Motorola MCF5281, MCF5282 user manual Driver/DMA Operation with Buffer Descriptors

Buffer Descriptors

17.6 Buffer Descriptors

This section provides a description of the operation of the driver/DMA via the buffer descriptors. It is followed by a detailed description of the receive and transmit descriptor fields.

17.6.1 Driver/DMA Operation with Buffer Descriptors

The data for the FEC frames must reside in memory external to the FEC. The data for a frame is placed in one or more buffers. Associated with each buffer is a buffer descriptor (BD) which contains a starting address (pointer), data length, and status/control information (which contains the current state for the buffer). To permit maximum user flexibility, the BDs are also located in external memory and are read in by the FEC DMA engine.

Software “produces” buffers by allocating/initializing memory and initializing buffer descriptors. Setting the RxBD[E] or TxBD[R] bit “produces” the buffer. Software writing to either the TDAR or RDAR tells the FEC that a buffer has been placed in external memory for the transmit or receive data traffic, respectively. The hardware reads the BDs and “consumes” the buffers after they have been produced. After the data DMA is complete and the buffer descriptor status bits have been written by the DMA engine, the RxBD[E] or TxBD[R] bit will be cleared by hardware to signal the buffer has been “consumed.” Software may poll the BDs to detect when the buffers have been consumed or may rely on the buffer/frame interrupts. These buffers may then be processed by the driver and returned to the free list.

The ECR[ETHER_EN] signal operates as a reset to the BD/DMA logic. When ECR[ETHER_EN] is deasserted the DMA engine BD pointers are reset to point to the starting transmit and receive BDs. The buffer descriptors are not initialized by hardware during reset. At least one transmit and receive buffer descriptor must be initialized by software before the ECR[ETHER_EN] bit is set.

The buffer descriptors operate as two separate rings. ERDSR defines the starting address for receive BDs and ETDSR defines the starting address for transmit BDs. The last buffer descriptor in each ring is defined by the Wrap (W) bit. When set, W indicates that the next descriptor in the ring is at the location pointed to by ERDSR and ETDSR for the receive and transmit rings, respectively. Buffer descriptor rings must start on a 32-bit boundary; however, it is recommended they are made 128-bit aligned.

17.6.1.1 Driver/DMA Operation with Transmit BDs

Typically a transmit frame will be divided between multiple buffers. An example is to have an application payload in one buffer, TCP header in a 2nd buffer, IP header in a 3rd buffer, Ethernet/IEEE 802.3 header in a 4th buffer. The Ethernet MAC does not prepend the Ethernet header (destination address, source address, length/type field(s)), so this must be

MOTOROLA Chapter 17. Fast Ethernet Controller (FEC) 17-45