AM3352: EDMA

Hi,

This is what I found in AM3358 TRM:

11.3.1.1
If events on different channels are detected simultaneously, the events are queued based on a fixed
priority arbitration scheme with the DMA channels being higher priority events than the QDMA channels.
Among the two groups of channels, the lowest-numbered channel is the highest priority.

11.3.11
Queue0 has highest priority and Queue2 has the lowest priority, if Queue0 and Queue1 both have at least
one event entry and if both TC0 and TC1 can accept transfer requests, then the event in Queue0 is dequeued
first and its associated PaRAM set is processed and submitted as a transfer request (TR) to TC0.

11.3.11.4
The main system bus infrastructure (L3) arbitrates bus requests from all of the masters (TCs, CPU(S), and
other bus masters) to the shared slave resources (peripherals and memories).
The priorities of transfer requests (read and write commands) from the EDMA3 transfer controllers with
respect to other masters within the system crossbar are programmed using the queue priority register
(QUEPRI). QUEPRI programs the priority of the event queues (or indirectly, TC0-TC2, because QueueN
transfer requests are submitted to TCN).

11.3.12.1.3
By default, reads are as issued as fast as possible. In some cases, the reads issued by the EDMA3TC
could fill the available command buffering for a slave, delaying other (potentially higher priority) masters
from successfully submitting commands to that slave. The rate at which read commands are issued by the
EDMA3TC is controlled by the RDRATE register. The RDRATE register defines the number of cycles that
the EDMA3TC read controller waits before issuing subsequent commands for a given TR, thus minimizing
the chance of the EDMA3TC consuming all available slave resources. The RDRATE value should be set
to a relatively small value if the transfer controller is targeted for high priority transfers and to a higher
value if the transfer controller is targeted for low priority transfers.
In contrast, the Write Interface does not have any performance turning knobs because writes always have
an interval between commands as write commands are submitted along with the associated write data.

11.3.14 EDMA3 Prioritization

The EDMA3 controller has many implementation rules to deal with concurrent events/channels, transfers,
etc. The following subsections detail various arbitration details whenever there might be occurrence of
concurrent activity. Figure 11-19 shows the different places EDMA3 priorities come into play.

11.3.14.1 Channel Priority
The DMA event registers (ER and ERH) capture up to 64 events; likewise, the QDMA event register
(QER) captures QDMA events for all QDMA channels; therefore, it is possible for events to occur
simultaneously on the DMA/QDMA event inputs. For events arriving simultaneously, the event associated
with the lowest channel number is prioritized for submission to the event queues (for DMA events, channel
0 has the highest priority and channel 63 has the lowest priority; similarly, for QDMA events, channel 0
has the highest priority and channel 7 has the lowest priority). This mechanism only sorts simultaneous
events for submission to the event queues.
If a DMA and QDMA event occurs simultaneously, the DMA event always has prioritization against the
QDMA event for submission to the event queues.

11.3.14.2 Trigger Source Priority
If a DMA channel is associated with more than one trigger source (event trigger, manual trigger, and chain
trigger), and if multiple events are set simultaneously for the same channel (ER.En = 1, ESR.En = 1 ,
CER.En = 1) , then the EDMA3CC always services these events in the following priority order: event
trigger (via ER) is higher priority than chain trigger (via CER) and chain trigger is higher priority than
manual trigger (via ESR).
This implies that if for channel 0, both ER.E0 = 1 and CER.E0 = 1 at the same time, then the ER.E0 event
is always queued before the CER.E0 event.

11.3.14.3 Dequeue Priority
The priority of the associated transfer request (TR) is further mitigated by which event queue is being used
for event submission (dictated by DMAQNUM and QDMAQNUM). For submission of a TR to the transfer
request, events need to be de-queued from the event queues. Queue 0 has the highest dequeue priority
and Queue 2 the lowest.

11.3.14.4 System (Transfer Controller) Priority
INIT_PRIORITY_0 and INIT_PRIORITY_1 registers in the chip configuration module are used to configure
the EDMA TC's priority through the system bus infrastructure.

Regards, Eric