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switch-l4t-atf/docs/cpu-specific-build-macros.md
Sandrine Bailleux d1b2b203a7 Cortex-A57: Add link to software optimization guide 
This patch adds a link to the Cortex-A57 Software Optimization Guide
in the ARM CPU Specific Build Macros document to justify the default
value of the A57_DISABLE_NON_TEMPORAL_HINT build flag.

Change-Id: I9779e42a4bb118442b2b64717ce143314ec9dd16
2016-02-18 15:38:18 +00:00

4.5 KiB
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ARM CPU Specific Build Macros

Contents

  1. Introduction

  2. CPU Errata Workarounds

  3. CPU Specific optimizations

  4. Introduction

This document describes the various build options present in the CPU specific operations framework to enable errata workarounds and to enable optimizations for a specific CPU on a platform.

  1. CPU Errata Workarounds

ARM Trusted Firmware exports a series of build flags which control the errata workarounds that are applied to each CPU by the reset handler. The errata details can be found in the CPU specific errata documents published by ARM. The errata workarounds are implemented for a particular revision or a set of processor revisions. This is checked by reset handler at runtime. Each errata workaround is identified by its ID as specified in the processor's errata notice document. The format of the define used to enable/disable the errata workaround is ERRATA_<Processor name>_<ID>, where the Processor name is for example A57 for the Cortex_A57 CPU.

All workarounds are disabled by default. The platform is reponsible for enabling these workarounds according to its requirement by defining the errata workaround build flags in the platform specific makefile. In case these workarounds are enabled for the wrong CPU revision then the errata workaround is not applied. In the DEBUG build, this is indicated by printing a warning to the crash console.

In the current implementation, a platform which has more than 1 variant with different revisions of a processor has no runtime mechanism available for it to specify which errata workarounds should be enabled or not.

The value of the build flags are 0 by default, that is, disabled. Any other value will enable it.

For Cortex-A53, following errata build flags are defined :

  • ERRATA_A53_826319: This applies errata 826319 workaround to Cortex-A53 CPU. This needs to be enabled only for revision <= r0p2 of the CPU.

  • ERRATA_A53_836870: This applies errata 836870 workaround to Cortex-A53 CPU. This needs to be enabled only for revision <= r0p3 of the CPU. From r0p4 and onwards, this errata is enabled by default.

For Cortex-A57, following errata build flags are defined :

  • ERRATA_A57_806969: This applies errata 806969 workaround to Cortex-A57 CPU. This needs to be enabled only for revision r0p0 of the CPU.

  • ERRATA_A57_813420: This applies errata 813420 workaround to Cortex-A57 CPU. This needs to be enabled only for revision r0p0 of the CPU.

  1. CPU Specific optimizations

This section describes some of the optimizations allowed by the CPU micro architecture that can be enabled by the platform as desired.

  • SKIP_A57_L1_FLUSH_PWR_DWN: This flag enables an optimization in the Cortex-A57 cluster power down sequence by not flushing the Level 1 data cache. The L1 data cache and the L2 unified cache are inclusive. A flush of the L2 by set/way flushes any dirty lines from the L1 as well. This is a known safe deviation from the Cortex-A57 TRM defined power down sequence. Each Cortex-A57 based platform must make its own decision on whether to use the optimization.

  • A53_DISABLE_NON_TEMPORAL_HINT: This flag disables the cache non-temporal hint. The LDNP/STNP instructions as implemented on Cortex-A53 do not behave in a way most programmers expect, and will most probably result in a significant speed degradation to any code that employs them. The ARMv8-A architecture (see ARM DDI 0487A.h, section D3.4.3) allows cores to ignore the non-temporal hint and treat LDNP/STNP as LDP/STP instead. Enabling this flag enforces this behaviour. This needs to be enabled only for revisions <= r0p3 of the CPU and is enabled by default.

  • A57_DISABLE_NON_TEMPORAL_HINT: This flag has the same behaviour as A53_DISABLE_NON_TEMPORAL_HINT but for Cortex-A57. This needs to be enabled only for revisions <= r1p2 of the CPU and is enabled by default, as recommended in section "4.7 Non-Temporal Loads/Stores" of the Cortex-A57 Software Optimization Guide.

Copyright (c) 2014-2016, ARM Limited and Contributors. All rights reserved.