Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

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

5.1.6.3.5. Enhanced Descriptor for Time-Based Scheduling

The time-based scheduling feature requires enhanced descriptors that are 32 bytes to be enabled on all the required DMA channels that intend to use the feature.
  1. Configure the number of channels that use the time-based scheduling feature. Use the DWCXG_NUM_DMA_TBS_CH parameter to specify the number of DMA channels that use the time-based scheduling feature. Specify the channels in the descending order.
  2. Set the EDSE bit of the DMA_CH(#i)_TX_Control register.

The structure of 32-byte descriptor for the context and the normal descriptor in read and write formats are described in the following sections.

Enhanced Normal Descriptor (Read Format)

The fields present in the first 16 bytes of the enhanced descriptor format of normal descriptor are called TDES4, TDES5, TDES6, and TDES7 as shown in figure below. TDES0-TDES3 remain the same as in the default normal descriptor format, as described in the Transmit Normal Descriptor (Read Format) section.

Figure 65. Enhanced Normal Descriptor - Read Format
Table 149.  TDES4 Enhanced Normal Descriptor - Read Format
Bit Name Description
31 LTV

Launch time valid

When FD = 0, this field indicates that the launch time (LT) and GSN fields present in the descriptor are valid. Set this field to 1, only if the FD field of the descriptor is 0.

11:8 GSN

GCL slot number

This field indicates the GCL slot number associated with the packet.

7:0 LT

Launch time[31:24]

This field indicates the launch time in seconds, associated with the packet. This value is compared with the least significant 8 bits of the seconds field of the time maintained in EMAC, for PTP protocol.

Table 150.  TDES4 Enhanced Normal Descriptor - Read Format
Bit Name Description
31:8 LT

Launch time[23:0]

This field indicates the launch time in seconds, associated with the packet. This value is compared with the least significant 8 bits of the seconds field of the time maintained in EMAC, for PTP protocol.

Enhanced Normal Descriptor (Write-Back Format)

When the DMA closes the enhanced descriptor, it updates only the TDES0, TDES1, TDES2, and TDES3 fields. So, the values in the TDES4-TDES7 fields are retained in the host memory.

Figure 66. Enhanced Normal Descriptor - Write-Back Format
Note:
  • All the enhanced descriptor examples, assume a 32-bit data structure. However, the same definitions are valid for 64 and 128 bit configurations.
  • In the write back format, no modifications are done to the extended 16 bytes. The rest of the 16 bytes (TDES0 to TDES3) are written back as per the previous 16 bytes descriptor format as described in Transmit Normal Descriptor (Write-Back Format) section.

Enhanced Context Descriptor (Read Format)

The first 16 bytes of the enhanced context descriptor (TDESC4 to TDESC7) are reserved and must be zeros. The fields present in last bytes of the descriptor (TDESC0 to TDESC3) are same as the context descriptor (TDESC0 to TDESC3) in 16 byte format. For details of the context descriptor format, see Transmit Context Descriptor section.

Figure 67. Enhanced Context Descriptor - Read Format

Enhanced Context Descriptor (Write Format)

In the write format, no modifications are done to the extended 16 bytes in the TDES4, TDES5, TDES6, and TDES7 fields. The rest of the 16 bytes (TDES0 to TDES3) are written back as per the previous 16 bytes of the default normal descriptor format, as described in the Transmit Context Descriptor section.

Figure 68. Enhanced Context Descriptor - Write Format