From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vladislav Bolkhovitin Subject: Re: [PATCH 2/3] sd: implement START/STOP management Date: Thu, 22 Mar 2007 13:21:49 +0300 Message-ID: <460258BD.9080606@vlnb.net> References: <20070320150718.GF6152@htj.dyndns.org> <20070320151359.GG6152@htj.dyndns.org> <460020DF.6090203@torque.net> <460099DD.2050706@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <460099DD.2050706@gmail.com> Sender: linux-scsi-owner@vger.kernel.org To: Tejun Heo Cc: dougg@torque.net, Jeff Garzik , linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org, James Bottomley , djwong@us.ibm.com, hmh@hmh.eng.br List-Id: linux-ide@vger.kernel.org Tejun Heo wrote: > Hello, Douglas. > > Douglas Gilbert wrote: > >>Tejun, >>I note at this point that the IMMED bit in the >>START STOP UNIT cdb is clear. [The code might >>note that as well.] All SCSI disks that I have >>seen, implement the IMMED bit and according to >>the SAT standard, so should SAT layers like the >>one in libata. >> >>With the IMMED bit clear: >> - on spin up, it will wait until disk is ready. >> Okay unless there are a lot of disks, in >> which case we could ask Matthew Wilcox for help >> - on spin down, will wait until media is >> stopped. That could be 20 seconds, and if there >> were multiple disks .... >> >>I guess the question is do we need to wait until a >>disk is spun down before dropping power to it >>and suspending. > > > I think we do. As we're issuing SYNCHRONIZE CACHE prior to spinning > down disks, it's probably okay to drop power early data-integrity-wise > but still... > > We can definitely use IMMED=1 during resume (needs to be throttled > somehow tho). This helps even when there is only one disk. We can let > the disk spin up in the background and proceed with the rest of resuming > process. Unfortunately, libata SAT layer doesn't do IMMED and even if > it does (I've tried and have a patch available) it doesn't really work > because during host resume each port enters EH and resets and > revalidates each device. Many if not most ATA harddisks don't respond > to reset or IDENTIFY till it's fully spun up meaning libata EH has to > wait for all drives to spin up. libata EH runs inside SCSI EH thread > meaning SCSI comman issue blocks till libata EH finishes resetting the > port. So, IMMED or not, sd gotta wait for libata disks. > > If we want to do parallel spin down, PM core needs to be updated such > that there are two events - issue and done - somewhat similar to what > SCSI is doing to probe devices parallelly. If we're gonna do that, we > maybe can apply the same mechanism to resume path so that we can do > things parallelly IMMED or not. Seems, there is another way of doing a bank spin up / spin down: doing it in two passes. On the first pass START_STOP will be issued with IMMED=1 on all devices, then on the second pass START_STOP will be issued with IMMED=0. So the devices will spin up / spin down in the parallel, but synchronously, hence the needed result will be achieved with minimal code changes, although it will indeed need upper layer changes in struct device_driver's suspend(), resume(), etc. callers. Vlad