Copy data to the memory descriptor's buffer from the specified buffer.
A byte offset into the memory descriptor's memory.
The caller supplied buffer to copy the data from.
The length of the data to copy.
The number of bytes copied, zero will be returned if the specified offset is beyond the length of the descriptor. Development/debug kernel builds will assert if the offset is beyond the length of the descriptor.
This method copies data to the memory descriptor's memory at the given offset, from the caller's buffer. The memory descriptor MUST have the kIODirectionIn direcction bit set and be prepared. kIODirectionIn means that this memory descriptor will be input from an external device, so writeBytes is used to write memory into the descriptor for PIO drivers.