Re: [PATCH 1/1] ipr: Fix HDIO_GET_IDENTITY oops for SATA devices

[Jeff Garzik <jeff@garzik.org>]

James Bottomley wrote:
> The quickest way seems to be to break the scsi_host <-> ata_port link by
> mapping scsi_host <-> ata_host instead.

Correct for newer controllers... not desirable for older master/slave


> Legacy controllers with only a
> single port can keep the apparent 1:1 mapping (we can even keep the
> hostdata stuff).

Not sure what you mean by this


> Unfortunately, the standard way of doing this is via
> the transport classes, but as long as you have a pointer from the port
> to the host and from the device to the port (which you do) it should be
> possible.

I've always been open to an ATA transport class and thought that was the 
best way to go long term.  But I confess to not knowing the best way to 
implement that goal in concrete terms.


> The downside I can see is that qc_defer handling changes non trivially
> because of this, but there don't seem to be many other issues.

There are a lot of little details that are easily fixable.

The main detail is not breaking queueing for master/slave. 
Master/slave, if you recall, _requires_ one-shost-per-port because it 
relies on scsi queueing to handle the balancing between master/slave. 
We set can_queue to 1, and for the 2-device case -- master and slave 
present -- SCSI does the heavy lifting to ensure proper arbitration.

Adding support for NCQ and qc_defer made it a bit easier to change the 
master/slave setup, though.

The other reason why we use one-short-per-port on master/slave is that, 
in many respects, each legacy IDE port really can behave like a 
completely separate controller, each port with its own interrupts and 
independent reset logic.

Thus ATA really has two models:  1-port-1-host and N-ports-1-host.

[meta:  I'm not disagreeing with you here, just explaining how all this 
came about...]


> The ideal (for us at least) would be to completely separate port
> operations from host operations, because libsas really wants to control
> the host and attach the port only to SATA devices.

Agreed, I've wanted to move in that direction for a while.

	Jeff