AM79C978VCSLASHW datasheet(241/261 Pages) AMD

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APPENDIX B

B-1

Am79C978

Look-Ahead Packet Processing

(LAPP) Concept

INTRODUCTION

A driver for the controller would normally require that

the CPU copy receive frame data from the controllers

buffer space to the applications buffer space after the

entire frame has been received by the controller. For

applications that use a ping-pong windowing style, the

traffic on the network will be halted until the current

frame has been completely processed by the entire ap-

plication stack. This means that the time between last

byte of a receive frame arriving at the client’s Ethernet

controller and the client’s transmission of the first byte

of the next outgoing frame will be separated by:

1. The time that it takes the client’s CPU interrupt pro-

cedure to pass software control from the current

task to the driver,

2. Plus the time that it takes the client driver to pass

the header data to the application and request an

application buffer,

3. Plus the time that it takes the application to gener-

ate the buffer pointer and then return the buffer

pointer to the driver,

4. Plus the time that it takes the client driver to transfer

all of the frame data from the controller’s buffer

space into the application’s buffer space and then

call the application again to process the complete

frame,

5. Plus the time that it takes the application to process

the frame and generate the next outgoing frame,

and

6. Plus the time that it takes the client driver to set up

the descriptor for the controller and then write a

TDMD bit to CSR0.

The sum of these times can often be about the same

as the time taken to actually transmit the frames on the

wire, thereby, yielding a network utilization rate of less

than 50 percent.

An important thing to note is that the controller’s data

transfers to its buffer space are such that the system

bus is needed by the controller for approximately 4 per-

cent of the time. This leaves 96 percent of the system

bus bandwidth for the CPU to perform some of the in-

terframe operations in advance of the completion of

network receive activity, if possible. The question then

becomes: how much of the tasks that need to be per-

formed between reception of a frame and transmission

of the next frame can be performed before the recep-

tion of the frame actually ends at the network, and how

can the CPU be instructed to perform these tasks dur-

ing the network reception time.

The answer depends upon exactly what is happening

in the driver and application code, but the steps that

can be performed at the same time as the receive data

are arriving include as much as the first three steps and

part of the fourth step shown in the sequence above.

By performing these steps before the entire frame has

arrived, the frame throughput can be substantially

increased.

A good increase in performance can be expected when

the first three steps are performed before the end of the

network receive operation. A much more significant

performance increase could be realized if the controller

could place the frame data directly into the applica-

tion’s buffer space; (i.e., eliminate the need for step 4.)

In order to make this work, it is necessary that the ap-

plication buffer pointer be determined before the frame

has completely arrived, then the buffer pointer in the

next descriptor for the receive frame would need to be

modified in order to direct the controller to write directly

to the application buffer. More details on this operation

will be given later.

An alternative modification to the existing system can

gain a smaller but still significant improvement in per-

formance. This alternative leaves step 4 unchanged in

that the CPU is still required to perform the copy oper-

ation, but is allows a large portion of the copy operation

to be done before the frame has been completely re-

ceived by the controller, i.e., the CPU can perform the

copy operation of the receive data from the controller’s

buffer space into the application buffer space before

the frame data has completely arrived from the net-

work. This allows the copy operation of step 4 to be

performed concurrently with the arrival of network data,

rather than sequentially, following the end of network

receive activity.

OUTLINE OF LAPP FLOW

This section gives a suggested outline for a driver that

utilizes the LAPP feature of the controller.

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