Re: Asynchronous scsi scanning, version 9

[Matthew Wilcox <matthew@wil.cx>]

On Mon, Jun 26, 2006 at 07:44:42AM -0700, Matthew Dharm wrote:
> On Mon, Jun 26, 2006 at 06:40:01AM -0600, Matthew Wilcox wrote:
> > I'm not 100% sure what the problem is with USB.  If it's that we may not
> > have discovered all the USB devices currently plugged in, then I think
> > we need to change the way USB works to use one Scsi_Host for all USB
> > storage devices, and then make each device either its own target or its
> > own channel (probably the former; the latter is less well-tested code).
> 
> There are a couple of things which make this difficult for USB.
> 
> First, some (many?) USB devices need to be left alone for several seconds
> after attachment in order to allow them to initialize to the point where
> they are usable.

That's OK.  Once we know they're there, we can reserve their place
and delay until they're ready to go.

> Second, depending on how many hubs are between the host and target, the
> time-to-discover the device is highly variable.

Can, or does, USB keep track of hub discovery, and hence know whether
or not it's completed USB discovery?

> Third, once discovered the device may still take a long time to be "ready".
> Think of this as a slow spin-up time.  This is different from my first
> point in that the device can actually accept commands, but all commands
> will fail with some sort of not-ready type error.

That's OK too.

> As for using one Scsi_Host... there are several usb-storage devices which
> attach to an entire SCSI bus (not just a single target), so can't make each
> device it's own target.

Oh.  My fault for reading the comment rather than the code.

                /* reject if target != 0 or if LUN is higher than
                 * the maximum known LUN
                 */
                else if (us->srb->device->id && 
                                !(us->flags & US_FL_SCM_MULT_TARG)) {

Do these devices stick to occupying only target IDs from 0-7?  If not,
you may wish to increase ->max_id for those devices.  I think it'd be worth
exporting scsi_scan_channel() from the midlayer (and rearranging it to have
__scsi_scan_channel() as was done with __scsi_scan_target) for USB's benefit.

> Also, at the time this was all written it wasn't
> possible to dynamically remove (with any stability) individual targets or
> channels.  Perhaps that has changed?

It's definitely possible to remove individual targets dynamically now;
Fibre Channel has sorted that out (and will complain loudly if it breaks).
The scsi core doesn't really have a channel object; channel is just an integer 
that describes a path to a target.  So I think there should be no problem in
converting USB to have one host and many channels.

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