On Wed, Feb 29, 2012 at 4:23 PM, Douglas Gilbert [off-list ref] wrote:

On 12-02-29 06:27 PM, Dan Williams wrote:

quoted

On Wed, Feb 29, 2012 at 2:40 PM, Douglas Gilbert[off-list ref]
 wrote:

quoted

On 12-02-29 04:55 PM, James Bottomley wrote:

quoted

On Fri, 2012-02-10 at 00:45 -0800, Dan Williams wrote:

quoted

It is possible for a host to get "locked out" from talking to sata
devices in the domain if, for example, its sas address changes but the
expander topology has existing affiliations with the old address.  If
the system is booted userspace can write to
/sys/class/sas_phy/<phy-X>/hard_reset to clear the affiliation, however
if this condition exists for the root device the module parameter can
be
used to promote all ata resets to hard resets.

A point of order: SAS has link resets and hard resets. The
hard reset is a superset of link reset. A "link reset sequence
serves as a hard reset for SATA devices" and hence is
sufficient to reset a SATA device. To reset a SAS device
(e.g. a SAS disk) you need a SAS hard reset. Therefore a link
reset is the appropriately sized "gun" to reset a SATA device.

I have a SAS-2 expander that annoyingly powers up with the
programmed maximum physical link rate of its phys at 3 Gbps
even though its hardware maximum rate is 6 Gbps. For expander
phys connected to SAS-2 disks I can up the programmed maximum
value to 6 Gbps on the expander phy then do a link reset on
that phy. So without upsetting Linux (or any other OS) I can
switch that path from 3 Gbps to 6 Gbps. Can't do that with a
SATA disk without the OS finding out.

At least now (with these pending patches) if you trigger a link-reset
via the sysfs interface libsas will manage the link recovery like any
other error-recovery initiated reset.

I can think of 4 cases for link reset. The other end
of the link is:
 a) a SAS target: not error recovery situation
 b) a SAS expander phy: not error recovery situation
 c) a SATA device: error recovery situation

sas_try_ata_reset() [1] is what promotes user requested resets into
error recovery managed resets if the other end of the link is sata.

[1]: http://git.kernel.org/?p=linux/kernel/git/djbw/isci.git;a=blob;f=drivers/scsi/libsas/sas_init.c;h=57e7ac97b3e3dba3091f83a64c0c32a6660390cb;hb=refs/heads/all#l222