Agilex™ 7 Hard Processor System Technical Reference Manual

ID 683567
Date 8/13/2024
Public
Document Table of Contents

17.6.3. Descriptor Overview

The DMA in the Ethernet subsystem transfers data based on a single enhanced descriptor, as explained in the DMA Controller section. The enhanced descriptor is created in the system memory. The descriptor addresses must be word-aligned.

The enhanced or alternate descriptor format can have 8 DWORDS (32 bytes) instead of 4 DWORDS as in the case of the normal descriptor format.

The features of the enhanced or alternate descriptor structure are:

  • The alternative descriptor structure is implemented to support buffers of up to 8 KB (useful for Jumbo frames).
  • There is a re-assignment of control and status bits in TDES0, TDES1, RDES0 (advanced timestamp or IPC full offload configuration), and RDES1. 
  • The transmit descriptor stores the timestamp in TDES6 and TDES7 when you select the advanced timestamp. 
  • The receive descriptor structure is also used for storing the extended status (RDES4) and timestamp (RDES6 and RDES7) when advanced timestamp, IPC Full Checksum Offload Engine, or Layer 3 and Layer 4 filter feature is selected. 
  • You can select one of the following options for descriptor structure:
    • If timestamping is enabled in Register 448 (Timestamp Control Register) or Checksum Offload is enabled in Register 0 (MAC Configuration Register), the software must to allocate 32 bytes (8 DWORDS) of memory for every descriptor by setting Bit 7 (Descriptor Size) of Register 0 (Bus Mode Register). 
    • If timestamping or Checksum Offload is not enabled, the extended descriptors (DES4 to DES7) are not required. Therefore, software can use descriptors with the default size of 16 bytes (4 DWORDS) by clearing Bit 7 (Descriptor Size) of Register 0 (Bus Mode Register) to 0.