pa_asio.cpp

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        void *buffer = stream->asioBufferInfos[ i + stream->inputChannelCount ].buffers[index];

        int bytesPerSample = BytesPerAsioSample( stream->asioChannelInfos[ i + stream->inputChannelCount ].type );

        memset( buffer, 0, stream->framesPerHostCallback * bytesPerSample );
    }
}


static unsigned long SelectHostBufferSize( unsigned long suggestedLatencyFrames,
        PaAsioDriverInfo *driverInfo )
{
    unsigned long result;

    if( suggestedLatencyFrames == 0 )
    {
        result = driverInfo->bufferPreferredSize;
    }
    else{
        if( suggestedLatencyFrames <= (unsigned long)driverInfo->bufferMinSize )
        {
            result = driverInfo->bufferMinSize;
        }
        else if( suggestedLatencyFrames >= (unsigned long)driverInfo->bufferMaxSize )
        {
            result = driverInfo->bufferMaxSize;
        }
        else
        {
            if( driverInfo->bufferGranularity == -1 )
            {
                /* power-of-two */
                result = 2;

                while( result < suggestedLatencyFrames )
                    result *= 2;

                if( result < (unsigned long)driverInfo->bufferMinSize )
                    result = driverInfo->bufferMinSize;

                if( result > (unsigned long)driverInfo->bufferMaxSize )
                    result = driverInfo->bufferMaxSize;
            }
            else if( driverInfo->bufferGranularity == 0 )
            {
                /* the documentation states that bufferGranularity should be
                    zero when bufferMinSize, bufferMaxSize and
                    bufferPreferredSize are the same. We assume that is the case.
                */

                result = driverInfo->bufferPreferredSize;
            }
            else
            {
                /* modulo granularity */

                unsigned long remainder =
                        suggestedLatencyFrames % driverInfo->bufferGranularity;

                if( remainder == 0 )
                {
                    result = suggestedLatencyFrames;
                }
                else
                {
                    result = suggestedLatencyFrames
                            + (driverInfo->bufferGranularity - remainder);

                    if( result > (unsigned long)driverInfo->bufferMaxSize )
                        result = driverInfo->bufferMaxSize;
                }
            }
        }
    }

    return result;
}


/* returns channelSelectors if present */

static PaError ValidateAsioSpecificStreamInfo(
        const PaStreamParameters *streamParameters,
        const PaAsioStreamInfo *streamInfo,
        int deviceChannelCount,
        int **channelSelectors )
{
	if( streamInfo )
	{
	    if( streamInfo->size != sizeof( PaAsioStreamInfo )
	            || streamInfo->version != 1 )
	    {
	        return paIncompatibleHostApiSpecificStreamInfo;
	    }

	    if( streamInfo->flags & paAsioUseChannelSelectors )
            *channelSelectors = streamInfo->channelSelectors;

        if( !(*channelSelectors) )
            return paIncompatibleHostApiSpecificStreamInfo;

        for( int i=0; i < streamParameters->channelCount; ++i ){
             if( (*channelSelectors)[i] < 0
                    || (*channelSelectors)[i] >= deviceChannelCount ){
                return paInvalidChannelCount;
             }           
        }
	}

	return paNoError;
}


/* see pa_hostapi.h for a list of validity guarantees made about OpenStream  parameters */

static PaError OpenStream( struct PaUtilHostApiRepresentation *hostApi,
                           PaStream** s,
                           const PaStreamParameters *inputParameters,
                           const PaStreamParameters *outputParameters,
                           double sampleRate,
                           unsigned long framesPerBuffer,
                           PaStreamFlags streamFlags,
                           PaStreamCallback *streamCallback,
                           void *userData )
{
    PaError result = paNoError;
    PaAsioHostApiRepresentation *asioHostApi = (PaAsioHostApiRepresentation*)hostApi;
    PaAsioStream *stream = 0;
    PaAsioStreamInfo *inputStreamInfo, *outputStreamInfo;
    unsigned long framesPerHostBuffer;
    int inputChannelCount, outputChannelCount;
    PaSampleFormat inputSampleFormat, outputSampleFormat;
    PaSampleFormat hostInputSampleFormat, hostOutputSampleFormat;
    unsigned long suggestedInputLatencyFrames;
    unsigned long suggestedOutputLatencyFrames;
    PaDeviceIndex asioDeviceIndex;
    ASIOError asioError;
    int asioIsInitialized = 0;
    int asioBuffersCreated = 0;
    int completedBuffersPlayedEventInited = 0;
    int i;
    PaAsioDriverInfo *driverInfo;
    int *inputChannelSelectors = 0;
    int *outputChannelSelectors = 0;

