View source for I/O Base Classes

The base classes in <code>src/dev/io_device.*</code> allow devices to be created with reasonable ease. The classes and virtual  functions that must be implemented are listed below. Before reading the following it will help to be familiar with the [[Memory_System]].

=== PioPort===
The PioPort class is a programmed I/O port that all devices that are sensitive to an address range use. The port takes all the memory access types and roles them into one <code>read()</code> and <code>write()</code> call that the device must respond to. The device must also provide the <code>addressRanges()</code> function with which it returns the address ranges it is interested in. An extra  <code>sendTiming()</code> function is implemented which takes an delay. In this way the device can immediately call <code>sendTiming(pkt, time)</code> after processing a request and the request will be handled by the port even if the port bus the device connects to is blocked. Because of this a PIO device should not call <code>Port::sendTiming()</code> only <code>PioPort::sendTiming()</code> should be used. <code>sendTiming()</code> causes a new <code>PioPort::SendEvent</code> is created and when the event time is reached the packet is sent. If the send is not successful then the packet is placed on a the transmit list and resent when <code>recvRetry()</code> is called.

If desired a device could have more than one PIO port. However in the normal case it would only have one port and return multiple ranges when the <code>addressRange()</code> function is called. The only time multiple PIO ports would be desirable is if your device wanted to have separate connection to two memory objects. 

=== PioDevice ===
This is the base class which all devices senstive to an address range inherit from. There are three pure virtual functions which all devices must implement <code>addressRanges()</code>, <code>read()</code>, and <code>write()</code>. The magic to choose which mode we are in, etc is handled by the PioPort so the device doesn't have to bother.

Parameters for each device should be in a Params struct derived from <code>PioDevice::Params</code>.

===BasicPioDevice===
Since most PioDevices only respond to one address range <code>BasicPioDevice</code> provides an <code>addressRanges()</code> and parameters for the normal pio delay and the address to which the device responds to. Since the size of the device normally isn't configurable a parameter is no used for this and anything that inherits from this class is expected to write it's size into <code>pioSize</code> in its constructor.

===DmaPort===
The DmaPort is used only for device mastered accesses. The <code>recvFunctional()</code>, and <code>recvAtomic()</code> methods are defined to panic as the device should never receive a request on this port. the <code>recvTiming()</code> method must be available to responses (nacked or not) to requests it makes. The port has two public methods <code>dmaPending()</code> which returns if the dma port is ''busy (e.g. It is still trying to send out all the pieces of the last request).'' All the code to break requests up into suitably sized chunks, collect the potentially multiple responses and respond to the device is accessed through <code>dmaAction()</code>. A command, start address, size, completion event, and possibly data is handed to the function which will then execute the completion events <code>process()</code> method when the request has been completed. Internally the code uses <code>DmaRequestState</code> to manage what blocks it has received and to know when to execute the completion event.

=== DmaDevice ===
This is the base class from which a DMA non-pci device would inherit from, however none of those exist currently within M5. The class does have some methods <code>dmaWrite(), dmaRead()</code> that select the appropriate command from a DMA read or write operation.