linux-kernel - Re: 2.6.20.3-rt8 - DMA suffers excessive delay periodically

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Message-ID: <460A297A.2030200@free.fr>
Date:	Wed, 28 Mar 2007 10:38:18 +0200
From:	John <linux.kernel@...e.fr>
To:	linux-kernel@...r.kernel.org
CC:	Lee Revell <rlrevell@...-job.com>, mingo@...e.hu, akpm@...l.org,
	linux-apps@...r.kernel.org, linux.kernel@...e.fr
Subject: Re: 2.6.20.3-rt8 - DMA suffers excessive delay periodically

Lee Revell wrote:

> John wrote:
>
>> I'm runnning 2.6.20.3 patched with -rt8 (and glibc 2.3.6).
>> http://people.redhat.com/mingo/realtime-preempt/older/patch-2.6.20-rt8
>>
>> I've written a program to highlight a phenomenon I don't understand.
>>
>> This system includes a PCI board that provides data at ~38 Mbit/s.
>> I request data in lumps of 1316 bytes.
>> In other words, a new request should complete every 277 µs.
>>
>> However, if I time stamp each packet as soon as I receive it, I notice
>> that almost exactly every 2 seconds, the packet is received 30-100
>> microseconds late.
> 
> Sounds like periodic SMM traps.  There's not much you can do about
> them other than to avoid such hardware if you have tight RT
> constraints.  The RTAI people have some code to disable SMM but it
> does not work on all chipsets and may render some of your hardware
> inoperable.

The motherboard is an EBC-2000T.
http://www.adlinktech.com/PD/web/PD_detail.php?pid=213

lspci -vvv output provided as an attachment.

VIA Pro133T chipset:
http://www.via.com.tw/en/products/chipsets/legacy/pro133/
http://www.via.com.tw/en/products/chipsets/legacy/pro133a/

dmesg mentions:
DMI 2.3 present.

Is this related to SMM?

As far as I can tell, the BIOS is Phoenix AwardBIOS v6.00PG.

Would someone know how to disable SMM in this BIOS?

> Is this a laptop?  They are plagued with SMM problems...

No it is an "indutrial" motherboard.
Although I don't know what makes it "industrial".

Regards.

View attachment "adlink.lspci" of type "text/plain" (9970 bytes)