Handling asynchronous requests in VS .NET

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12.4.7.6 Handling asynchronous requests
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Similarly, asynchronous requests are usually handled by a DoRequest() method which is responsible for setting up the hardware to create an event that will complete the request at some point in the future.
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Depending on the nature of the device driver, it may be possible to handle several outstanding asynchronous requests simultaneously. Consider my serial port example for a moment a duplex link allows simultaneous transmission and reception of data, so I want to allow both a read and a write request to be outstanding simultaneously. However, I want to prevent a client from requesting two simultaneous operations of the same type. The serial port driver handles this by maintaining a copy of the outstanding read/write request status objects (iRxStatus and iTxStatus), and panicking the client if it receives a second request of the same type as an outstanding one. (Panicking is the right thing to do here, as the client s behavior indicates that it has a serious bug.) Other device drivers, such as the local media sub-system, do allow simultaneous requests of the same type to be issued, since a single instance of a media driver may be servicing several le systems which access different partitions on the disk. Such scenarios are handled by forwarding the requests to an internal queue, from which any deferred requests are handled when it is convenient to do so. Here s how asynchronous requests are handled in my example serial driver:
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TInt DSimpleSerialChannel::DoRequest(TInt aReqNo, TRequestStatus* aStatus, TAny* a1, TAny* a2) { if(iStatus==EOpen) Start(); else return(KErrNotReady) TInt r=KErrNone; TInt len=0; switch (aReqNo) { case RSimpleSerialChannel::ERequestRead: { if(iRxStatus) { Kern::ThreadKill(iClient,EExitPanic, ERequestAlreadyPending,KLitKernExec); return(KErrNotSupported); } if(a2) r=Kern::ThreadRawRead(iClient,a2, &len,sizeof(len)); if(r==KErrNone) { iRxStatus=aStatus; InitiateRead(a1,len); } break; } case RSimpleSerialChannel::ERequestWrite: {
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if(iTxStatus) { Kern::ThreadKill(iClient,EExitPanic, ERequestAlreadyPending,KLitKernExec); return(KErrNotSupported); } if(!a1) a1=(TAny*)1; r=Kern::ThreadRawRead(iClient,a2,&len,sizeof(len)); if(r==KErrNone) { iTxStatus=aStatus; InitiateWrite(a1,len); } break; } default: return KErrNotSupported; } return r; }
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Both ERequestRead and ERequestWrite requests follow the same basic pattern. First, the status of the device is checked to determine if the channel is currently open, and if so the hardware is prepared for data transfer by calling ::Start():
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void DSimpleSerialChannel::Start() { if (iStatus!=EClosed) { PddConfigure(iConfig); PddStart(); iStatus=EActive; } }
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Since the con guration of a port is speci c to the underlying hardware, a call is made to the PDD to set up the required con guration:
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void DComm16550::Configure(TCommConfigV01 &aConfig) { // wait for uart to stop transmitting Kern::PollingWait(FinishedTransmitting,this,3,100); // Select the UART, clear bottom two bits iUart->SelectUart(); TUint lcr=0; switch (aConfig.iDataBits) { case EData8:
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lcr = T16550UartIfc::K16550LCR_Data8; break; // ... etc } switch (aConfig.iStopBits) { case EStop1: break; case EStop2: lcr |= T16550UartIfc::K16550LCR_Stop2; break; } switch (aConfig.iParity) { case EParityEven: lcr |= T16550UartIfc::K16550LCR_ParityEnable | T16550UartIfc::K16550LCR_ParityEven; break; // ... etc } iUart->SetLCR(lcr|K16550LCR_DLAB); iUart->SetBaudRateDivisor(BaudRateDivisor[(TInt)aConfig.iRate]); iUart->SetLCR(lcr); iUart->SetFCR(T16550UartIfc::K16550FCR_Enable | T16550UartIfc::K16550FCR_RxReset | T16550UartIfc::K16550FCR_TxReset | T16550UartIfc::K16550FCR_TxRxRdy | T16550UartIfc::K16550FCR_RxTrig8); }
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Notice the use of the Kern::PollingWait() API. I don t want to change the port con guration while the UART is transmitting, as this may lead to lost or badly formed data. Since there can be at most 16 bytes of data outstanding (the size of my TX FIFO), then I may simply poll the FIFO until it is fully drained. But rather than waste power and CPU cycles doing this in a code loop, I would prefer that the current thread be put to sleep for a while before checking the status again. The Kern::PollingWait() API allows me to do this. It rst checks the supplied polling function (FinishedTransmitting()) before sleeping the current thread for the speci ed poll interval (100 mS). This process is repeated until the poll period expires or the polling function returns ETrue. Be aware that if you are using this API (or any other method of polling which sleeps the current thread) then all the other drivers sharing the DFC thread will also be blocked until the poll is complete. You should take care to ensure that you don t inadvertantly affect the operation of other drivers in the system particularly if you are running within any of the standard kernel threads. Similarly, it is the responsibility of the PDD to set up and enable any interrupt-speci c con guration:
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