Boaz Harrosh wrote:
Vladislav Bolkhovitin wrote:
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.
Where is that SG coming from? is it from Network stack? or is it an
SG prepared by scsi-ml with call to blk_rq_map_sg()?
It's prepared by SCST core. A request comes from target LDD and reply
finally sent there. Everything goes in the kernel mode only. It is
simple pass-through, exactly as it sounds.
Thus, looks like for SCST in the pass-through mode there is no
alternative to scsi_execute_async().
Tomo what do you think is it logical to add a blk function that will
accept an SG list and map it to a request. Like, for example, an SG
from Network? opposite of what blk_rq_map_sg does.
Thanks,
Vlad
Boaz
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