linux-kernel - Re: Strange problem with tty layer

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Message-ID: <20070124214840.GC7584@csclub.uwaterloo.ca>
Date:	Wed, 24 Jan 2007 16:48:40 -0500
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	Paul Fulghum <paulkf@...rogate.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Strange problem with tty layer

On Wed, Jan 24, 2007 at 03:20:53PM -0600, Paul Fulghum wrote:
> In 2.6.16 the tty buffering pushes data to the line
> discipline without regard to tty->receive_room.
> If the line discipline can't keep up, the data gets dropped.
> I observed this data loss at higher speeds when
> placing the system under heavy load.
> 
> 2.6.18 added code to respect tty->receive_room.
> 
> This may or may not be your problem, but you should
> be able to check by adding a conditional printk
> to drivers/char/tty_io.c:flush_to_ldisc()
> 
> If tty->receive_room is less than the size of the buffer
> passed to disc->receive_buf() then you are losing data.

Sounds plausible.  Certainly higher cpu load makes the problem occour
more often, and making the system slower made it worse too.  I will take
a look at the tty_io.c in 2.6.18 and compare it against 2.6.16.

--
Len Sorensen
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