Configuring and connecting the MMU-500

This SoC allows external system masters to connect to it, which requires the inclusion of an MMU-500. The MMU-500 adds a flexibility to the SoC, which can be used as required.

Functionality

The MMU-500 is a System MMU. For system masters, a System MMU performs a functionality similar to the functionality that CPU MMUs perform for CPU cores. This functionality involves the translation of virtual addresses into physical addresses. Both types of MMU use the same translation tables, which are defined during memory virtualization. These translation tables are stored in the main memory of the SoC.

The MMU-500 effectively enables one or more system coherent masters to operate in the same virtual address space as the Cortex-A53 clusters. The system masters could be a:

  • DMA
  • Display processor
  • GPU
  • Custom accelerator
  • PCIe interface

The MMU-500 can perform multiple address translations from different system masters at the same time. Context switching is not required.

The MMU-500 is composed of two main blocks, the Translation Buffer Unit (TBU) and the Translation Control Unit (TCU). One MMU-500 instance can have up to 32 TBUs but always has a single TCU.

TBU master interfaces are paired with a slave interface. Each system master requires a TBU master and slave pair. The TBU receives system master transactions, which are tagged with a virtual address, on the allocated slave interface. After translating the addresses, the TBU accesses the memory using the corresponding master interface.

In an SoC, the TBU is expected to be physically near the connected system master. This requirement means that the TBU is in the same clock or power domain as the system master. The TBU also includes a Translation Lookaside Buffer (TLB) to cache translation table information. The purpose of the buffer is to speed up the translations by caching translation table descriptors.

The TCU is a central control block that is responsible for reading translation tables from memory through its main master interface. The translation information is sent to the TBU, which puts them in the TLB. The TCU also includes extra caches and implements the MMU-500 programmers view, which is a set of configurable registers.

Configuration

The main configuration parameters for each TBU are:

  • The TLB depth. Each TLB can have up to 128 entries. One entry is used for each translation table descriptor.
  • The write data buffer depth. For a memory write, this buffer holds the data to write.
  • The number of transactions that can be handled in parallel. The maximum value is 16.

The main configuration parameters for the TCU are:

  • The depth of the various caches
  • The maximum number of translations (translation table walks) that can be handled in parallel
Connections
Interface Description 
TBU ACE-Lite slave The MMU-500 has up to 32 ACE-Lite slave interfaces. Coherent masters connect to these interfaces and send virtual address requests.
TBU ACE-Lite master

The MMU-500 has up to 32 ACE-Lite master interfaces that pair with the slave interfaces. The master interfaces connect to the CCI-500 and can read and write to memory using the translated virtual addresses.

Each coherent master uses one slave and one master interface on the MMU-500. In this way, the MMU-500 intercepts traffic driven to it by each coherent master and enables access to the correct physical address in memory.
TCU ACE-Lite master
The MMU-500 has a single ACE-Lite master interface that obtains translated addresses from the translation tables, which are stored in the main memory. 
TCU ACE-Lite configuration The MMU-500 has an AXI4 slave interface that enables software to configure the MMU-500. The configuration process involves setting registers.
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