Re: Wording proposal for request_safe_update

I personally don't think the idea of a channel calling both request
mechanisms in the same cycle is a good idea. The idea is to create special
channels for the *async_request_update*. The async version will always have
a performance penalty to ensure safety.

When I looked at the code and considered the issue, I realized it would be
bad if that happened with the implementation I had; however, it was easy to
make it resilient. I added a separate pointer for the safe vs normal
sc_prim_channel mechanisms. This adds a small space overhead with no
performance penalty (unless you count construction time during elaboration).
So you can call both mechanisms with the net result that update will be
called twice. Since I think this should be discouraged, I don't think it is
a bad thing. At least the code is now "safe" from that scenario.

Of course an sc_prim_channel derived class could have an internal flag that
it uses to determine which of the two it will use (for its duration).

*BOTTOM LINE*: I suggest adding text to the effect:

An * sc_prim_channel* object shall not call both *request_update* and
*async_request_update
*. The results of violating this are undefined.

On Thu, Nov 25, 2010 at 4:06 AM, <john.aynsley@doulos.com> wrote:

> Philipp, David,
>
> Philip wrote "I think, we can allow both as long as they result in a
> _single_ call to 'update' in the corresponding update phase as it is for
> multiple, regular 'request_update' calls, too. (although I haven't found any
> explicit wording covering this?)."
>
> In my mind I was assuming the regular request_update had this behavior,
> because I remember writing that the update request queue is a "set". I need
> to tweak the LRM to make this unambiguous.
>
> Given that, we have two choices:
>
> * Allow request_update and aync_request_update to be mixed for the same
> primitive channel instance. This means that the implementation has to be
> able to handle this case.
>
> * Disallow it, which may simplify the implementation a little in the sense
> that async_request_update can assume there are no regular update requests in
> the queue for that primitive channel.
>
> What does the current implementation do, David? Which way should we go?
>
> John A
>
>
>
> From:
> "Philipp A. Hartmann" <philipp.hartmann@offis.de>
> To: David C Black <dcblack@xtreme-eda.com>, john.aynsley@doulos.com Cc:
> systemc-p1666-technical@eda.org
> Date: 24/11/2010 16:09 Subject: Re: Wording proposal for
> request_safe_update
> ------------------------------
>
>
>
> John, David,
>
> simultaneous calls to '(async_)request_thread_safe_update' and
> 'request_update' and are not really the key issue.
>
> I think, we can allow both as long as they result in a _single_ call
> to 'update' in the corresponding update phase as it is for multiple,
> regular 'request_update' calls, too. (although I haven't found any
> explicit wording covering this?).
>
> The real issue is here, that the user can't arbitrarily access the
> state of the current channel that actually provides this asynchronous
> access (and therefore calls request_thread_safe_update internally).
> Such concurrent accesses from inside the SystemC model and from other
> parts of the application might need to be protected by additional,
> application defined locking techniques. But this is out of the scope of
> the standard. It could warrant a note, though.
>
> Greetings from Oldenburg,
> Philipp
>
> On 24/11/10 15:08, David C Black wrote:
> >
> > On Wed, Nov 24, 2010 at 4:31 AM, <john.aynsley@doulos.com> wrote:
> [snip]
> >> 2nd question: Are there any dependencies or interactions between
> >> request_update and request_thread_safe_update? Can you have both for the
> >> same primitive channel instance? What happens?
> >
> > Good point, I think it should be stated that it is undefined if both are
> > called. Choose one or the other.
>
>
> --
> Philipp A. Hartmann
> Hardware/Software Design Methodology Group
>
> OFFIS Institute for Information Technology
> R&D Division Transportation · FuE-Bereich Verkehr
> Escherweg 2 · 26121 Oldenburg · Germany · http://offis.de/en/
> Phone/Fax: +49-441-9722-420/282 · PGP: 0x9161A5C0 · Skype: phi.har
>
>
>
>

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