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arm64: Documentation: add list of software workarounds for errata
It's not immediately obvious which hardware errata are worked around in the Linux kernel for an arbitrary kernel tree, so add a file to keep track of what we're working around. Acked-by: Catalin Marinas <[email protected]> Signed-off-by: Will Deacon <[email protected]>
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Silicon Errata and Software Workarounds | ||
======================================= | ||
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Author: Will Deacon <[email protected]> | ||
Date : 27 November 2015 | ||
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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: | ||
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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. | ||
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For more information, consult one of the "Software Developers Errata | ||
Notice" documents available on infocenter.arm.com (registration | ||
required). | ||
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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). | ||
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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. | ||
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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. | ||
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| 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 | |