From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: SAS overview Date: Sat, 27 Mar 2004 22:19:17 -0500 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <40664435.1060409@pobox.com> References: <20040316174713.C424976C4F@isis.visi.com> <40664024.2070700@torque.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from parcelfarce.linux.theplanet.co.uk ([195.92.249.252]:41914 "EHLO www.linux.org.uk") by vger.kernel.org with ESMTP id S262065AbUC1DTa (ORCPT ); Sat, 27 Mar 2004 22:19:30 -0500 In-Reply-To: <40664024.2070700@torque.net> List-Id: linux-scsi@vger.kernel.org To: dougg@torque.net Cc: "Scott M. Ferris" , Matthew Wilcox , linux-scsi Douglas Gilbert wrote: > Scott M. Ferris wrote: > >> Matthew Wilcox wrote: >> >>> On Tue, Mar 16, 2004 at 09:46:31PM +1000, Douglas Gilbert wrote: >>> >>> >>>> Now things start to get interesting. SAS can aggregate >>>> individual phy interconnects to form "wide" links. So >>>> if two (single phy) cables are run from a HBA (occupying >>>> two of its phys) to phys on the same expander then that >>>> is a wide link. Seen from Linux driving that HBA that is >>>> two PCI devices (each HBA phy) but only one SCSI initiator >>>> port. Up to 256 phy interconnects can be ganged to make >>>> a very wide SAS link (a parallel bus you might say). >>> >>> >>> It's different though (and I suspect you know this, but let's clarify >>> for >>> the audience). A parallel link would send each bit down a different >>> phy. >>> PCI-E (and probably SAS) send each byte down a different phy. >> >> >> >> The assumption that each phy will be a different PCI device is likely >> to be false, at least for initiators. > > > Scott, > Whether two phys on a SAS HBA are two separate SCSI initiators > or one initiator on a wide link depends on what those phys are > attached to. That seems pretty dynamic so it seems to me the HBA > driver needs to look at phys individually. So if the phys are > not separate PCI devices then they at least need to be individually > addressable (and the SAS address isn't going to help since it's > the same for all HBA phys). At the moment for SPI PCI HBAs sysfs > has a one to one link between a PCI device and a SCSI host. > Such sysfs links might look a little different in the presene of > SAS wide links. If it's anything like SATA, you have one phy per point-to-point link. Thus, 8 sata phys on a single 8-port SATA board. Each is individually controlled and reset-able, and can throw transport error conditions independent of the other phys. Jeff