Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 11/27/2024
Public
Document Table of Contents

5.1.6.5.5. Context Descriptor Sequence

You can use the context descriptor to provide the MSS value for segmentation. The application must provide the context descriptor before the normal descriptor to be used for the corresponding TCP packet. If the application wants to provide a new MSS, it must create the context descriptor in the descriptor list before the first normal descriptor of the packet to be segmented with the new MSS value. The MSS value in the context descriptor is valid only if the TCMSSV bit of TDES3 in the context descriptor is set and the OSTC bit is reset.

When the application provides a context descriptor with a valid MSS value, the DMA internally stores the MSS value and uses this MSS value for all subsequent packets that have the TSO enabled through the TSE bit of TDES3 normal descriptor.

If the application places a context descriptor in the middle of a packet (between the first and last descriptors of a packet), the DMA does the following:
  1. The DMA ignores the context and closes the descriptor.
  2. The DMA indicates the error in the descriptor status
  3. The DMA generates an interrupt if the CDEE bit is set in the Interrupt_Enable register of the corresponding DMA channel.

The application can read the interrupt status through CDE bit of the status register corresponding to a DMA channel/