Re: Multiple writers to sc_signal

I agree with Dave here: If it's simple enough to be added, it would be
really good to have.

That said, two more comments on this:

- I like the proposal to add a "multiple writer" policy template
  parameter to sc_signal/sc_buffer.

  If we still check-for and disallow conflicting writes in the
  same delta, we would even avoid to define who is the "last
  one winning" and retain determinism.

  I would also vote for keeping the static check of a single writing
  port here. But this can be decided separately.

- I'm not sure about specific multiple-writer ports, though. This
  doesn't seem to add "real" safety, as it stands now:

  sc_inout<bool> port;
  sc_inout_mw<bool> mw_port;

  mw_port.bind( port ); // would not work
  port.bind( mw_port ); // compiles (and fail at runtime)

  So the assumed IP vendor would still need to handle support requests
  due to this issue and add appropriate documentation to the model.

Greetings from Oldenburg,
Philipp

On 08/11/10 18:55, david.long@doulos.com wrote:
> If there is a simple proposal that can be agreed upon without lots of
> discussion then I vote YES, otherwise I do not think it a high priority at
> this time.
>
> Dave L
>
>
>
>
> From:
> john.aynsley@doulos.com
> To:
> P1666 Technical WG <systemc-p1666-technical@eda.org>
> Date:
> 08/11/2010 17:24
> Subject:
> Re: Multiple writers to sc_signal
> Sent by:
> owner-systemc-p1666-technical@eda.org
>
>
>
> Okay, let's have a straw poll. Please vote whether you would like the
> P1666 WG to pursue the proposal for having multiple writers to an
> sc_signal at this time - YES or NO.
>
> John A
>
>
>
> From:
> David C Black <dcblack@xtreme-eda.com>
> To:
> Jerome CORNET <jerome.cornet@st.com>
> Cc:
> "Philipp A. Hartmann" <philipp.hartmann@offis.de>,
> "john.aynsley@doulos.com" <john.aynsley@doulos.com>, Bishnupriya
> Bhattacharya <bpriya@cadence.com>, Stuart Swan <stuart@cadence.com>, P1666
> Technical WG <systemc-p1666-technical@eda.org>
> Date:
> 08/11/2010 16:13
> Subject:
> Re: Multiple writers to sc_signal
>
>
>
>
> I would agree there are definite uses for such a model. I was sort of
> hoping Cadence would take up the cause. I really like Cadence previous
> proposal because it ensure the user is very aware of the issue. I was sort
> of hoping Cadence would take up the cause.
>
>
> On Nov 8, 2010, at 9:43 AM, Jerome CORNET wrote:
>
>> To concur with Philipp, we are currently evaluating internally the
> benefit of the proposal.
>> In many cases indeed, this could ease the implementation, say of
> TLM->RTL transactors.
>>
>> Regards,
>>
>> Jerome
>>
>>
>> -----Original Message-----
>> From: owner-systemc-p1666-technical@eda.org [
> mailto:owner-systemc-p1666-technical@eda.org] On Behalf Of Philipp A.
> Hartmann
>> Sent: Monday, November 08, 2010 4:37 PM
>> To: john.aynsley@doulos.com
>> Cc: Bishnupriya Bhattacharya; Stuart Swan; P1666 Technical WG
>> Subject: Re: Multiple writers to sc_signal
>>
>> All,
>>
>> I find it somewhat sad, that this proposal is dropped for now.
>>
>> When writing transactors / adapters to RTL blocks with signal-level
>> interfaces, a (restricted) policy for multiple drivers of a signal can
>> ease the implementation in many cases.
>>
>> If the transactor already arbitrates concurrent requests, there may be
>> technically multiple writers to a signal (due to different initiators),
>> but the behaviour is still well-defined since there are no conflicting
>> updates during the same delta cycle. Thus, a policy that allows
>> multiple drivers in separate deltas would both be useful and safe.
>>
>> But since there seems to be no strong interest in P1666 in such a
>> feature, I think we can still pursue this topic inside OSCI after
>> P1666-2011.
>>
>> Greetings from Oldenburg,
>> Philipp
>>
>>
>> On 08/11/10 12:55, john.aynsley@doulos.com wrote:
>>> Okay. I'll close the issue of the sc_multiple_sig_writers_policy class.
