Re: SCSI commands order when using scsi_do_req

[James Bottomley <James.Bottomley@SteelEye.com>]

On Mon, 2005-06-06 at 11:32 +0300, Liran Schour wrote:
> In order to run our OSD SCSI initiator on top of the Linux SCSI layer
> we have needed to do the following changes to the SCSI code:
> 1. Redefine MAX_COMMAND_SIZE to 256 byte.

There's more than just you interested in this one, so separating this
feature out from all the rest would be helpful.

> 2. Define a new scatter gather structures that includes two virtual buffers
>    on for data out and one for data in. (implementing bidirectional
> commands)
>    The SCSI middle layer doesn't now this new structure (only ULD and LLD).

Yes, this is the nasty one ... and why OSD hasn't been incredibly
popular; bidirectional commands necessitate fairly sweeping changes to
all LLDs to support.  However, I don't see how we can get away without
the mid-layer knowing, since it's the entity that constructs the sglists
and the sglists have to be separated between the in and out pieces.

> 3. The problem above - define a new scsi_do_req that inserts our commands
> to
>    the tail of the Q.

Well, the solution, I think, is going to be the block execute one, which
will allow head or tail insertion.

James