Intel® Stratix® 10 Hard Processor System Technical Reference Manual

ID 683222
Date 11/28/2022
Public

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17.6.2.1. Descriptor Lists and Data Buffers†

The DMA transfers data frames received by the MAC to the receive Buffer in the Host memory, and transmit data frames from the transmit Buffer in the Host memory. Descriptors that reside in the Host memory act as pointers to these buffers.

There are two descriptor lists: one for reception and one for transmission. The base address of each list is written into Register 3 (Receive Descriptor List Address Register) and Register 4 (Transmit Descriptor List Address Register), respectively. A descriptor list is forward linked (either implicitly or explicitly). The last descriptor may point back to the first entry to create a ring structure. Explicit chaining of descriptors is accomplished by setting the second address chained in both receive and transmit descriptors (RDES1[14] and TDES0[20]). The descriptor lists resides in the Host physical memory address space. Each descriptor can point to a maximum of two buffers. This enables two buffers to be used, physically addressed, rather than contiguous buffers in memory.

A data buffer resides in the Host physical memory space, and consists of an entire frame or part of a frame, but cannot exceed a single frame. Buffers contain only data, buffer status is maintained in the descriptor. Data chaining refers to frames that span multiple data buffers. However, a single descriptor cannot span multiple frames. The DMA skips to the next frame buffer when end-of-frame is detected. Data chaining can be enabled or disabled.

Descriptor Ring Structure
Descriptor Chain Structure
Note: The control bits in the descriptor structure are assigned so that the application can use an 8 KB buffer. All descriptions refer to the default descriptor structure.