>>>
>>> John A
>>>
>>>
>>>
>>> From:
>>> Bishnupriya Bhattacharya <bpriya@cadence.com>
>>> To:
>>> "john.aynsley@doulos.com" <john.aynsley@doulos.com>, Stuart Swan
>>> <stuart@cadence.com>
>>> Cc:
>>> P1666 Technical WG <systemc-p1666-technical@eda.org>
>>> Date:
>>> 08/11/2010 08:36
>>> Subject:
>>> RE: Multiple writers to sc_signal
>>>
>>>
>>>
>>> John,
>>>
>>> As Stuart also opined, there is currently no interest or motivation in
>>> pursuing a sc_multiple_sig_writers_policy class.
>>>
>>> Thanks,
>>> -Bishnupriya
>>>
>>>
>>> From: owner-systemc-p1666-technical@eda.org [
>>> mailto:owner-systemc-p1666-technical@eda.org] On Behalf Of
>>> john.aynsley@doulos.com
>>> Sent: Friday, November 05, 2010 5:00 AM
>>> To: john.aynsley@doulos.com; Stuart Swan
>>> Cc: P1666 Technical WG
>>> Subject: Re: Multiple writers to sc_signal
>>>
>>> Bishnupriya, Stuart, All,
>>>
>>> In the past, Cadence made a proposal to add an
>>> sc_multiple_sig_writers_policy as a template parameter to sc_signal
> (see
>>> below). Do you, Bisnupriya and/or Stuart, still wish to champion this
>>> proposal? Do others think it is a good idea?
>>>
>>> Thanks,
>>>
>>> John A
>>>
>>> -----owner-systemc-p1666-technical@eda.org wrote: -----
>>> To: bpriya@cadence.com, systemc-p1666-technical@eda.org
>>> From: john.aynsley@doulos.com
>>> Sent by: owner-systemc-p1666-technical@eda.org
>>> Date: 11/02/2010 04:09PM
>>> Subject: Multiple writers to sc_signal
>>>
>>> Bishnupriya, All,
>>>
>>> Next on the list is the issue of multiple writers to sc_signal.
>>>
>>> As a reminder, I include some historical notes below.
>>>
>>> John A
>>>
>>>
>>> [BB] 6.4.4 The LRM states that it is an error for multiple processes
> to
>>> write to an sc_signal. The LRM also says it is ok to write to a
> sc_signal
>>> during elaboration in order to initialize it. The category of writes
> that
>>> the LRM does not specify are mainly those coming from sc_main and from
>>> other unusual places, like phase callbacks (e.g, start_of_simulation),
> a
>>> channel's update phase etc. This can all be generalized under the
> umbrella
>>> of writing to a signal from a non-process context (the current process
> is
>>> 0). We think extrapolating what the LRM says, all such writes to a
> signal
>>> can be considered ok, and do not contribute as a driver. It seem
>>> reasonable to have a consistent rule like sc_main (generally, any
>>> non-process context) never counts as a writer when writing to a signal.
>>>
>>> [JA] My take on it is that the LRM is explicit that sc_signal::write()
>
>>> calls request_update(), that update() changes the value and notifies an
>
>>> event. Moreover, request_update() can be called during elaboration (it
> is
>>> explicitly allowed in the callbacks). When multiple writes() are made
> in
>>> the same phase, the most recent write() wins. Writes from multiple
>>> processes instances are forbidden, the (implicit, unstated) reason
> being
>>> that the behaviour would be non-deterministic. Writes from outside
>>> processes are permitted, the (implicit, unstated) reason being that the
>
>>> behavior would be deterministic (because elaboration is
> single-threaded).
>>> I think that hangs together?
>>>
>>> However, I don't think there is a strong reason to change the OSCI
>>> simulator. Old code should still work (because the OSCI sim supports
> many
>>> deprecated features). New code should comply with 1666 (whether or not
> a
>>> tool implements the checks properly). In time, the OSCI sim may get
>>> changed to be less tolerant of legacy code.
>>>
>>> [AG] The existing simulator ignores initializations by sc_main if they
>
>>> occur before any process writes, but reports an error if sc_main tries
> to
>>> change a value that has been written by the process. I think I should
> just
>>> fix things to ignore writes out of the simulator's purview, e.g. by
>>> sc_main.
>>>
>>> [BB/JA] Need an LRM clarification to that effect
