Jeff Garzik wrote:
Mark Lord wrote:
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.
It depends on the task.
The miscdev (i.e. chrdev) interface found in drivers/scsi/scsi_tgt_if.c
of repo [1] seems pretty generic, simple, small and applicable to
portions of the problem presented here... The basic task in
scsi_tgt_if's case is just shoveling packets to/from userspace.
..
Except it's rather SCSI specific, and the userspace frontend even more so.
The code expects a SCSI command block, LUN, TAG, and other fields that
a SATA FIS won't have. Seems clumsy, particularly when we (in theory)
are trying to decouple libata from SCSI. But if that's the way,
then I can clumsily wrap each FIS in a fake ATA_16 header or something.
SATA packet capture: highly useful, but implies _copying_ the packets
before passing them on to regular channels. So, does this imply packets
will be copied kernel->userspace->kernel ? kernel->kernel? The
interface will be vastly different in each case.
..
Absolutely, which is why I'm leaving that for (much) later,
and looking for a simple (userspace) comms method to begin with,
using the special (non-SATA compliant) "initiator" on one end,
and a "target" on the other end. This is different from the
SATA interceptor/emulator configurations.
..
Also, in general, it sounds like we need a general way to put a port
into a specific mode (initiator, target, vendor special) during runtime.
Then create a module parameter that allows boot-time initialization of
this port mode selector.
..
That part's easy, and already implemented: module parameter + sysfs attrs.
Cheers
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