UM0851 datasheet(54/245 Pages) STMICROELECTRONICS

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Communication device drivers

UM0851

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Doc ID 16604 Rev 2

The data shown above is a result of the following configuration features supported by the

driver.

●

Jumbo frames

Since its creation (around 1980), Ethernet has used 1500 byte frame sizes. To maintain

backward compatibility, 100 Mbps Ethernet used the same size, and today "standard"

gigabit Ethernet also uses 1500 byte frames. This is so a packet to/from any

combination of 10/100/1000 Mbps Ethernet devices can be handled without any layer

two fragmentation or reassembly.

"Jumbo frames" extends Ethernet to 9000 bytes.

For SPEAr600, the maximum MTU size has been limited to 4000, but for SPEAr300 the

maximum MTU size is kept at 9000. The advantage of using bigger MTU size can be

estimated from the fact that smaller frames usually mean more CPU interrupts and

more processing overhead for a given data transfer size. Often the per-packet

processing overhead sets the limit of TCP performance in the LAN environment. The

experiments results obtained above prove that the jumbo frames provided 50% more

throughput with 50% less CPU load than 1500 byte frames.

●

Checksum offloading during reception

Checksum offloading is used to relieve the kernel (and thus the CPU) from the burden

of calculating transport-PDU checksums (TCP and UDP checksums) when the SPEAr

MAC hardware can perform these calculations itself. As it can be seen in the results

above, when the checksum offloading is enabled the reception performance has

increased by almost 60 Mbps.

●

CRC stripping

This option enables GMAC to strip the PAD/FC S on the incoming frames only when

the length of the frame is less than or equal to 1500 bytes. The results show that

enabling this particular option does not impact the performance significantly, as the

execution time for the same option in the software is also on the lower side.

Kernel configuration options

Below is a list of the kernel configuration options supported by the driver using make

menuconfig.

Table 16.

Menuconfig options

Configuration options

Comment

CONFIG_NET_ETHERNET

Kernel networking support

CONFIG_MII

Generic media independent interface provided by

kernel.

CONFIG_ETH_SPEAR_SYN

This option is used to enable the GMAC driver

support (CONFIG_ETH_SPEAR_SYN=Y).

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