Re: Adaptec SAS integration notes

[Jeff Garzik <jgarzik@pobox.com>]

Stefan Richter wrote:
> All transports implement HCIL mappings. (SPI drivers enjoy the easiest 
> mapping, hcil<->HCIL.) They are all faced with the necessity of mapping 
> and solve it in different ways, for different reasons. Why stop at FC + 
> SAS and try to find a unified solution for just those two? Also, when 
> you are working out how FC and SAS could interface to a common HCIL 
> mapping library (or HCIL mapping layer?), you are effectively designing 
> aspects of how an interface between FC|SAS and SCSI core should look 
> like. Or, why not provide the HCIL mapping helpers in the core and 
> subsequently make users of this new interface completely unaware of the 
> existence of HCIL...?

Ignoring some legacy ioctls and bits of existing code...   The only 
mapping that's _really_ required is an internal C pointer, used by the 
device class to send messages to the transport class.

Adaptec's code does a bit of that on the transport side, but we'd need 
to update all the existing device classes (sd, sr, st, ...) before they 
had been converted to a purely using pointers to connect the transport 
to the device.

Getting to the point where device classes need only to do

	transport_instance->send_message(scsi command | TMF)

and where the transport class replies with

	device_instance->send_message(scsi response | TMF response)

may be a long term goal.  Depends on where evolution takes us...


> Moreover, HCIL mapping is but one aspect of a common problem. As an 
> example, I think yet another aspect of the same problem is that device 
> properties like target names and LUNs get lost below the 
> transport<->core interface. We want to present these device properties 
> to userspace in a unified manner. Here it becomes even more clear: 
> Neither is it a problem for FC and SAS alone, nor are transport-level 
> helpers a good solution to the problem.

Target names are already in the generic device.  Luben has a good point, 
though, that there may be multiple "labels" for the same addressible object.


> I freely admit that I don't have a picture of where the real 
> difficulties lie; I am far too unfamiliar with the innards of scsi core. 
> But time spent on transport-level helpers which are meant to ease 
> symptoms of the transport<->core difficulties seems to be spent rather 
> ineffectively to me. More concrete, what is the ratio of benefit to cost 
> of HCIL mapping code sharing between SAS and FC? Especially since that 
> mapping is deemed obsolete already (in the long term or perhaps even mid 
> term). And to completely go back to the subject: Will attempts on code 
> sharing between FC layer and SAS layer bring things forward in the 
> short-term goal of SAS integration?

HCIL mapping is a simple and very small part of this task :)  There are 
a bunch of line items to tackle, including SMP and SDI details, all 
bundled up in this.

	Jeff