linux-kernel - Re: Serial problem and request for guidance..

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Message-ID: <bec9a5870806171433gf74e251m6742e066960d6ab4@mail.gmail.com>
Date:	Wed, 18 Jun 2008 00:33:21 +0300
From:	"Nameer Yarkon" <nameer.yarkon@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Serial problem and request for guidance..

On Mon, Jun 16, 2008 at 3:03 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> Some of my serial rx data gets lost.
>
> Which hardware ?

It's a range of platforms/kernel versions and scenarios.
Currently it happens with a bluetooth device connected to standard UART
on ARM (OMAP) with an old 2.6.14 kernel, but only when I enable the
debug macros of the line discipline code (hci_h4, hci_ldisc,..).
When a big chunk of data arrives, more than
half of it is lost. even on 115200. But when I turn of the debug macros (BT_DBG
macros which are simply a bunch of prinks) the problem disappears. So I suspect
there are some nasty timing issues which lead to data loss.

I encountered similar problems last year even on a PC, with pre 2.6.21 kernels
(around 2.6.19 or even earlier IIRC) but it was all gone with 2.6.21 and 2.6.22.
Currently I have totally different machines, and these devices tend now to come
with USB-to-serial connectors so I'm not quite sure about up-to-date
kernels yet.

All I can say is that it is nasty and very hard to debug, and I'm not quite sure
how can I debug it as I find the serial/tty subsystems a bit perplexing.

I know this is not necessarily relevant to the tip of the kernel, and it still
lacks many details, but any guidance towards how can I debug it will
be much appreciated. If I will successfully reproduce it with a current
kernel I promise a full and detailed bug report.

Thank you for your time and patience !
Nameer.
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