From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vladislav Bolkhovitin Subject: Re: [Scst-devel] [PATCH 2.6.25.1] Add scsi_execute_async_fifo() Date: Wed, 14 May 2008 20:49:41 +0400 Message-ID: <482B1825.9050806@vlnb.net> References: <200805021638.42972.bart.vanassche@gmail.com> <20080502153306.GB7376@infradead.org> <20080502155525.GA16353@infradead.org> <481DA2A4.10607@vlnb.net> <481DF822.7090905@panasas.com> <4829C653.9050205@vlnb.net> <4829D14E.10909@panasas.com> <482B0C32.3010100@vlnb.net> <482B1582.7050709@panasas.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-relay-02.mailcluster.net ([77.221.130.214]:47011 "EHLO mail-relay-02.mailcluster.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750887AbYENQto (ORCPT ); Wed, 14 May 2008 12:49:44 -0400 In-Reply-To: <482B1582.7050709@panasas.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Boaz Harrosh Cc: FUJITA Tomonori , linux-scsi@vger.kernel.org, James Bottomley , Christoph Hellwig , scst-devel@lists.sourceforge.net 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 >