linux-kernel - Re: [PATCH] libata: rewrite SCSI host scheme to be one per ATA host

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Message-ID: <49EF4A45.8040901@garzik.org>
Date:	Wed, 22 Apr 2009 12:48:05 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
CC:	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH] libata: rewrite SCSI host scheme to be one per ATA host

Boaz Harrosh wrote:
> On 04/22/2009 12:23 PM, 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.
>>
>>
>>> I was able to successfully boot the following patch on
>>> AHCI/x86-64/Fedora.
>>>
>>> It may work with other controllers -- TRY AT YOUR OWN RISK.  It will
>>> probably fail for master/slave configurations, and SAS & PMP also
>>> need looking at.  It yielded this lsscsi output on my AHCI box:
>>>
>>> [0:0:0:0]    disk    ATA      ST3500320AS      SD15  /dev/sda
>>> [0:2:0:0]    disk    ATA      G.SKILL 128GB SS 02.1  /dev/sdb
>>> [0:5:0:0]    cd/dvd  PIONEER  BD-ROM  BDC-202  1.04  /dev/sr0
>> For comparison, here is unmodified 2.6.30-rc3:
>>
>> [jgarzik@bd ~]$ lsscsi
>> [0:0:0:0]    disk    ATA      ST3500320AS      SD15  /dev/sda
>> [2:0:0:0]    disk    ATA      G.SKILL 128GB SS 02.1  /dev/sdb
>> [5:0:0:0]    cd/dvd  PIONEER  BD-ROM  BDC-202  1.04  /dev/sr0
>>
> 
> Could the master/slave be simply solved by emulating a SCSI LUN
> for example below is my machine today:
> []$ lsscsi
> [0:0:0:0]    disk    ATA      ST3160023A       3.01  /dev/sda
> [1:0:0:0]    cd/dvd  _NEC     DVD_RW ND-3550A  1.05  /dev/sr0
> [4:0:0:0]    disk    ATA      WDC WD1600JS-60M 10.0  /dev/sdb
> 
> the /dev/sda and /dev/sr0 share a master/slave wide cable (sdb is sata)
> 
> it could be made to scan as:
> []$ lsscsi
> [0:0:0:0]    disk    ATA      ST3160023A       3.01  /dev/sda
> [0:0:0:1]    cd/dvd  _NEC     DVD_RW ND-3550A  1.05  /dev/sr0
> [1:0:0:0]    disk    ATA      WDC WD1600JS-60M 10.0  /dev/sdb
> 
> So we need to emulate the REPORT_LUN (or what ever else) to return
> two LUNs. Or do you want to report a separate target for the master/slave?

Mapping master/slave is not difficult -- each should be a separate 
target, just like with parallel SCSI.

The issue with master/slave and simplex is guaranteeing that only _one_ 
command may be executing at a time, for a given set of targets, i.e. 
only one command per master/slave pair, only one command per pair of 
simplex ports (== 4 ATA devices max).

Originally this was done by setting can_queue==1, cmd_per_lun==1, and 
assigning each master/slave pair to a different Scsi_Host.

	Jeff




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