>>>
>>> [BB] 6.4.4 The LRM clearly states that having multiple processes
> write
>>> to a sc_signal is an error. In the refsim, the implementation is
> somewhat
>>> flaky. The multiple writer check is performed only if the DEBUG_SYSTEMC
>
>>> macro is defined and is liable to break for pre-defined signals like
>>> sc_signal<bool> etc., if the systemc library is not compiled with
>>> DEBUG_SYSTEMC defined. It would be nice to have a cleaner solution.
>>>
>>> From user experience, it turns out that people hardly ever have
>>> DEBUG_SYSTEMC on, and are often surprised in the case that
> DEBUG_SYSTEMC
>>> is on, and the check reports an error. It is not uncommon for a model
> to
>>> use to two different processes that write to the same signal in a
> mutually
>>> exclusive fashion, e.g., at the posedge and negedge of a clock,
>>> respectively. People expect such models to work and are surprised that
> the
>>> check rejects this.
>>>
>>> In view of all of the above, in the past, Stuart has proposed that this
>
>>> fact be made explicit in the signal declaration by adding a second
>>> template parameter to sc_signal that specifies if multiple writers are
> ok
>>> or not, with the default value being not ok. I'm trying out an
>>> implementation at my end that explores this option.
>>>
>>> enum sc_multiple_sig_writers_policy {
>>> SC_MULTIPLE_WRITERS_NOT_OK, // default
>>> SC_MULTIPLE_WRITERS_OK
>>> };
>>>
>>> template <class T,sc_multiple_sig_writers_policy POL =
>>> SC_MULTIPLE_WRITERS_NOT_OK>
>>> class sc_signal { ... };
>>>
>>> If the template parameter value is SC_MULTIPLE_WRITERS_NOT_OK, then
>>> sc_signal::write() checks for multiple writers and reports an error if
>>> more than one process writes to it. This check always happens
> irrespective
>>> of the DEBUG_SYSTEMC flag.
>>>
>>> If the template parameter value is SC_MULTIPLE_WRITERS_OK, then
>>> sc_signal::write() allows multiple writers and the last one wins.
>>>
>>> This is the basic proposal for sc_signal. Typically, users won't bother
>
>>> with the second template parameter, and by default it would perform the
>
>>> check, thus being LRM compliant. In the cases where users do want to
> have
>>> the convenience of multiple writers, they can specify the second
> template
>>> parameter value to be SC_MULTIPLE_WRITERS_OK.
>>>
>>> A related topic is for IP vendors writing a module with output ports -
>>> these output ports will be bound to signals externally and is not
>>> controlled by the IP vendor. However, the IP vendor may also choose to
>>> have the convenience of using multiple processes to write to an output
>>> port, in which case it will be an error if the sc_signal bound to that
>>> port does not allow multiple writers. There should then be a way for an
> IP
>>> vendor to specify that you can only connect a sc_signal that allows
>>> multiple writers to this output port. This is similar to the concept of
>
>>> the sc_out_resolved port, which can only be bound to a
> sc_signal_resolved.
>>> The port proposal is
>>>
>>> template <class T> class sc_out_mw : public sc_out<T> { ... };
>>>
>>> template <class T> class sc_inout_mw : public sc_inout<T> { ... };
>>>
>>> Such a port cannot be bound to a resolved signal (sc_signal_resolved or
>
>>> sc_signal_rv) because it is inconsistent for the port to say I've
> multiple
>>> processes writing to me and the last one wins, and at the same time ask
>
>>> for resolution
>>>
>>> Such a port cannot be bound to a
> sc_signal<T,SC_MULTIPLE_WRITERS_NOT_OK>
>>> because it will generate a run time error due to multiple processes
>>> writing to the signal via the port
>>>
>>> Such a port can only be bound to a sc_signal<T,SC_MULTIPLE_WRITERS_OK>
>>> In its end_of_elaboration, this port type will check that it is bound
> to
>>> the correct type of sc_signal

-- 
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
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