SATA Target mode & libata

[Mark Lord <liml@rtr.ca>]

Jeff Garzik wrote:
> On Thu, Nov 08, 2007 at 03:19:52PM -0500, Mark Lord wrote:
..
>> MMmm..  I wonder what the most common use case is for target mode?
>>
>> Everybody I've dealt with thus far uses it as a high-speed local comms 
>> interface,
>> which would suggest that it might be done as a network interface (ethernet 
>> emulation).
>>
>> But that would confusingly go across driver subsystems,
>> despite that this is how it actually is used.
> 
> The low-level driver itself will just be a dumb DMA send/receive engine,
> with submit/completion APIs highly similar to the existing ones.  Then
> you can easily provide a network interface interface (not a typo) on top
> of that.
..

The obvious BIG difference is that in host mode, *we* initiate communcations,
whereas in target mode, it has to just sit there waiting for a host to say something.

That's a pretty big change from how libata operates today,
in just about every respect.

> The biggest use case I've seen is in the embedded space, where you
> really are creating a SCSI (or ATA) target, that appears to the
> initiator/client to be a real SCSI-or-ATA device.
> 
> There are certainly other uses:  networking, creating a cheap SATA bus
> analyzer, creating a cheap SATA bridge, ...
> 
> My main goal is to ensure that the low-level driver is as simple as
> possible, which permits upper layers to actually figure out what
> purposes it shall use.
> 
> Modern SATA is just a DMA engine with PHY control anyway (just like
> networking), so we really just need to be sure to abstract away
> initiator-mode (aka host mode) specifics in drivers that support target
> mode.