Level 2 Memory Interface
ARM DDI 0388I
Copyright © 2008-2012 ARM. All rights reserved.
8-7
ID073015
Non-Confidential
8.2
Optimized accesses to the L2 memory interface
This section describes optimized accesses to the L2 memory interface. These optimized
accesses can generate non-AXI compliant requests on the Cortex-A9 AXI master ports. These
non-AXI compliant requests must be generated only when the slaves connected on the
Cortex-A9 AXI master ports can support them. The L2 cache controller supports these kinds of
requests. The following subsections describe the requests:
•
Prefetch hint to the L2 memory interface
•
•
•
8.2.1
Prefetch hint to the L2 memory interface
The Cortex-A9 processor can generate prefetch hint requests to the L2 memory controller. The
prefetch hint requests are non-compliant AXI read requests generated by the Cortex-A9
processor that do not expect any data return.
You can generate prefetch hint requests to the L2 by:
•
Enabling the L2 Prefetch Hint feature, bit [1] in the ACTLR. When enabled, this feature
enables the Cortex-A9 processor to automatically issue L2 prefetch hint requests when it
detects regular fetch patterns on a coherent memory. This feature is only triggered in a
Cortex-A9 MPCore processor, and not in a uniprocessor.
•
Programming PLE operations, when this feature is available in the Cortex-A9 processor.
In this case, the PLE engine issues a series of L2 prefetch hint requests at the programmed
addresses. See
L2 prefetch hint requests are identified by having their ARUSER[5] bit set.
Note
No additional programming of the L2C-310 is required.
8.2.2
Early BRESP
BRESP
answers on response channels must be returned to the master only when the last data
has been sent by the master. The Cortex-A9 processor can also deal with
BRESP
answers
returned as soon as address has been accepted by the slave, regardless of whether data is sent or
not. This enables the Cortex-A9 processor to provide a higher bandwidth for writes if the slave
can support the Early BRESP feature. The Cortex-A9 processor sets the
AWUSER[8]
bit to
indicate to the slave that it can accept an early
BRESP
answer for this access. This feature can
optimize the performance of the processor, but the Early
BRESP
feature generates non-AXI
compliant requests. When a slave receives a write request with
AWUSER[8]
set, it can either
give the
BRESP
answer after the last data is received, AXI compliant, or in advance, non-AXI
compliant. The L2C-310 cache controller supports this non-AXI compliant feature.
The Cortex-A9 processor does not require any programming to enable this feature, that is
always on by default.
Note
You must program the L2 cache controller to benefit from this optimization. See the
CoreLink
Level 2 Cache Controller (L2C-310) Technical Reference Manual
.