Re: [PATCH v8 08/13] libsas: libsas.force_hard_reset module parameter

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On Wed, Feb 29, 2012 at 2:40 PM, Douglas Gilbert <dgilbert@xxxxxxxxxxxx> wrote:
> On 12-02-29 04:55 PM, James Bottomley wrote:
>>
>> On Fri, 2012-02-10 at 00:45 -0800, Dan Williams wrote:
>>>
>>> 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.

Something like a libsas.force_max_phys_link_rate module parameter
might not be a bad idea for this scenario, since libsas sata discovery
always forces at least one reset of the disk after the phy reports
"attached sata device".

--
Dan
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