From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: [PATCH] libata: rewrite SCSI host scheme to be one per ATA host Date: Thu, 23 Apr 2009 06:39:39 -0400 Message-ID: <49F0456B.2050502@garzik.org> References: <20090422090929.GA14928@havoc.gtf.org> <49EEE225.3010700@garzik.org> <20090423063542.GK4593@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from srv5.dvmed.net ([207.36.208.214]:38959 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754571AbZDWKjo (ORCPT ); Thu, 23 Apr 2009 06:39:44 -0400 In-Reply-To: <20090423063542.GK4593@kernel.dk> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Jens Axboe Cc: linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org, LKML , Tejun Heo Jens Axboe wrote: > On Wed, Apr 22 2009, Jeff Garzik wrote: >> Jeff Garzik wrote: >>> Currently, libata creates a Scsi_Host per port. This was originally >>> done to leverage SCSI's infrastructure to arbitrate among master/slave >>> devices, but is not needed for most modern SATA controllers. And I >>> _think_ it is not needed for master/slave if done properly, either. >> BTW note the above, with regards to the libata SCSI->block conversion. >> libata currently relies on SCSI for some amount of generic device >> arbitration, in several situations (see ->qc_defer, >> SCSI_MLQUEUE_.*_BUSY). libata expects SCSI to be intelligent and not >> starve devices, etc. > > Defer looks like internal policy, I don't see that functioning any > different in the block layer. SCSI_MLQUEUE_*_BUSY in SCSI is primarily > using the block layer functionality of BLKPREP_DEFER to begin with, so I > think we're pretty close to providing all that already. It's not quite that simple. I am referring mainly to arbitration across multiple request_queue's. SCSI has useful code in place to deal with target-busy and host-busy conditions, both of which could potentially be blocking and unblocking multiple request queues. mlqueue is much more than just a wrapper over block requeueing functions. Read scsi_next_command() and scsi_run_queue(), and grep for starved_list, host_{busy,blocked}, target_{busy,blocked}, device_{busy,blocked}. In our master/slave case, we must choose between queue A and queue B, making sure to starve neither. For simplex DMA, we potentially have queues A, B, C and D serving requests across the "bus bottleneck," and must ensure no starvation of A, B, C or D. Although I have no code to back this up, my gut feeling is that a "request queue group" object, with associated functions, that would be the appropriate place for cross-queue or "host-wide" (as in, struct Scsi_Host or struct ata_host) functionality. Whatever the solution, libata definitely makes use of SCSI's cross-request_queue arbitration, so any move to block will require similar functionality. Jeff