Re: [lwip-users] sys_timeout

[Peter Graf]

Hi Curt,

> > > I think it's an architectural deficiency that any of 
> > > > sys_arch is implemented in the lwip core.
> >
> > I found it a benefit.
>
> Peter,
>
> To be clear: I'm not denigrating the idea of sys_arch, which is a typical 
> example of a system abstraction layer and is critical for
> portability.  All of the API definitions it provides are fine.
>
> However, sys_arch is not doing its job.  It needs to be a clean layer between 
> lwip and the underlying operating system, but instead
> it's inextricably tangled into lwip.  For portability, it should be possible to 
> completely replace the implementation of that layer.

But portability to what? If an OS can not provide the requirements lwIP 
has now, I can not see how it'll meet what is required for a 
multitasking networking implementation anyway.

> No part of sys_arch belongs in lwip core.  All of the sys_arch implementations 
> should reside fully in the contrib platform ports.
> To wit, half of sys_arch is implemented in one particular way (core/sys.c), 
> severely limiting what can be done to port the other
> half.
>
> What about those portions of code that are thought to be common across *all* 
> platforms?  First, there are none, as the troubling
> examples below will illustrate.  Second, as you noted, in a proper multitasking 
> O/S it is an easy matter to map each call.  Third,
> even if there is common code between some of the ports, that's no reason to put 
> it in lwip core instead of contrib.
>
> My operating system has its own perfectly good timeout function.  However, I'm 
> unable to use it because the core of lwip depends
> (incorrectly) on specific internal functionality of sys_timeout.  If I were able to provide my own sys_timeout, I would be fine. 

If your multitasking O/S has just semaphores with timeouts and shared 
memory, you _are_ fine. In my case, I didn't even have semaphores in the 
OS, just timeouts, and I implemented the semaphores with timeouts myself.

> The mere existence of the pseudo-API calls sys_timeouts(), sys_arch_sem_wait() 
> and sys_arch_mbox_fetch() provide further evidence
> that something is broken.  These calls are unclean and should not be necessary.

They are not broken (at least not in the somewhat older version I'm 
using). The internal mechanism is just a bit hard to understand and not 
elegant at some points. But someone who just wants to port lwIP doesn't 
need to care.

> > > Worse, any application that wants to use lwip is forced to 
> > be based on lwip's sys_arch primitives.
> >
> > If lwIP is intialized and the main+tcp+interface threads are 
> > active (this can be set up outside the application), the 
> > application just has to link itself into lwIP's thread list 
> > once. Afterwards it is free to call the lwIP networking 
> > API(s) or not, but it's not required to use the sys_arch primitives.
>
> The latter is not a true statement.  Say your O/S has its own semaphore wait, 
> which nearly all do.  Your application can never call
> it instead of the sys_arch version, or sys_timeouts will cease.

I guess this is a misunderstanding which comes up here every now and then.

If your thread calls the semaphore wait of the OS directly, it is 
already implicitely sure that _this_ thread doesn't have timeouts in 
lwIPs list. (It could only have timeouts in the list if a blocking call 
into lwIP's API was ongoing in it's own context). The timeouts of the 
other threads are not affected if this thread uses the OS's mechanisms 
in parallel (unless the OS is broken).

> This forces your whole application to be rewritten based on
> sys_arch.

No :-) I have several applications running fine, none was rewritten 
except for the networking calls and a startup function which makes sure 
that lwIP is up and I link my thread(s) in it's list.

> That is unreasonable, and in my (large) application, not possible.  If sys_arch 
> is properly separated out, this would no
> longer be a problem.

If it was separated out, the risk of misunderstandings and erratic 
architecure layers rises. This could cause maintainance problems.

All thge best
Peter