Data +Transfer across Memory Boundaries

When a driver reads or writes data from or to a user-side program, the +data must be copied between user address space and Kernel address space. The +Kernel provides a number of APIs that allow this to be done safely. The API +to use depends on whether the request is serviced in a user thread context, +or in a Kernel thread context.

From +kernel space

To read and write data to the user space from +a kernel thread context, use Kernel APIs such as Kern::ThreadDesRead(), Kern::ThreadRawRead(), Kern::ThreadDesWrite(), and Kern::ThreadRawWrite(). The data can be handled +as descriptors or as raw memory.

The Kernel also provides other APIs +to safely read the information about a descriptor in another thread's address +space. Kern::ThreadGetDesInfo() returns the length, maximum +length, and a pointer to the descriptor's buffer. The length and maximum length +can also be obtained specifically by using Kern::ThreadGetDesLength() and Kern::ThreadGetDesMaxLength().

TInt DExDriverLogicalChannel::DoControl(TInt aFunction, TAny* a1, TAny* /*a2*/) + { + ... + // To read the buffer descriptors from user space + r=Kern::ThreadDesRead(iClient,a1,iTxBuffer,0,0); + ... + // To write the buffer descriptors to user space + r=Kern::ThreadDesWrite(iClient,(TDes8*)a1,iRxBuffer,0); + ... + } // Read the descriptor maximum length from the user thread (iClient) +// using the kernel API Kern::ThreadGetDesMaxLength(). This API +// gets the maximum length of a descriptor in another thread's +// address space. Kern::ThreadGetDesInfo() can also be used to +// get length, maximum length and pointer to the descriptor's +// buffer. +// +iCount=Kern::ThreadGetDesMaxLength(iClient,aData);

From user space

When +executing a read or write operation in the context of a user thread, use the +following APIs:

kumemget()
kumemput()
umemget()
umemput()

Kernel APIs are also available to do copy operations using descriptors:

Kern::KUDesGet()
Kern::KUDesPut()
Kern::KUDesInfo()
Kern::KUdesSetLength()

Kern::InfoCopy() can be used to copy a descriptor +safely in a way that enables forward and backward compatibility. It is typically +used for copying capability packages.

void DExDriverLogicalDevice::GetCaps(TDes8& aCaps) const + { + // device capabilities are packaged to a buffer + TPckgBuf<RExDriver::TUartCaps> caps; + caps().iVersion=iVersion; + // Copy the device capabilities to the user thread using the + // Kernel API + Kern::InfoCopy(aCaps,caps); + }