Freescale Semiconductor PXR4030, Reference Manual

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Enhanced Direct Memory Access Controller (eDMA)
Freescale Semiconductor 21-59
PXR40 Microcontroller Reference Manual, Rev. 1
21.5.8 Dynamic Programming
This section provides recommended methods to change the programming model during channel execution.
21.5.8.1 Dynamic Channel Linking and Dynamic Scatter-Gather Operation
Dynamic channel linking and dynamic scatter-gather operation is the process of changing the
EDMA
_x _TCD.MAJOR.E_LINK or EDMA _x _TCD.E_SG bits during channel execution. These bits are
read from the TCD local memory at the end of channel execution thus allowing the user to enable either
feature during channel execution.
Because the user is allowed to change the configuration during execution, a coherency model is needed.
Consider a scenario where the user attempts to execute a dynamic channel link by enabling the
EDMA
_x _TCD.MAJOR.E_LINK bit at the same time the eDMA engine is retiring the channel. The
EDMA _x _TCD.MAJOR.E_LINK would be set in the programmer’s model, but it would be unclear
whether the actual link was made before the channel retired.
The following coherency model is recommended when executing a dynamic channel link or dynamic
scatter-gather request:
1. Set the EDMA _x _TCD.MAJOR.E_LINK bit.
2. Read back the EDMA _x _TCD.MAJOR.E_LINK bit
3. Test the EDMA _x _TCD.MAJOR.E_LINK request status:
a) If the bit is set, the dynamic link attempt was successful.
b) If the bit is cleared, the attempted dynamic link did not succeed, the channel was already
retiring.
This same coherency model is true for dynamic scatter-gather operations. For both dynamic requests, the
TCD local memory controller forces the EDMA
_x _TCD.MAJOR.E_LINK and EDMA _x _TCD.E_SG
bits to zero on any writes to a channel’s TCD after that channel’s EDMA _x _TCD.DONE bit is set
indicating the major loop is complete.
Table 21-26. Channel Linking Parameters
Desired Link
Behavior
TCD Control Field Name Description
Link at end of
minor loop
citer.e_link Enable channel-to-channel linking on minor loop
completion (current iteration).
citer.linkch Link channel number when linking at end of minor
loop (current iteration).
Link at end of
major loop
major.e_link Enable channel-to-channel linking on major loop
completion.
major.linkch Link channel number when linking at end of major
loop.