    /* unless we move to using lower level ASIO calls, we can only have
        one device open at a time */
    if( asioHostApi->openAsioDeviceIndex != paNoDevice ){
        PA_DEBUG(("OpenStream paDeviceUnavailable\n"));
        return paDeviceUnavailable;
    }

    if( inputParameters && outputParameters )
    {
        /* full duplex ASIO stream must use the same device for input and output */

        if( inputParameters->device != outputParameters->device ){
            PA_DEBUG(("OpenStream paBadIODeviceCombination\n"));
            return paBadIODeviceCombination;
    }
    }

    if( inputParameters )
    {
        inputChannelCount = inputParameters->channelCount;
        inputSampleFormat = inputParameters->sampleFormat;
        suggestedInputLatencyFrames = (unsigned long)((inputParameters->suggestedLatency * sampleRate)+0.5f);

        /* unless alternate device specification is supported, reject the use of
            paUseHostApiSpecificDeviceSpecification */
        if( inputParameters->device == paUseHostApiSpecificDeviceSpecification )
            return paInvalidDevice;

        asioDeviceIndex = inputParameters->device;

        PaAsioDeviceInfo *asioDeviceInfo = (PaAsioDeviceInfo*)hostApi->deviceInfos[asioDeviceIndex];

        /* validate hostApiSpecificStreamInfo */
        inputStreamInfo = (PaAsioStreamInfo*)inputParameters->hostApiSpecificStreamInfo;
        result = ValidateAsioSpecificStreamInfo( inputParameters, inputStreamInfo,
            asioDeviceInfo->commonDeviceInfo.maxInputChannels,
            &inputChannelSelectors
        );
        if( result != paNoError ) return result;
    }
    else
    {
        inputChannelCount = 0;
        inputSampleFormat = 0;
        suggestedInputLatencyFrames = 0;
    }

    if( outputParameters )
    {
        outputChannelCount = outputParameters->channelCount;
        outputSampleFormat = outputParameters->sampleFormat;
        suggestedOutputLatencyFrames = (unsigned long)((outputParameters->suggestedLatency * sampleRate)+0.5f);

        /* unless alternate device specification is supported, reject the use of
            paUseHostApiSpecificDeviceSpecification */
        if( outputParameters->device == paUseHostApiSpecificDeviceSpecification )
            return paInvalidDevice;

        asioDeviceIndex = outputParameters->device;

        PaAsioDeviceInfo *asioDeviceInfo = (PaAsioDeviceInfo*)hostApi->deviceInfos[asioDeviceIndex];

        /* validate hostApiSpecificStreamInfo */
        outputStreamInfo = (PaAsioStreamInfo*)outputParameters->hostApiSpecificStreamInfo;
        result = ValidateAsioSpecificStreamInfo( outputParameters, outputStreamInfo,
            asioDeviceInfo->commonDeviceInfo.maxOutputChannels,
            &outputChannelSelectors
        );
        if( result != paNoError ) return result;
    }
    else
    {
        outputChannelCount = 0;
        outputSampleFormat = 0;
        suggestedOutputLatencyFrames = 0;
    }

    driverInfo = &asioHostApi->openAsioDriverInfo;

    /* NOTE: we load the driver and use its current settings
        rather than the ones in our device info structure which may be stale */

    result = LoadAsioDriver( asioHostApi->inheritedHostApiRep.deviceInfos[ asioDeviceIndex ]->name,
            driverInfo, asioHostApi->systemSpecific );
    if( result == paNoError )
        asioIsInitialized = 1;
    else{
        PA_DEBUG(("OpenStream ERROR1\n"));
        goto error;
    }

    /* check that input device can support inputChannelCount */
    if( inputChannelCount > 0 )
    {
        if( inputChannelCount > driverInfo->inputChannelCount )
        {
            result = paInvalidChannelCount;
            PA_DEBUG(("OpenStream ERROR2\n"));
            goto error;
        }
    }

