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

[Jeff Garzik]

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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