Argument to __builtin_dmaoffset() is not the address of an object in a dma section;

Question

I have a dsPIC33 and programming it using MPLAB X.

I have three files: main.c ADC.c ADC.h

In ADC.h I do a bunch of variable initialisation as follows:

typedef struct pinPair{
    unsigned CH0:5,CH123:1,STAT:8,:2;
    unsigned I:16;
    unsigned V0:16;
    unsigned V1:16;
    unsigned V2:16;
}pinPair;

typedef pinPair PINPAIRS [5];   
extern PINPAIRS pinPairs __attribute__((space(dma)));

then in my main.c file I realise the pinPairs variable:

//Define pinPairs data buffers
PINPAIRS pinPairs __attribute__((space(dma),aligned(256)));

int main(void) {

...

}

Now in my ADC.c file comes the confusing part, The variable pinPair is clearly aligned somewhere in the dam memory because if I do this:

DMA0STA=__builtin_dmaoffset(pinPairs);

the compiler just does its thing but if I do it in the way I was intending:

DMA0STA=__builtin_dmaoffset(pinPairs[pairCounter]);

I get the error:

error: Argument to __builtin_dmaoffset()  is not the address of an object in a dma section;

But it is though !!!

Does anyone know what it is that this error means and how I could use the variable in the way I want ?

Answer 1

I can imagine multiple possibilities

• The subentities of the array don't have the proper attributes set, and the compiler doesn't lift pinpairs[pairCounter] (a pinpair struct)

• due to Paircounter being a variable, the compiler can't compute a compiletime address for value.

If the latter, a fixed value would work, if the former, then a compiler limitation that is pretty hard to workaround.

I'd try something like

DMA0STA=__dmaoffset(pinpairs)+dmacounter*uint16(&pinpairs[1]-&pinpairs[0])

(plus maybe some typecasts to get it assigned)