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From: joerg@mwtech.UUCP (Joerg Werner)
Newsgroups: comp.os.minix
Subject: Re: Minix ST 1.5 printer driver problems
Message-ID: <1095@mwtech.UUCP>
Date: 22 Apr 91 09:24:09 GMT
References: <30@oski.toppoint.de>
Reply-To: joerg@mwtech.UUCP (Joerg Werner)
Organization: MIKROS Systemware, Darmstadt/W-Germany
Lines: 36

In article <30@oski.toppoint.de> schlut@oski.toppoint.de (Olaf Schlueter) writes:

>When I wrote a HP Deskjet + printer driver for mroff on Minix ST 1.5, I
>got problems with the kernel printer driver. It looked like on the way from
>the mroff driver to the printer somewhere bytes had been added or removed
>so I got nasty bit shifts and misinterpreted graphics data on the
>page occasionally. What had been most weird, it was only partially
>reproducable, i.e. I could count on getting at least two destroyed lines
>on the page, but could not predict where it would happen, even with
>essantially the same output. The problem is definitely in the kernel:

I've had the same problem with NEC P6+!

>Every once and again during the output the kernel reported "printer still
>busy". To cite Mr. Spock "That's fascinating", since the place where this
>message is produced, is entered on response to a printer not busy interrupt.

Delete the print statement :-)

>Just a layer above in piaint() I guess a race condition: there is
>a sequence of code looking like:
>
>  while(left > 0) { outchar(*addr++); left--; ... }
>

Yes, there is a race condition. But the solution is very simple.
The Problem is, that piaint() is reentered before left is decremented.
My solution (since MINIX ST 1.1) is:

  while(left > 0) { left--; outchar(*addr++); ... }

Decrement left _before_ outchar() is called!

Ciao Joerg
-- 
Joerg Werner, email: joerg@mwtech.UUCP, voice: 49-(0)6151-37 33 66