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Re: [lwip-users] lwip-users Digest, Vol 157, Issue 27

From: Robert Sexton
Subject: Re: [lwip-users] lwip-users Digest, Vol 157, Issue 27
Date: Sat, 24 Sep 2016 16:58:27 +0000
Re: prioritizing of active connections

Norbert,

The Cortex-M CPU that you are using has architectural features that can solve your problem.  

The trick is to install a SVC handler, and use that to call lwIP.      If the SVC handler has the same IRQ priority as the ethernet interrupt, the interrupt controller will enforce exclusive access to the lwIP stack.     Then you wrap the lwIP functions that you need in system calls, and it will all just work.   The only place where things get complicated is getting data out of lwIP structures in an interrupt-safe method.   That may not be an issue in your case, or you can just use system calls when accessing lwIP structures.

Keep in mind that you cannot use these system calls from interrupt context, only the lower-priority worker tasks that you describe.

Here's a sample syscall handler: svchandler.S   with a few sample lwIP calls at the bottom.

If you don't want to resort to compiler trickery, you will have to define some simple assembly functions to serve as the stand-ins for the final lwIP functions:

Declaration:
extern err_t SAPI_tcp_write(struct tcp_pcb * pcb, void *,uint16_t len, uint8_t flags);

systemcalls.S:
....
.global SAPI_tcp_write
SAPI_tcp_write:
svc #22
bx lr
.... 

The latter function could also be done with a compiler inline.

You can use this technique to safely access any lwIP function from main().

- Robert






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Today's Topics:

   1. Re: prioritizing of active connections (Noam Weissman)


----------------------------------------------------------------------

Message: 1
Date: Sat, 24 Sep 2016 12:58:10 +0000
From: Noam Weissman <address@hidden>
To: "address@hidden" <address@hidden>
Subject: Re: [lwip-users] prioritizing of active connections
Message-ID:
        <address@hidden>

Content-Type: text/plain; charset="iso-8859-1"

Hi,


Doing some "dirty" trick will not help but cause other problems. IwIP can back-fire and

you will get unpredictable results. Problems that can be difficult to find.


sys_timeout is an LwIP function that runs inside the context of LwIP. Time interval is in milliseconds.


That means that if you set it to 100ms it will call your function (only once) when time elapses.


So instead of using the poll call back that is triggered at best every 0.5 seconds ... you can set your

own timeout function that will handle some house keeping at a faster rate. Do remember that if you

need it to periodically do something you need to set it again and again... until you finish the process.


BR,

Noam.


________________________________
From: lwip-users <lwip-users-bounces+noam=address@hidden> on behalf of Norbert Kleber <address@hidden>
Sent: Saturday, September 24, 2016 11:20 AM
To: address@hidden
Subject: Re: [lwip-users] prioritizing of active connections


Hi Noam,


thank you for your suggestion. I guess I will start with an OS for the next project. For this one it's allready to late I am afraid (thesis due date next month). I didn't fully understand the explanation in the wiki regarding the sys_timeout(TMR_INTERVAL, function , NULL); function. Which timer triggers this function and how can i calculate the exact timing of the sys_timeout?

For now i found an quick and dirty approach which i can't recommand to anyone. I just call 2x tcp_tmr() directly after my computing work is done. This improves my performance. Where i needed 36 seconds for one run I now only need 16 seconds.


sincerly,

Norbert

Am 23.09.2016 um 18:30 schrieb Noam Weissman:

Hi Norbert,


First of all I would suggest changing your design and use an OS. I am running FreeRTOS

on STM micro's for 6 years now and I do not see myself doing it any other way.


The STM32F4 is a strong micro with sufficient power to do much more then you do now.


If you run an OS there will be a small overhead but your system design will be much simpler

to menage.


If you use Socket API you can send data outside of the LwIP context. If you use RAW API

you cannot send data from outside of the LwIP context and must take that into consideration:


First option protect the code that is called from outside of the LwIP context. Either by using a

critical section (OS).... or using the poll call back.... or triggering LwIP own system_timer call-back:


    sys_timeout(TMR_INTERVAL, function , NULL);


The above is an LwIP internal timer handling. You pass the function you want (see prototype)

with or without parameters and it will be triggered when time expires.



BR,

Noam.

________________________________
From: lwip-users <lwip-users-bounces+noam=address@hidden><mailto:lwip-users-bounces+noam=address@hidden> on behalf of Norbert Kleber <address@hidden><mailto:address@hidden>
Sent: Friday, September 23, 2016 6:58 PM
To: address@hidden<mailto:address@hidden>
Subject: [lwip-users] prioritizing of active connections

Hi evereyone,

I got some questions again.

I am using the lwIP on a STM32F4 mikrocontroller without OS. A client
will connect to the stack at two ports for transmission. First port will
be used for controlsignals and second for datatransmission. The whole
System works sequentially.

Firstly 1 package will be received on the ctrl connection afterwards we
receive many packages on the dataconnection. Now I was wondering that
the stack acknowlegded all data packages before he acknowledges the ctrl
package. Due to the operation of the ?C I can tell that he received the
package right away and ofcause i use the acknowledgement function in the
receive function.  But somehow it is severly delayed approx. 200ms but
the later received data packages all get acknowledged right away.

Does the stack some prioritizing between open connections? Or is it due
to the Ctrl Package being quite short?

Also I am doing some computational work outside of the callback
functions and want to transmit some of the results asap over the ctrl
connection. Is there a way to do that without waiting for the polling
function? Or is there a way to trigger the polling function somehow for
a instant call? Also it seemed like i can't use tcp_write if i am
outside of a callback function (i was storing the pointer to the pcb in
a global variable).

sincerly,

Norbert


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