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

[Vladislav Bolkhovitin <vst@vlnb.net>]

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