sc_signal cannot have more than one driver error

[Khushi]

Hi

I have a use case where multiple threads are writing on a signal port and I get the sc_signal<T> cannot have more than one driver error.

To avoid that I started to use sc_mutex and use lock/unlock., but still I get the same error.

Here is what I tried to reproduce my use case with a small example.

Can you help me to understand what is going wrong even after using mutex and how to avoid this issue.

#include"systemc.h"

using namespace std;
using namespace sc_core;

class module : public sc_module {
    public:
    sc_signal<bool> sig;
    sc_mutex m;
    explicit module(sc_module_name name): sc_module(name),sig("sig"){
        SC_HAS_PROCESS(module);
        SC_THREAD(p1);
        SC_THREAD(p2);
    }
    void p1(){
        while(1){
            wait(10,SC_NS);
            m.lock();
            sig.write(1);
            m.unlock();
        }
    }

    void p2(){
        while(1){
            wait(10,SC_NS);
            m.lock();
            sig.write(0);
            m.unlock();
        }
    }

    ~module(){
    }
};

int sc_main(int, char**){
    module m("m");
    sc_start();
    return 0;
}

Thanks 

Khushi

[karthickg]

Check the documentation of SC_ONE_WRITER and SC_MANY_WRITERS in the LRM. The default value of the "WRITER_POLICY" in sc_signal is SC_ONE_WRITER.

[Khushi]

Just to add, in my use case it is guaranteed that the different threads write to same port(signal) at different time stamp.

Thanks

Khushi

[Khushi]

my point is that if multiple threads write to same port at same time than this error make sense but when multiple threads write to same port at different time than it should be ok as there is no race in that case. It is as good as same thread write to that port at different time stamp.

I am trying to understand the logic behind this error when threads are competing for the same resource in different time points.

[karthickg]

1 minute ago, Khushi said:

Just to add, in my use case it is guaranteed that the different threads write to same port(signal) at different time stamp.

Thanks

Khushi

That still is a problem with SC_ONE_WRITER policy. Quoting from LRM (my emphasis):

Quote

 If the template argument WRITER_POLICY has the value SC_ONE_WRITER, it shall be an error to write to a given signal instance from more than one process instance at any time during simulation.

See the change if you declare sig as:

sc_signal<bool, SC_MANY_WRITERS> sig;

 

[Khushi]

Yes agreed wrt LRM

But why "at any time" instead of "at same time" ?

Thanks

Khushi

[David Black]

If you want multiple drivers, use sc_signal_resolved or sc_signal_rv<N>. The reason for SC_ONE_WRITER is to catch errors since the normal use case for sc_signal is single drivers. The SC_MANY_WRITERS was intended to allow one process to initialize and then hand-off to another process. Your use case likely the resolved signal situation. Verilog uses wires for the resolved case. VHDL has you to be more explicit.

[Moreshwar Salpekar]

I had a similar question only difference is that in my case, I have declared:

sc_signal<bool, SC_MANY_WRITERS>

on one side

and

sc_port<sc_signal_inout_if<bool>, 2> 

on other side.  I used this because on the side of DUT, i need an inout port and there is only one wire for bidirectional data.

Still i am getting same error:  sc_signal<T> cannot have more than one driver.

I wanted to know why the above code (using SC_MANY_WRITERS with sc_signal) cannot work. if you need any files or output, please let me know.

 

Please note that using

sc_signal_resolved 

and

 sc_inout_resolved

works fine though I had to change all true to SC_LOGIC_0 and false to SC_LOGIC_1 to make it work.

[David Black]

Many writers detects race conditions.