ARM Technical Support Knowledge Articles

How do we determine the AXI parameters of DMA transfers (particularly the length) ?

Applies to: ARM1176

Answer

The SIZE will depend on the transfer size (TS) parameter in the control register, if the memory
type is device/strongly ordered. Size may be modified for normal memory to make use of more
efficient 64-bit transfers.

BURST will be INCR or, if the stride is zero, will be FIXED for accessing FIFOs.

LEN depends on the size of the DMA data buffer. It is possible to transfer up to 8 words in a
burst, so bursts can be 4 beats of 64 bits.

Address will be aligned to the size of the transfer.

In Summary:

SIZE : 1, 2, 4, 8 bytes
BURST : INCR or FIXED
LEN : 0-3 (i.e. up to 4 beats)
ADDR: aligned to the SIZE

Consider a DMA transfer with TS = 2 bytes, stride = 2 bytes.

If the memory type is normal, the transfer is done using 64bit sized transfers, 4-beat burst
(LEN=3).

If the memory type is DEVICE, the transfer is done using 16bit sized transfers, single (LEN=0).

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