[Linux-parport] Ping - bug report with fix... printer hangs on parallel port

[tim]

Hi,

See below for my original note from some years ago (2003!). The fix
never got installed, so printers will still be hanging. The problem
affects printers with slow response times back to the parallel port eg
HP1100 and causes them to appear to hand in the middle of a page.

I heard there is a new maintainer for the parallel port drivers so I
thought I would have another go at getting this fixed.

The original bug report with fix is at
http://cyberelk.net/tim/parport/archive/current/5486.html

Tim Waugh also suggested a different version of the patch - his version
is also in the thread. I tested Tim W's version and it worked OK too.

Regards,
Tim Josling

Original Note
*************
[PARPORT] [PATCH] to drivers/parport/ieee1284_ops.c to fix timing
dependent printer hang
From: Tim Josling (tej at melbpc.org.au)
Date: Thu Mar 20 2003 - 14:32:16 EST 

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________________________________________________________________________

Hopefully I have at last foudn the right place to post this... 


I have an HP1100 printer and since I upgraded to a faster CPU the 
printer has started hanging. The problem persisteed across 2.0 2.2 and 
2.4 kernel versions. I am running Red Hat Linux 8.0 on a Compaq Armada
E500. 


The problem occurs intermittently. The symptom is that the 'buffer 
contains data' light stays on on the printer, but data transfer stops. 


I traced the problem to drivers/parport/ieee1284_ops.c function 
parport_ieee1284_write_compat. The problem occurs when the parallel
port 
is not using interrupts. If the printer takes a while to respond the 
routine parport_wait_event gets called, if count == 0. However this 
routine generally waits almost no time, as shown in my traces, if the 
port does not have interrupts enabled. Looking at that code, it seems 
that parport_wait_event is only meant to be called when interrupts are 
available on the port. 


Anyway, if 32 repeats of this occur e.g. for a complex document where 
the printer is slow, 'wait' ends up as a negative number from repeated 
doublings due to the way twos complement arithmetic works in C. In this 
case the routine never returns, or at least waits for a very long time 
i.e. hours. So no more data gets sent to the printer. 


Originally I fixed the problem by adding code to ensure that 'wait' 
never got set to anything above 10 seconds (10 * HZ). However the patch 
I have sent you does something different, it just ensures that 
parport_wait_event never gets called for printer without interrupt. I 
have tested this on documents which reproduce the problem and the hangs 
go away. 


Clearly I am not an expert on the parport code, so my patch may be 
incorrect. I have traces using extra printks I put in the code, showing 
the wait variable being doubled to negative value, available on
request. 


Definitely my patch does fix a real problem on my system. 


As far as I can tell from browsing the patches since 2.4.18, there has 
not been any other fix for this problem to date. 


Regards, 
Tim Josling 


--- ChangeLog.original 2003-03-16 09:18:07.000000000 +1100 
+++ ChangeLog 2003-03-16 09:20:35.000000000 +1100 
@@ -1,3 +1,9 @@ 
+2003-03-16 Tim Josling <tej at melbpc.org.au> 
+ 
+ * ieee1284_ops.c (parport_ieee1284_write_compat): Avoid calling 
+ parport_wait_event if interrupts are not enabled for device, avoid 
+ output hang. 
+ 
   2002-04-25 Tim Waugh <twaugh at redhat.com> 


       * parport_serial.c, parport_pc.c: Move some SIIG cards around. 


--- ieee1284_ops.c.original 2003-03-16 08:27:33.000000000 +1100 
+++ ieee1284_ops.c 2003-03-16 09:17:23.000000000 +1100 
@@ -93,7 +93,7 @@ 
                              first time around the loop, don't let go
of 
                              the port. This way, we find out if we
have 
                              our interrupt handler called. */ 
- if (count && no_irq) { 
+ if (count || no_irq) { 
                   parport_release (dev); 
                   __set_current_state (TASK_INTERRUPTIBLE); 
                   schedule_timeout (wait);