e.g. a SATA disk could be directly attached to a SAS HBA). So
> some extra work may be required.

That would be up to userspace in the "none" view of the world.  I could
see people wanting to ignore the self-configuring expander and impose
a new (incorrect) topology on the system.

BTW, there'll be a lucky version 13 in a few minutes ...
shost_for_each_device_safe isn't.
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