[fa.info-vax] posting to fa.info-vax, Vax/controller hardware question

info-vax@ucbvax.ARPA (05/02/85)

From: decwrl!ios!richa@BERKELEY


I have a question on Vax hardware operation.

I am having trouble understanding the correct operation of disk drivers
in handling asynchronous Attention Status interrupts.  I have looked at
the 4.1/4.2 up.c (Emulex Unibus SC21), hp.c (Emulex Massbus SC750),
and Sys V hm.c (Massbus RM05).  In each case, on occurance of an
interrupt, the driver merely checks whether it had last started
a transfer operation, if not then process Attention Status, else
transfer is complete and check error bits.  My problem is I can see
no assurance that since a transfer was started, that the interrupt
occuring must be due to transfer complete.

It would be required that a Unibus controller which is issuing Bus Request
to try to cause an interrupt (e.g. AS, due to Seek complete on another drive)
but is not being Granted due to driver interrupt masking,
would cancel or defer that interrupt request when it received
a command (transfer).  Do all Unibus controllers behave this way?
This means the device drops Bus Request without having received
a Bus Grant.

It is also implied here that Bus Request/Grant must propagate all the
way to the processor to be resolved against processor IPL.  The Adapter
may not do its own priority arbitration and queueing of vector(s)
to be forwarded to the processor.  This seems to cause interesting
effects such as the processor executing a MTPR to mask interrupts
possibly having to pause execution for a bus timeout period if
a Grant had just been issued (the arbiter must wait for the incoming
vector or timeout).  Otherwise there is a possibility of accepting
a vector when the processor will not take the interrupt (violating
the sequence assumption above).

The reason I am wondering if things really work this way is the whole
structure seems rather delicate.  Especially when I don't find it described
anywhere.  Even the excellent Emulex manuals don't say exactly when an
asynchronous interrupt might be issued.
Thanks for any help.  Please respond directly to me by mail.

{wjvax, decwrl, oliveb, qubix, durango, idi, apple, pesnta, sun}!ios!richa
Rich Altmaier, Integrated Office Systems
2111 Grant Rd
Los Altos, CA 94022