Intel PCI-X, Intel Gigabit Ethernet Controllers manual Transmit Data Storage, Transmit Descriptors

Receive and Transmit Description

•The protocol stack calculates the number of packets required to transmit this block based on the MTU size of the media and required packet headers.

•For each packet of the data block:

—Ethernet, IP and TCP/UDP headers are prepared by the stack.

—The stack interfaces with the software device driver and commands the driver to send the individual packet.

—The driver gets the frame and interfaces with the hardware.

—The hardware reads the packet from host memory (via DMA transfers).

—The driver returns ownership of the packet to the Network Operating System (NOS) when the hardware has completed the DMA transfer of the frame (indicated by an interrupt).

Output packets are made up of pointer–length pairs constituting a descriptor chain (so called descriptor based transmission). Software forms transmit packets by assembling the list of pointer– length pairs, storing this information in the transmit descriptor, and then updating the on–chip transmit tail pointer to the descriptor. The transmit descriptor and buffers are stored in host memory. Hardware typically transmits the packet only after it has completely fetched all packet data from host memory and deposited it into the on-chip transmit FIFO. This permits TCP or UDP checksum computation, and avoids problems with PCI underruns.

3.3.1Transmit Data Storage

Data are stored in buffers pointed to by the descriptors. Alignment of data is on an arbitrary byte boundary with the maximum size per descriptor limited only to the maximum allowed packet size (16288 bytes). A packet typically consists of two (or more) descriptors, one (or more) for the header and one for the actual data. Some software implementations copy the header(s) and packet data into one buffer and use only one descriptor per transmitted packet.

3.3.2Transmit Descriptors

The Ethernet controller provides three types of transmit descriptor formats.

The original descriptor is referred to as the “legacy” descriptor format. The two other descriptor types are collectively referred to as extended descriptors. One of them is similar to the legacy descriptor in that it points to a block of packet data. This descriptor type is called the TCP/IP Data Descriptor and is a replacement for the legacy descriptor since it offers access to new offloading capabilities. The other descriptor type is fundamentally different as it does not point to packet data. It merely contains control information which is loaded into registers of the controller and affect the processing of future packets. The following sections describe the three descriptor formats.

The extended descriptor types are accessed by setting the TDESC.DEXT bit to 1b. If this bit is set, the TDESC.DTYP field is examined to control the interpretation of the remaining bits of the descriptor. Table 3-7shows the generic layout for all extended descriptors. Fields marked as NR are not reserved for any particular function and are defined on a per-descriptor type basis. Notice that the DEXT and DTYP fields are non-contiguous in order to accommodate legacy mode operation. For legacy mode operation, bit 29 is set to 0b and the descriptor is defined in Section 3.3.3.

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