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SubjectRe: IDE tape and SMP
On Fri, 23 Oct 1998, Andre M. Hedrick wrote:

> The tape driver defaults to "ATAPI overlap supported" and is not flag
> tested and updated. Since the "HP COLORADO 5GB" reports that this is
> not supported hardware via the logs and the driver thinks that it is
> support, here is the first suspect in causing timing faults.

I've just eyeballed the ide-tape sources. It looks like DSC is most likely
the culprit of the problems we've been seeing with our tape devices.
Switching this off should *hopefully* fix it. The programmer who left in
the comments about it seemed to imply that DSC should be replaced with the
newer ATAPI overlap protocol for all new IDE tape devices.

Strangely enough, I think we're missing something here. This works just
fine on a UP machine as we've all said. Therefore there must be some
deeper bug in the IDE-tape code somewhere that rears its ugly head in SMP
situations. Let's dig a bit deeper. (PS: I'll turn on the debugging stuff
in ide-tape tonight, and activate SMP, retest to generate some logs -
should be fun!)

Here's the relevant comments from ide-tape.c
-- begins

* In order to maximize bus throughput, we currently use the DSC
* overlap method which enables ide.c to service requests from the
* other device while the tape is busy executing a command. The
* DSC overlap method involves polling the tape's status register
* for the DSC bit, and servicing the other device while the tape
* isn't ready.
*
* In the current QIC development standard (December 1995),
* it is recommended that new tape drives will *in addition*
* implement the ATAPI overlap protocol, which is used for the
* same purpose - efficient use of the IDE bus, but is interrupt
* driven and thus has much less CPU overhead.
*
* ATAPI overlap is likely to be supported in most new ATAPI
* devices, including new ATAPI cdroms, and thus provides us
* a method by which we can achieve higher throughput when
* sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
*/
-- ends

Cheers,
Alex.

---
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Linux lo-pc3035a 2.1.125 #6 Fri Oct 9 13:53:00 EDT 1998
One Intel Pentium 75+ processor, 66.36 total bogomips, 16M RAM
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