This section defines the fields of the S2MM (Receive) and MM2S (Transmit) Scatter Gather Descriptors for when the AXI DMA is configured for Scatter / Gather Mode. The descriptor is made up of eight 32-bit base words and 0 or 5 User Application words. The descriptor has future support for 64-bit addresses and support for user application data. Multiple descriptors per packet are supported through the Start of Frame and End of Frame flags. Completed status and Interrupt on Complete are also included. The Buffer Length can describe up to 67, 108, 863 bytes of data buffer per descriptor. Two descriptor chains are required for the two data transfer directions, MM2S and S2MM.
Table 2-25:
Descriptor Fields (Non-multichannel Mode)
|
Address Space Offset
(1)
|
Name
|
Description
|
|
00h
|
NXTDESC
|
Next Descriptor Pointer
|
|
04h
|
NXTDESC_MSB
|
Upper 32 bits of Next Descriptor Pointer
|
|
08h
|
BUFFER_ADDRESS
|
Buffer Address
|
|
0Ch
|
BUFFER_ADDRESS_MSB
|
Upper 32 bits of Buffer Address.
|
|
10h
|
RESERVED
|
N/A
|
|
14h
|
RESERVED
|
N/A
|
|
18h
|
CONTROL
|
Control
|
|
1Ch
|
STATUS
|
Status
|
|
20h
|
APP0
|
User Application Field 0
(2)
|
|
24h
|
APP1
|
User Application Field 1
|
|
28h
|
APP2
|
User Application Field 2
|
|
2Ch
|
APP3
|
User Application Field 3
|
|
30h
|
APP4
|
User Application Field 4
|
|
Notes:
1.
Address Space Offset is relative to 16 - 32-bit word alignment in system memory, that is, 0x00, 0x40, 0x80 and so forth.
2.
User Application fields (APP0, APP1, APP2, APP3, and APP4) are only used when the Control / Status Streams are included, When the Control/Status Streams are not included, the User Application fields are not fetched or updated by the Scatter Gather Engine.
3.
The MSB fields or the upper 32 bit addresses are used only when DMA is configured for an address space greater than 32.
|