28.4.3. Descriptor Fields Definition
The following topics describe each of the fields of Descriptor with Prefetcher enabled.
Next Descriptor Pointer
The next descriptor pointer field specifies the address of the next descriptor in the linked list. The address of the descriptors must be aligned on a descriptor read/write data width boundary.
Actual Bytes Transferred
Specifies the actual number of bytes that has been transferred to the write host with packet support enabled. This field counts the number of bytes between the start-of-packet (SOP) and end-of-packet (EOP) received by the write host. This field is only applicable if mSGDMA is configured as Streaming to Memory-Mapped transfer with Packet Support Enable parameter enabled.
Status
| Bits | Fields | Description |
|---|---|---|
| 15:9 | Reserved | Reserved fields. |
| 8 | Early Termination | Indicates early termination condition where write host is performing a packet transfer and does not receive EOP before pre-determined amount of bytes are transferred. This status bit is similar to status register bit 8 of the dispatcher core. For more details, refer to dispatcher core CSR definition. |
| 7:0 | Error | Indicates an error has arrived at the write host streaming sink port with error support enabled. |
Control
During descriptor write back, only the Owned by Hardware bit is updated by the mSGDMA IP. Refer to the following table for details about Owned by Hardware bit.
| Bits | Fields | Description |
|---|---|---|
| 31 | reserved | Reserved. |
| 30 | Owned by Hardware | This field determines whether hardware or software has write access to the current descriptor. When this field is set to 1 by software, the mSGDMA IP can process and update the descriptor, while software should not access the descriptor due to the possibility of race conditions. Otherwise, it is safe for software to update the descriptor when this field is 0, and mSGDMA IP does not process the current descriptor. This field is cleared by the hardware during descriptor write back if Park mode is disabled in the Prefetcher Control Register. If Park mode is enabled in the Prefetcher Control Register, the Prefetcher does not clear this field during descriptor write back. |
Sideband Signal
mSGDMA IP provides the option for user to add sideband information (such as timestamp), to be written back by Prefetcher to the host memory.
- Sideband signal [31-0] is mapped to Avalon® -ST response sink [191:160].
- Sideband signal [63-32] is mapped to Avalon® -ST response sink [223:192].
- Sideband signal [95-64] is mapped to Avalon® -ST response sink [255:224].
If Expose response port to enable sideband support parameter is disabled, address offset 0x30, 0x34 and 0x38 of the Prefetcher’s extended responder descriptor format are reserved fields. Prefetcher does not perform write back to these 3 offset fields.