Re: [Scst-devel] [PATCH 2.6.25.1] Add scsi_execute_async_fifo()

Vladislav Bolkhovitin <vst@xxxxxxxx> · Wed, 14 May 2008 19:58:42 +0400

Boaz Harrosh wrote:

Vladislav Bolkhovitin wrote:

Boaz Harrosh wrote:

Vladislav Bolkhovitin wrote:

Christoph Hellwig wrote:

On Fri, May 02, 2008 at 05:53:22PM +0200, Bart Van Assche wrote:

Regarding scsi_execute_async(): I didn't know that this API is on its
way out. What will it be replaced by, and when ?

blk_execute_rq/blk_execute_rq_nowait plus the block level helpers built
ontop to build requests.

scsi_execute_async() is already a nice and simple helper function on top 

blk_execute_rq_nowait(). What's the point then to remove it? Do you 

consider that exposing scsi_execute_async() internals to its users is 

better?

The problem with it is the use of sg list to *hack* in bio's. Which totally 

ignores/duplicates block layer mechanisms. There is pending a large patchset

that removes the use of scsi_execute_async from sg/sr and friends to use blk_map_*
members and directly call blk_execute_*. The original patchset was written by
Mike Christie but is now brought up to date by Tomo. It should be submitted soon I think.
If you need good example of usage check out bsg.c it maps user-space data in all kind
of combinations. If you have kernel space memory it is even simpler.

Thanks for pointing on it. But it still remained unclear for me what's 

the point in the scsi_execute_async() removal. Function 

scsi_req_map_sg() looks pretty simple and straightforward, so I don't 

see how the overall code can be simplified.

Well No, scsi_req_map_sg() is a complete hack. If you have user memory
or kernel memory you better go through blk_map_* which will take care of
device masks, alignment and all, where here the ULD does that. So you have
2 places of waisted code both at ULD to build the SG right, and here to
translate SG to BIO. Where at block layer you have one function call.

Try it out you see that not using scsi_execute_async() is much more simple 

at ULD then using it.

If you do mmap then Tomo has code for block layer to support that.

Seems, I'm starting understanding you. You mean that all ULDs (User 

Level Devices, i.e. sg, st, etc.) deal with user supplied buffers, i.e. 

pointers to virtually continuous memory, which at the moment it has to 

convert to SG vector, which then will be translated to BIOs for the 

corresponding LDD by scsi_execute_async() (and then back to SG vector on 

the queuecommand() time). So, it will be simpler to supply that buffer 

pointer directly to block functions. Correct?

But the problem is that in SCST in each data transfer 2 LDDs 

participate: one target and one backstorage (initiator). And the target 

LDD deals with SG vectors only. So, SCST never deals with buffers, it 

always deals with SG vectors and pass them between target LDDs and 

backstorage as necessary.

Thus, looks like for SCST in the pass-through mode there is no 

alternative to scsi_execute_async().

Thanks,
Vlad

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