Arbitration Types
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
14-19
14.3
Arbitration Types
There are two types of DMA arbitration: round-robin and early-deadline-serve-first (EDF). The
type of arbitration is selected via the DGCR[AT] bit (see Table 14-17, DMAGCR Field
Descriptions, on page 14-27).
14.3.1 Round-Robin Arbitration
The round-robin arbitration between channels is a least recently used (LRU) schema. Following
is a list of the arbitration parameters according to their weight:
DMA port. Each channel is assigned to one DMA port. Each time a channel request is
serviced, the channels assigned to its port are masked for three clock cycles. When there
are requesting channels for both ports, they are serviced intermittently.
Fixed-priority among round-robin groups. Each channel is assigned to one of the four
priority groups as defined by the DCHCRx[RRPG] bit. Each priority group can contain
from 0 (empty) to all 16 channels. Pending requests from the highest-priority group are
serviced first.
Bandwidth control. Each channel has credit for maximum consecutive grants according to
BD_ATTR [TSZ] and BD_ATTR[BTSZ]. If TSZ is greater than BTSZ bytes, the channel
wins consecutively in BTSZ byte portions until TSZ is reached. The channel may stop
requesting before it is continuously granted the maximum transfer size. The channel keeps
its priority until the address is aligned. The channel priority is updated when the buffer or
dimension ends.
Least recently used round-robin. A linear queue defines the channel priority. After reset,
each channel has a unique priority according to its number. After a channel is serviced, it
gets the lowest priority. All the other channels with a priority lower than the winning
priority upgrade their priority. All the channels with higher priority on this cycle keep
their priority so that they are guaranteed service. Some channels may get the same priority
by the time. Table 14-12 lists the priority of the channels for two successive cycles.
Table 14-12. Round-Robin Arbitration Example
Channel Number
Channel Request
Clock n Priority
Clock n+1 Priority
1
Deasserted
0 (Highest)
0 (Highest)
8
Deasserted
1
1
3
Asserted
2
Channel 3 win
31 (Lowest)
2
Asserted
3
2
6
Asserted
4
3
5
Deasserted
5
4
4
Deasserted
6
5
7
Deasserted
7 (Lowest)
6
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