Channel Preemption - NXP Semiconductors freescale KV4 Series Reference Manual

23.4.3 Channel preemption

Channel preemption is enabled on a per-channel basis by setting the DCHPRIn[ECP] bit.
Channel preemption allows the executing channel's data transfers to temporarily suspend
in favor of starting a higher priority channel. After the preempting channel has completed
all its minor loop data transfers, the preempted channel is restored and resumes
execution. After the restored channel completes one read/write sequence, it is again
eligible for preemption. If any higher priority channel is requesting service, the restored
channel is suspended and the higher priority channel is serviced. Nested preemption, that
is, attempting to preempt a preempting channel, is not supported. After a preempting
channel begins execution, it cannot be preempted. Preemption is available only when
fixed arbitration is selected.
A channel's ability to preempt another channel can be disabled by setting
DCHPRIn[DPA]. When a channel's preempt ability is disabled, that channel cannot
suspend a lower priority channel's data transfer, regardless of the lower priority channel's
ECP setting. This allows for a pool of low priority, large data-moving channels to be
defined. These low priority channels can be configured to not preempt each other, thus
preventing a low priority channel from consuming the preempt slot normally available to
a true, high priority channel.
23.4.4 Performance
This section addresses the performance of the eDMA module, focusing on two separate
metrics:
• In the traditional data movement context, performance is best expressed as the peak
data transfer rates achieved using the eDMA. In most implementations, this transfer
rate is limited by the speed of the source and destination address spaces.
• In a second context where device-paced movement of single data values to/from
peripherals is dominant, a measure of the requests that can be serviced in a fixed time
is a more relevant metric. In this environment, the speed of the source and destination
address spaces remains important. However, the microarchitecture of the eDMA also
factors significantly into the resulting metric.
23.4.4.1 Peak transfer rates
The peak transfer rates for several different source and destination transfers are shown in
the following tables. These tables assume:
Freescale Semiconductor, Inc.
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Chapter 23 Direct Memory Access Controller (eDMA)
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