    /* check that output device can support outputChannelCount */
    if( outputChannelCount )
    {
        if( outputChannelCount > driverInfo->outputChannelCount )
        {
            result = paInvalidChannelCount;
            PA_DEBUG(("OpenStream ERROR3\n"));
            goto error;
        }
    }


    // check that the device supports the requested sample rate 

    asioError = ASIOCanSampleRate( sampleRate );
    PA_DEBUG(("ASIOCanSampleRate(%f):%d\n",sampleRate, asioError ));

    if( asioError != ASE_OK )
    {
        result = paInvalidSampleRate;
        PA_DEBUG(("ERROR: ASIOCanSampleRate: %s\n", PaAsio_GetAsioErrorText(asioError) ));
        goto error;
    }


    // retrieve the current sample rate, we only change to the requested
    // sample rate if the device is not already in that rate.

    ASIOSampleRate oldRate;
    asioError = ASIOGetSampleRate(&oldRate);
    if( asioError != ASE_OK )
    {
        result = paInvalidSampleRate;
        PA_DEBUG(("ERROR: ASIOGetSampleRate: %s\n", PaAsio_GetAsioErrorText(asioError) ));
        goto error;
    }
    PA_DEBUG(("ASIOGetSampleRate:%f\n",oldRate));

    if (oldRate != sampleRate){

        PA_DEBUG(("before ASIOSetSampleRate(%f)\n",sampleRate));
        asioError = ASIOSetSampleRate( sampleRate );
        /* Set sample rate */
        if( asioError != ASE_OK )
        {
            result = paInvalidSampleRate;
            PA_DEBUG(("ERROR: ASIOSetSampleRate: %s\n", PaAsio_GetAsioErrorText(asioError) ));
            goto error;
        }
        PA_DEBUG(("after ASIOSetSampleRate(%f)\n",sampleRate));
    }
    else
    {
        PA_DEBUG(("No Need to change SR\n"));
    }


    /*
        IMPLEMENT ME:
            - if a full duplex stream is requested, check that the combination
                of input and output parameters is supported
    */

    /* validate platform specific flags */
    if( (streamFlags & paPlatformSpecificFlags) != 0 ){
        PA_DEBUG(("OpenStream invalid flags!!\n"));
        return paInvalidFlag; /* unexpected platform specific flag */
    }


    stream = (PaAsioStream*)PaUtil_AllocateMemory( sizeof(PaAsioStream) );
    if( !stream )
    {
        result = paInsufficientMemory;
        PA_DEBUG(("OpenStream ERROR5\n"));
        goto error;
    }

    stream->completedBuffersPlayedEvent = CreateEvent( NULL, TRUE, FALSE, NULL );
    if( stream->completedBuffersPlayedEvent == NULL )
    {
        result = paUnanticipatedHostError;
        PA_ASIO_SET_LAST_SYSTEM_ERROR( GetLastError() );
        PA_DEBUG(("OpenStream ERROR6\n"));
        goto error;
    }
    completedBuffersPlayedEventInited = 1;


    stream->asioBufferInfos = 0; /* for deallocation in error */
    stream->asioChannelInfos = 0; /* for deallocation in error */
    stream->bufferPtrs = 0; /* for deallocation in error */

    if( streamCallback )
    {
        PaUtil_InitializeStreamRepresentation( &stream->streamRepresentation,
                                               &asioHostApi->callbackStreamInterface, streamCallback, userData );
    }
    else
    {
        PaUtil_InitializeStreamRepresentation( &stream->streamRepresentation,
                                               &asioHostApi->blockingStreamInterface, streamCallback, userData );
    }


    PaUtil_InitializeCpuLoadMeasurer( &stream->cpuLoadMeasurer, sampleRate );


    stream->asioBufferInfos = (ASIOBufferInfo*)PaUtil_AllocateMemory(
            sizeof(ASIOBufferInfo) * (inputChannelCount + outputChannelCount) );
    if( !stream->asioBufferInfos )
    {
        result = paInsufficientMemory;
        PA_DEBUG(("OpenStream ERROR7\n"));
        goto error;
    }


    for( i=0; i < inputChannelCount; ++i )
    {
        ASIOBufferInfo *info = &stream->asioBufferInfos[i];

        info->isInput = ASIOTrue;

        if( inputChannelSelectors ){
            // inputChannelSelectors values have already been validated in
            // ValidateAsioSpecificStreamInfo() above
            info->channelNum = inputChannelSelectors[i];
        }else{
            info