8xx SCC and SMC uart latency

[Eli Brin]

Dear Jerry and Wolfgang,

Thank you for your reply.

I have changed the kernel clock rate to 1000 (/include/asm-ppc/param.h,
#define HZ 1000). It helped.
We reply in about 2mSec and we are within the 4mSec timeout.

As we are new to Linux, we imigrated from windows :), it took us some time.

We have a C++ program that listens to the serial port and reads the data.
If a packete has the address of our program we parse the data, and reply.

In parallel the program listens on a TCP port and waits for commands.  Those
commands are sent through the serial port.
We use goap (soap xml for c/c++).

The initial tests where done witout all the parsing and TCP overhead.  We
just replied imidiatly to packets addressed to us, and we got 5-8mSec delay
from the last char we received and the first char we send.  Now (HZ=1000)
it's 2mSec with the parsing and TCP/IP.

Can we leave the HZ to 1000 ?

Now that is works we can commit our project to Linux and use the RTAI.

Thanks again, and
Best Regards
Eli Brin



-----Original Message-----
From: Jerry Van Baren [mailto:gerald.vanbaren at smiths-aerospace.com]
Sent: Monday, January 13, 2003 5:40 PM
To: linuxppc-embedded at lists.linuxppc.org
Subject: Re: 8xx SCC and SMC uart latency



Your last line on the i386 makes me think you are polling the UART and
immediately sending back the response.  This is a totally different model
than what you are running under linux.

You can approach that model under linux only by writing a custom device
driver that understand your communications protocol and, in the Rx handler,
processes the received packet, generates data, and queues it to be sent.  Of
course, this is going to take some effort and has a pretty good sized
learning curve to climb.  There are some device driver toolkits that can
help here.

Your 10mSec timeout and 5-8mSec latency numbers sound suspiciously like task
switch (clock tick) times of 100Hz.  You need to tell us more about how your
code is structured and what happens along the path from when the first byte
comes in to when the first byte goes out, including all the separate tasks
that get involved and how they communicate with each other.  As a quick
test, increase your kernel clock rate to 1000Hz and see if it improves your
timings by about 10x.

gvb


At 04:14 PM 1/13/2003 -0500, wd at denx.de wrote:

>Dear Eli,
>
>in message <75DF04AC5ED4D511A9810090273CB4163A66F0 at ROKONET-E> you
>wrote:
> >
> > We have a device that communicates over RS232 at 9600 bps.  This
> > device has a 4 mSec timeout, i.e we have to reply in less then
> > 4mSec.
>
>Which parts of your software are included in this timeout?
>
>Do you mean something like that:
>
>* driver receives data
>* application reads data from driver
>* application generates response
>* application writes response to driver
>* driver sends data
>
>?
>
> > The packets are usually small (4-6 Bytes).  We communicate
> > successfully
> with
> > the device, but only if we change the timeout to above 10mSec. It
> > seems that it takes 5-8 mSec for us to reply.  This is the same if
> we use
>
>You might try to run your code with LTT instrumentation so  that  you
>can  see  where  it  takes  the  time.  I  guess  it's just that your
>application takes that long before it gets scheduled.
>
> > SMC or SCC.  We tried to play with the MAX_IDL and the Tx/Rx buffer
> > sized, but with no success.
> >
> > Has anyone an idea on how to minimize the uart latency?
>
>I don't think that this has anything to do with the UART driver.
>
> > If we use the same code on a PC (i386) we reply in 300 microSec.
>
>If you have to guarantee a response time, you will have  to  consider
>using a real-time extension like RTAI. The standard Linux kernel does
>not guarantee any latencies.
>
>Best regards,
>
>Wolfgang Denk
>
>--
>Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
>Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd at denx.de
>You can love it, change it, or leave it.    There is NO other option.
>But do not complain - it is your own choice...                  -- wd
>


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