linux-kernel - Re: Strange problem with tty layer

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Message-ID: <45B7CDB5.7020909@microgate.com>
Date:	Wed, 24 Jan 2007 15:20:53 -0600
From:	Paul Fulghum <paulkf@...rogate.com>
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Strange problem with tty layer

Lennart Sorensen wrote:
> I have confirmed that the driver does in fact receive all the
> characters, and that they are correct, and that they are being passed to
> the tty layer using tty_insert_flip_string, and that it returns that all
> the characters have been passed to the tty layer.  The user space
> application however still doesn't see the last few characters (when it
> fails).
> 
> The problem seems to occour every few hours of testing on a 266MHz Geode
> SC1200.  When I change the clock to 133MHz, it happens every few minutes
> instead (so much more frequently).  I suspect there is some race
> condition that allows the tty layer to not get around to processing all
> the data in the buffer, even when asked for data by the application
> (which is waiting on the serial port using select, with a 4s timeout).

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.

-- 
Paul Fulghum
Microgate Systems, Ltd.
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