Control Mode page (0xA) QERR bit setting

It looks as though the midlayer assumes that QERR is 0. Is that correct?

If QERR is 0, then a check condition for one command will not affect other commands, whereas if QERR is 1, then when one command gets a check condition, all others for that ITL are silently aborted.

Since I wasn't able to find any code (other than in the ipr driver) that completed all outstanding commands to a Lun when a check condition is received, I figure that we depend on QERR=0.

thanks

jeremy

Actually, no, the mid-layer is agnostic to the setting of QERR. On error, it simply quiesces the host and waits for all the commands to timeout or complete.

Yes, so we see them time out in the error handler.

No, we work for either. QERR != 0 is very rare in mode pages, primarily because it's almost impossible to predict what tags actually got dropped on the floor because you don't know the status of the head scheduler (only tasks accepted and scheduled behind the failing tag actually get dropped; executing tasks ahead of it are allowed to complete as long as the failing condition doesn't impact them).

James

Well okay. They are handled, just not too quickly :-)
Not optimal behavior if lots of devices have QERR set to 1. Fortunately, they are rare, as you mention below. Though some OSes actually set that bit when they can.

Right. It kind of has to be handled by the host driver or perhaps the adapter itself, since in many cases only the adapter knows what the device has actually accepted. Even then, some complex devices may theoretically have race conditions in their SCSI engine (though one would hope not).

thanks,

jeremy

The ipr driver actually depends on QERR=1 and adapter firmware to do the right thing. TCQing doesn't get setup by the ipr driver until userspace enables it, and the userspace tool to enable it takes care of setting up QERR=1 before enabling it. When I get a check condition I send down a cancel all requests to the device, which is similar to an abort task set. The end result is that all commands that were silently aborted get returned to me with aborted status. Meanwhile, the device is in a state where it will always return a "not ready - synchronization required" response (pseudo ACA state), which I translate to DID_IMM_RETRY. I do the request sense and then clear this sync required state on the next command issued.

-Brian

Brian King
eServer Storage I/O
IBM Linux Technology Center