Level 2 Memory System
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
7-15
ID021414
Non-Confidential
7.4.3
CHI channel properties
Table 7-13
shows the properties of the CHI channels.
See the
ARM
®
AMBA
®
5 CHI Protocol Specification
for more information about the CHI
channel.
Table 7-13 CHI channel properties
Property
Value
Comment
Snoop acceptance
capability
10
The SCU can accept and process a maximum of 10 snoop requests from the system.
DVM acceptance
capability
4
The SCU can accept and process a maximum of four DVM transactions from the system. Each of
these four transactions can be a two part DVM message.
Note
The interconnect must be configured to never send more than four DVM messages to a Cortex-A53
processor, otherwise the system might deadlock.
Snoop latency
Hit
When there is a hit in L2 cache, the best case for response and data is 11 processor cycles. When
there is a miss in L2 cache and a hit in L1 cache, the best case for response and data is 14 processor
cycles.
Note
Latencies can be higher if hazards occur or if there are not enough buffers to absorb requests.
Miss
Best case six processor cycles when the SCU duplicate tags and L2 tags indicate the miss.
DVM
The cluster takes a minimum of six cycles to provide a response to DVM packets.
Snoop filter
Supported
The cluster provides support for an external snoop filter in an interconnect. It indicates when clean
lines are evicted from the processor by sending Evict transactions on the CHI write channel.
However there are some cases where incorrect software can prevent an Evict transaction from
being sent, therefore you must ensure that any external snoop filter is built to handle a capacity
overflow that sends a back-invalidation to the processor if it runs out of storage.
Supported
transactions
-
All transactions described by the CHI protocol:
•
Are accepted on the CHI master interface from the system.
•
Can be produced on the CHI master interface except:
—
ReadClean.
—
WriteBackPtl.
—
WriteCleanPtl.