Re: [hylafax-users] Request For Linux Kernel Help: Was 2.6.26 Breaks USB Modem

[Lee Howard <lee.howard@xxxxxxxxxxxx>]

Frank Peters wrote:
> Last week I reported that the latest Linux kernel 2.6.26  breaks
> faxing with a USB modem.  The problem has been acknowledged (flow
> control is broken) 

Are you sure that the problem is limited to USB modems?  Have you tested 
with a different hardware modem?  Who acknowledged the problem as a 
flow-control issue in the kernel?

The reason I ask:

http://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg188348.php

If you read through that entire thread you may find that there is some 
degree of ambiguity in the presumed diagnosis of "flow control 
problem"... because it actually seemed more-likely that the buffer 
overrun was due to interrupt latency caused by the heavy-used ATA driver 
getting preferential treatment on its interrupts.  Note that the thread 
eventually got Alan Cox's attention - who I believe is responsible for 
the serial driver these days - however the problem was never 
pinpointed... or, rather, a solution was never identified (even after 
spending a lot of effort with real-time kernel support, see: 
http://hylafax.sourceforge.net/howto/install.php#ss2.6).

Here is another potential Linux kernel serial driver issue to consider...

http://www.spinics.net/lists/linux-serial/msg01180.php  (see the whole 
thread)

Here the symptom appears to be dissimilar (or extremely aggravated), but 
the finger-pointing is again directed at interrupt handling.

> and a possible way to pinpoint the cause is to
> bisect the kernel versions with git.
>   

So, basically step through commits or daily check-outs from 2.6.25 where 
you didn't have the problem until 2.6.26 where you do have the problem 
to see if you can determine the commit or day at which the problem is 
introduced.  Then you track-down the person who approved the commit who 
will then either fix the problem or refer it on to the person who 
developed the patch, etc.

That's a lot of work, and I wish you the best of luck with it.  My 
advice to you would be step back and stay with 2.6.25 until a larger 
mass of users are running 2.6.26 such that it could attract the 
attention of someone capable of actually fixing the problem head-on (and 
they would most-likely be those you can see responding to serial 
questions on linux-serial or linux-kernel mailing lists).

In my experience bug fixes to the Linux kernel either come very quickly 
in response to the bug report or they become very lengthy and laborious 
processes which really require the assistance from, if not advocacy of, 
someone with a good deal of Linux kernel development experience.

Sometimes issues such as interrupt latency become very complicated, and 
it's really impossible to undo a commit because it was part of a whole 
transformation of internal kernel API.  In those cases a fix has to come 
from direct inspection and creative problem-solving, rather than trying 
to back-step through commits in order to "fix" something that isn't 
really broken per the contributor.

My guess is that if you wait for 2.6.27 you'll see the problem is gone.  
And if not, then you'll almost certainly have much more company and a 
louder voice with which to get the necessary attention.

Thanks,

Lee.

--
*Lee Howard*
*Mainpine, Inc. Software Development Lead*
Tel: +1 866 363 6680 ext 5 | Fax: +1 360 462 8160
lee.howard@xxxxxxxxxxxx | www.mainpine.com


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