Computer Hardware Algorithm Standard User's Guide

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6.14.2 Minimizing Logical Channel Reconfiguration Overhead
6.14.3 Addressing Automatic Endianism Conversion Issues
6.15 Inter-Algorithm Synchronization
6.15.1 Non-Preemptive System
Inter-Algorithm Synchronization
Some common C55x DMA devices impose additional restrictions that affect when a channel needs to be
reconfigured. A logical channel needs to be reconfigured when the source or destination addresses refer
to different memory ports (SARAM, DARAM, EMIF) compared with the most recently configured channel
settings.
Additionally, utilizing the reload registers is not possible when the source or destination addresses
correspond to different memory ports currently being used by the ongoing transfer.
DMA Guideline 5
C55x algorithms should minimize channel configuration overhead by requesting a separate logical
channel for each different transfer type. They should also call ACPY2_configure when the source or
destination addresses belong in a different type of memory (SARAM, DARAM, External) as compared
with that of the most recent transfer.
Some C55x/OMAP architectures perform on-the-fly endianism conversion during DMA transfers between
DSP internal Memory (SARAM/DARAM) and external memory (via EMIF). Certain coherency problems
may arise due to automatic enabling/disabling of endianism conversion by the hardware, based on DMA
transfer settings, CPU access modes, and address alignments. In order to ensure correct operation of
general C55x algorithms on hardware with automatic endianism conversion following rules regarding
alignment, size, and access, all rules for data buffers that may reside in external memory must be
followed.
DMA Rule 10
C55x algorithms must request all data buffers in external memory with 32-bit alignment and sizes in
multiples of 4 (bytes).
DMA Rule 11
C55x algorithms must use the same data types, access modes and DMA transfer settings when reading
from or writing to data stored in external memory, or in application-passed data buffers.
An ideal system with unlimited DMA resources would assign a physical DMA channel to each logical
channel requested by the algorithms comprising the system. Unfortunately, the DMA resource is limited
and some of the physical DMA channels may be used for other system functions such as servicing serial
ports etc. As such, a variety of application scenarios are possible with regards to sharing physical DMA
channels. Let's consider two scenarios to illustrate how this can be dealt with: a non-preemptive system
and a preemptive system.
Assume a system with one physical DMA channel that has been assigned to be used by two algorithms.
The algorithms require one logical channel each. The algorithms do not preempt each other.
We know from DMA Rule 1 that upon return from the algorithm functions, the DMA is not active. The
system can easily share this single DMA channel among the two algorithms, since they will run
sequentially and use the DMA channel sequentially. See Section 6.15.2 .
SPRU352G – June 2005 – Revised February 2007 Use of the DMA Resource 71
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