AM79C970 datasheet(155/168 Pages) AMD

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Am79C970

Look-Ahead Packet Processing (LAPP) Concept

APPENDIX D

Introduction of the LAPP Concept

A driver for the PCnet-PCI controller would normally re-

quire 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 control-

ler. 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 application stack. This means that the time be-

tween last byte of a receive frame arriving at the clients

Ethernet controller and the clients transmission of the

first byte of the next outgoing frame will be separated by:

1. The time that it takes the clients CPUs interrupt

procedure to pass software control from the cur-

rent 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 trans-

fer all of the frame data from the controllers buffer

space into the applications buffer space and then

call the application again to process the complete

frame

5. plus the time that it takes the application to proc-

ess the frame and generate the next outgoing

frame

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%.

An important thing to note is that the PCnet-PCI control-

lers data transfers to its buffer space are such that the

system bus is needed by the PCnet-PCI controller for

approximately 4% of the time. This leaves 96% of the

system bus bandwidth for the CPU to perform some of

the inter-frame 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

performed between reception of a frame and transmis-

sion of the next frame can be performed before the re-

ception of the frame actually ends at the network, and

how can the CPU be instructed to perform these tasks

during 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 ar-

riving include as much as the first 3 steps and part of the

4th 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 3 steps are performed before the end of the net-

work receive operation. A much more significant per-

formance increase could be realized if the PCnet-PCI

controller could place the frame data directly into the ap-

plications 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 PCnet-PCI 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 opera-

tion, but it allows a large portion of the copy operation to

be done before the frame has been completely received

by the controller; i.e., the CPU can perform the copy op-

eration of the receive data from the PCnet-PCI control-

lers buffer space into the application buffer space before

the frame data has completely arrived from the network.

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.

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