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: Wed, 22 Apr 2009 12:52:40 -0400 Message-ID: <49EF4B58.8000204@garzik.org> References: <20090422090929.GA14928@havoc.gtf.org> <49EF16F5.5080909@rtr.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from srv5.dvmed.net ([207.36.208.214]:39108 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752495AbZDVQwn (ORCPT ); Wed, 22 Apr 2009 12:52:43 -0400 In-Reply-To: <49EF16F5.5080909@rtr.ca> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Mark Lord Cc: linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org, LKML Mark Lord 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. >> >> The patch below converts libata such that there is now a 1:1 >> correspondence between struct Scsi_Host and struct ata_host. ATA ports >> are represented as SCSI layer 'channels', which is more natural. >> >> This patch is an experiment, and not meant for upstream anytime soon. > .. > > Could you perhaps explain how error handling would behave in this scheme? > > Currently, one SATA port can have failures without any impact whatsoever > on concurrent operation of other ports, in part because each port is > treated > as a completely independent SCSI host. > > I wonder if that changes with the new (better) scheme proposed here? It changes, yes, most definitely. We just have to pay close attention, and make sure to indicate which EH actions are host-wide, channel-wide (== per port, in ATA parlance) or per-device. SCSI handles all these cases, because e.g. you might not want to disrupt all 1,000 SAN devices actively talking to a single SCSI host in Linux. So... error handling should behave how it needs to behave ;-) There might be an issue with concurrent error handling, because of potential sharing of EH threads (== one port's EH must wait for another's, I think)....but not with concurrent and independent operation. You should be able to reset an AHCI port without affecting data xfer on the other ports. Jeff