Hello,
I am developing an in-kernel usb device driver. When I call the
following method, defined in IOUSBPipe.h, the "reqCount" parameter is
ignored. Instead, it uses buffer->GetLength().
virtual IOReturn Write(
IOMemoryDescriptor * buffer,
UInt32 noDataTimeout,
UInt32 completionTimeout,
IOByteCount reqCount,
IOUSBCompletion * completion = 0);
I think I traced the problem down to the
AppleUSBOHCI::CreateGeneralTransfer() method in AppleUSBOHCI_UIM.cpp
(from the Darwin source). In this method, the "reqCount" parameter is
passed in as "bufferSize". The following is an abridged excerpt from the
CreateGeneralTransfer() function:
transferOffset = 0;
while (transferOffset < bufferSize)
{
// Figure out what pages compromise the memory descriptor,
// and get the phsical address of those pates
if (pageCount == 2)
{
// ...
transferOffset += physicalAddresses[1].length;
}
transferOffset += physicalAddresses[0].length;
// ...
}
It appears (to me) that the length of the transfer is set by the
memory descriptor length, not the "reqCount" (or "bufferSize")
parameters. Even though the while() condition depends on the
"bufferSize" variable, the body of the while loop uses on the size
of the memory descriptor. Is this the intended behavior?
-Dan
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