arm64: Documentation: add list of software workarounds for errata

                Silicon Errata and Software Workarounds

                =======================================

Author: Will Deacon <will.deacon@arm.com>

Date  : 27 November 2015

It is an unfortunate fact of life that hardware is often produced with

so-called "errata", which can cause it to deviate from the architecture

under specific circumstances.  For hardware produced by ARM, these

errata are broadly classified into the following categories:

  Category A: A critical error without a viable workaround.

  Category B: A significant or critical error with an acceptable

              workaround.

  Category C: A minor error that is not expected to occur under normal

              operation.

For more information, consult one of the "Software Developers Errata

Notice" documents available on infocenter.arm.com (registration

required).

As far as Linux is concerned, Category B errata may require some special

treatment in the operating system. For example, avoiding a particular

sequence of code, or configuring the processor in a particular way. A

less common situation may require similar actions in order to declassify

a Category A erratum into a Category C erratum. These are collectively

known as "software workarounds" and are only required in the minority of

cases (e.g. those cases that both require a non-secure workaround *and*

can be triggered by Linux).

For software workarounds that may adversely impact systems unaffected by

the erratum in question, a Kconfig entry is added under "Kernel

Features" -> "ARM errata workarounds via the alternatives framework".

These are enabled by default and patched in at runtime when an affected

CPU is detected. For less-intrusive workarounds, a Kconfig option is not

available and the code is structured (preferably with a comment) in such

a way that the erratum will not be hit.

This approach can make it slightly onerous to determine exactly which

errata are worked around in an arbitrary kernel source tree, so this

file acts as a registry of software workarounds in the Linux Kernel and

will be updated when new workarounds are committed and backported to

stable kernels.

| Implementor    | Component       | Erratum ID      | Kconfig                 |

+----------------+-----------------+-----------------+-------------------------+

| ARM            | Cortex-A53      | #826319         | ARM64_ERRATUM_826319    |

| ARM            | Cortex-A53      | #827319         | ARM64_ERRATUM_827319    |

| ARM            | Cortex-A53      | #824069         | ARM64_ERRATUM_824069    |

| ARM            | Cortex-A53      | #819472         | ARM64_ERRATUM_819472    |

| ARM            | Cortex-A53      | #845719         | ARM64_ERRATUM_845719    |

| ARM            | Cortex-A53      | #843419         | ARM64_ERRATUM_843419    |

| ARM            | Cortex-A57      | #832075         | ARM64_ERRATUM_832075    |

| ARM            | Cortex-A57      | #852523         | N/A                     |

| ARM            | Cortex-A57      | #834220         | ARM64_ERRATUM_834220    |

|                |                 |                 |                         |

| Cavium         | ThunderX ITS    | #22375, #24313  | CAVIUM_ERRATUM_22375    |

| Cavium         | ThunderX GICv3  | #23154          | CAVIUM_ERRATUM_23154    |