Intel® Agilex™ Hard Processor System Technical Reference Manual

ID 683567
Date 2/14/2023
Public

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17.6.2.4. Buffer Size Calculations

The DMA does not update the size fields in the transmit and receive descriptors. The DMA updates only the status fields (RDES and TDES) of the descriptors. The driver must perform the size calculations.

The transmit DMA transfers the exact number of bytes (indicated by the buffer size field of TDES1) to the MAC. If a descriptor is marked as the first (FS bit of TDES1 is set), then the DMA marks the first transfer from the buffer as the start of frame. If a descriptor is marked as the last (LS bit of TDES1), then the DMA marks the last transfer from that data buffer as the end-of-frame to the MTL.

The receive DMA transfers data to a buffer until the buffer is full or the end-of-frame is received from the MTL. If a descriptor is not marked as the last (LS bit of RDES0), then the descriptor’s corresponding buffer(s) are full and the amount of valid data in a buffer is accurately indicated by its buffer size field minus the data buffer pointer offset when the FS bit of that descriptor is set. The offset is zero when the data buffer pointer is aligned to the data bus width. If a descriptor is marked as the last, then the buffer may not be full (as indicated by the buffer size in RDES1). To compute the amount of valid data in this final buffer, the driver must read the frame length (FL bits of RDES0[29:16]) and subtract the sum of the buffer sizes of the preceding buffers in this frame. The receive DMA always transfers the start of next frame with a new descriptor.

Note: Even when the start address of a receive buffer is not aligned to the data width of system bus, the system should allocate a receive buffer of a size aligned to the system bus width. For example, if the system allocates a 1,024‑byte (1 KB) receive buffer starting from address 0x1000, the software can program the buffer start address in the receive descriptor to have a 0x1002 offset. The receive DMA writes the frame to this buffer with dummy data in the first two locations (0x1000 and 0x1001). The actual frame is written from location 0x1002. Thus, the actual useful space in this buffer is 1,022 bytes, even though the buffer size is programmed as 1,024 bytes, because of the start address offset.