data overruns while changing virtual screens

[John Kennedy]

I've come across an interesting phenomenon in AT&T SVR3.2.2
and Interactive 2.2.

When switching virtual screens (alt-SysRq F1, etc), I've observed
that either interrupts are shut off for very long periods of time,
or a DMA channel is tied up for excessively long.

The net result is a loss of incoming serial data even at speeds
down to 2400 baud.  I am already using 16550's, so I know that
over 15 characters are lost.  A rough estimate of 4 ms/char x 15
tells me that interrupts are off for at least 60 ms.

Test:

Provide a stream of incoming data, preferably one which generates a
recognizable pattern on a screen, or one that feeds a program that
can compare differences in incoming patterns.  Switch virtual consoles
and observe the character losses.

-or-

Watch your modem lights on a steady UUCP transfer.  Watch the lights when
you change virtual consoles.


John

-- 
John Kennedy                     johnk at opel.COM
Second Source, Inc.
Annapolis, MD