7.7.11. L2 Auxiliary Control Register settings

This section describes the recommended performance settings for the Cortex-A57 L2 Auxiliary Control Register in various system configurations.

Evict transactions

Evict and WriteEvict transactions indicate that a shareable cache line has been evicted from the master's local caches. The downstream snoop filter can use this information to update its directory to indicate that the issuing master no longer contains a copy of the cache line.

WriteEvict carries data and can be used to allow allocation into a system or Level 3 cache. In general, ARM recommends the following:

  • A system that contains a snoop filter enables Evict transactions.

  • A system that contains a L3 cache that wants to behave like a victim cache for cache lines in the Unique state enables WriteEvict transactions.

The Cortex-A57 L2ACTLR_EL1 register contains bits that can enable or disable Evict and WriteEvict transactions individually. See L2ACTLR_EL1[3] and L2ACTLR_EL1[14], respectively, in the L2 Auxiliary Control Register, EL1.

When the Cortex-A57 MPCore processor is used with the ARM CCI-400 in an ACE-based system, ARM recommends that you set L2ACTLR_EL1[3] to 1 to disable Evict transactions. The reset value of L2ACTLR[3] is 0 in Cortex-A57 ACE configurations. WriteEvict transactions are disabled by default in Cortex-A57 ACE configurations.

When the Cortex-A57 MPCore processor is used with the ARM CCN-504 in a CHI-based system, no change is required from the default reset value of L2ACTLR_EL1. By default, Cortex-A57 CHI configurations generate WriteEvict transactions for allocating into the CCN-504 L3 cache but do not generate Evict transactions because the CCN-504 snoop filter does not require them.

WriteUnique and WriteLineUnique transactions

If the Cortex-A57 MPCore processor is implemented in an ACE-based system that does not contain a snoop filter, WriteUnique and WriteLineUnique transactions can be enabled to provide a small increase in the performance of the processor. To enable WriteUnique and WriteLineUnique transactions, clear L2ACTLR_EL1[4]. The reset value of L2ACTLR[4] is 1, see L2 Auxiliary Control Register, EL1.

If the Cortex-A57 MPCore processor is implemented in an ACE-based system that does contain a snoop filter or in a CHI based system, no change is required from the default value.

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