convert sg to use block layer helpers

sg duplicates a lot of block layer dio and copying code. The block layer is missing some things like mmap helpers and a way to support sg and st's reserve buffer. The next two patches move move functionality from sg to the block layer so later st and osst can use it, and then coverts sg to use the block layer helper.

The goal is to kill scsi_execute_async and friends and covert the ULDs to use the block layer functions directly. This is nice because scsi_execute_async sucks and it actually duplicates block layer code, and in the end the scsi layer does not have to do any bio processing (which irks Jens :)), and we have common code for all ULDs.

The patches were made over Jens's bsg branch in his block layer git tree on kernel.org. Sorry for the large patches. The sg conversion patch rips out alot of code duplicated in the block layer and the patch to add some missing functionality to the block layer has conversions to bsg, tgt, etc so git bisect works on it.

Mike,
Might I suggest ripping all the scatter gather list handling out of the block layer and putting it in a library (a bit like libata). That way when the block layer paradigm doesn't apply, we don't lose all the useful things that have been put in there.

OSD and char devices (e.g. SSC, SES, wlun, security) are not block devices in any reasonable sense. It would be good to get away from "devices" that need a "/dev/" or even a sysfs representation. A REPORT LUNS well known LU is only needed during a LU discovery process on a target. So if that is done from the user space why do we need a "/dev" or sysfs entry hanging around clogging up the device space?

Pushing SCSI pass-through functionality further and further up the stack diminishes its utility and may lead to a re-introduction of useful pass-throughs at a lower level (actually there are some already there).

Think about a SCSI INQUIRY command, it is implicit "head of queue" which means it should bypass all the block layer queuing and other impediments. It can be sent to logical units that don't even exist! Task management functions should not be queued. SMP functions should not be queued. And so it goes on.

Doug Gilbert

Don't think block layer, think block IO helpers. That's essentially what large parts of the block layer is.

Jens Axboe

I think this might be a good idea. We want to do something similar for target requests. For tgt, we allocate a request_queue, and at one point were using it to queue requests from the host to the userspace code (the queue was to communicate with the host instead of specific device). We now only use the queue for scatterlist building, so if you are talking about making a new struct which holds those q limits that a request_queue or some other object could use, then making some things like max_sectors less block layerish, then we might be talking about the same thing.

But we may not be thinking about the same thing or maybe we are :) When I saw convert sg to block layer helpers, I do not mean exactly block layer. The code we are using is based off the request_queue and that does not need something like a block_device. You could interact with a request_queue through sysfs or netlink (tgt did both at different points :)). The queue, as we use it for scatterlist building, has some values which are blockish like max_sectors, but for values like that we need a way to for the LLDs to tell upper layers the max size command (any command) a card can take. Maybe some people would say it is unfortunate it has to be based on values like sectors which do not make sense for a command like inquiry and maybe others will say what other values should we use. I asked about just expressing it in bytes the other day when asking about transport level commands but did not get any replies/ideas. When going over the block sg code I noticed we assume 512 byte sectors in some places so the code needs some changes.

What about moving the queue limits that are used for scatterlist
building: like max_phys_segments, max_hw_segments, dma alignments, max_sectors, max_segment size, etc to a new struct "struct sg_limits". The request queue could have one and any other objects like tgt could use one. The block layer helpers we use would take a sg_limit struct instead of a request_queue. And the functions like blk_rq_*_user, blk_rq_map_sg, etc could be renamed to something without the blk prefix and moved to some new file. Would this be a cleaner abstraction or do you guys think it would just be a lot of code movement for just a big name change? And of course max_sectors can be made less block layerish.