Date: Wed, 15 Jan 97 10:33:31 PST
From: wmb@FirmWorks.COM (Mitch Bradley)
Subject: Item #404: SCSI Annex retry improvements

P1275 Open Firmware Working Group Proposal -- Proposal #404 Ver 0

Title:		SCSI Retry Improvements

Author:		Mitch Bradley

Date:		1/15/97

Ed/Tech:	Technical

Synopsis:	"medium not present" shouldn't be retryable

Doc & Version:	1275-1996 and SPI binding

Problem:

Refer to 1275-1994, Annex E (SCSI Host Adapter Package Class), Sections
E3.2.2, definition of "retry-command", and the sample implementation of
"classify-sense" in E6.3 "hacom.fth".

The classification of the "not ready" sense key (sense key value 2) as
"retryable" is too general.  One of the subcases of "not ready" is
"medium not present", as indicated by an Additional Sense Code (the byte
at sense data offset decimal 12) value of hex 3A and an Additional Sense
Code Qualifier (the byte at sense data offset decimal 13) value of 0.

In order for a sense condition to be properly classified as "retryable",
it should be self-correcting, so that the overall operation is likely to
eventually succeed if it is retried enough times.  "Medium not present"
is not a self-correcting condition, so it should not be retried.

One specific situation in which this problem shows up is when the list
of boot devices (i.e. the value of the "boot-file" configuration variable)
contains a tape drive at some position other than the end of the list.
Typically, the desired behavior in such a situation is for the system to
boot from the tape drive if a tape is present, otherwise to proceed to the
next device in the list.  If "medium not present" is retryable, the system
will instead wait indefinitely for a tape to be inserted.

In addition to this problem, the example code for "classify-sense" in E6.3
also fails to handle the "9) Transaction aborted ..." case listed on P223
L33.

Proposal:	

In 1275-1994 P223 L31, and the identical line in the SPI binding:

Replace:

	7) sense key = Not Ready is retryable

With:

	7) sense key = Not Ready is:
	    - not retryable if Additional Sense Code = 0x3A and Additional
              Sense Code Qualifier = 0 (thus indicating "medium not present")
	    - retryable otherwise

In 1275-1994 P229 L54-55:

Replace:

	\ not-ready(2) and attention(6) are retryable
	dup 2 =  swap 6 =  or  if 1  else  -1  then

With:

	\ not-ready(2) may be retryable
	dup 2 =  if
	   \ check (tapes, especially) for MEDIUM NOT PRESENT: if the
	   \ medium's not there the command is not retryable
	   drop sense-buf h# c + c@  h# 3a =  sense-buf h# d + c@ 0=  and
	   if  -1  else  1  then
	   exit
	then

	\ attention(6), and target aborted (b) are retryable.
	dup 6 =  swap 0b =  or if  1  else  -1  then

[ P1275 Item #404 -- Received: Wed Jan 15 10:31:56 PST 1997 ]