SATA "target mode" (or "Channel-to-Channel" comm mode)

[Mark Lord <liml@rtr.ca>]

Guys,

Within the next couple of weeks, I would like to submit patches
for 2.6.30 for a simple form of what Marvell likes to call "target mode",
or C2C (channel-to-channel communications).

This is for sata_mv.

The question is, how to expose an interface to actually access it?

Quick background on Marvell C2C:

1. C2C is only for Gen2 and Gen2e chipsets.

2. Requires a special SATA cross-over (simple twist) cable
    between two SATA ports.  Ports can be on the same host
    adaptor or on separate adaptors and/or machines.

3. Each sata_mv port can be either a (0) normal SATA host,
    or (1) special SATA C2C initiator, or (2) a SATA target device.

4. A Gen2e mode (2) target can connect/communicate with either
    a mode (0) host or a mode (1) initiator.  I'm not yet sure
    whether an older Gen2 target can connect with a mode (0) host.

5. Mode (1) initiator appears to relax requirements such as waiting
    for a device BUSY bit to clear etc., and is intended for simple
    channel-to-channel communications.

6. A boot/module parameter seems to be the best way to enable
    this feature, as otherwise libata wastes a lot of time and
    effort probing for non-existant drives and slowing down
    the boot process.

7. Initially, all that we want is a way to use two SATA ports
    (on the same or different machines) as a simple byte-stream
    communications channel, between a mode (1) inititiator
    and a mode (2) target.  This is used in real-life as a high-speed
    local comm channel between halves of split server machines.

8. Transparently emulating a SATA drive is possible on Gen2e chips
    at least, and perhaps also on Gen2 chips.  This is not being
    worked on at this time.

9. Using two ports in tandem, one mode (0) host and one mode (2) target,
    one can construct a quite capable SATA capture/analyzer device
    which could be inserted in between any other SATA host and device.
    Quite useful, and something I intend to work on later this year.

So, starting with simple stuff, I want to expose an interface for
point 7 above.  The thought is to use netlink for this, on both ends.

An alternative might be to tie it into the SCSI Target Framework (tgt).
But that is more for full target device emulation than for simple comms.
And SATA is not SCSI, so it could really restrict/prevent us from doing
a full SATA emulation (eg. point 9) in the end.

Time is short, so I'd like to spend it on something that Jeff would
actually accept.  Thus this email.

Jeff?