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| author | Soby Mathew <soby.mathew@arm.com> | 2019-09-25 14:03:41 +0100 |
|---|---|---|
| committer | Olivier Deprez <olivier.deprez@arm.com> | 2019-10-04 10:19:35 +0200 |
| commit | c97cba4ea44910df1f7b1af5dba79013fb44c383 (patch) | |
| tree | 9d2965a2b9610413dea548a95e2f5094fc84b6e8 /docs | |
| parent | ace23683beb81354d6edbc61c087ab8c384d0631 (diff) | |
Fix the CAS spinlock implementation
Make the spinlock implementation use ARMv8.1-LSE CAS instruction based
on a platform build option. The CAS-based implementation used to be
unconditionally selected for all ARM8.1+ platforms.
The previous CAS spinlock implementation had a bug wherein the spin_unlock()
implementation had an `sev` after `stlr` which is not sufficient. A dsb is
needed to ensure that the stlr completes prior to the sev. Having a dsb is
heavyweight and a better solution would be to use load exclusive semantics
to monitor the lock and wake up from wfe when a store happens to the lock.
The patch implements the same.
Change-Id: I5283ce4a889376e4cc01d1b9d09afa8229a2e522
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/design/firmware-design.rst | 7 | ||||
| -rw-r--r-- | docs/getting_started/user-guide.rst | 4 |
2 files changed, 9 insertions, 2 deletions
diff --git a/docs/design/firmware-design.rst b/docs/design/firmware-design.rst index dc082082..2cbd9c94 100644 --- a/docs/design/firmware-design.rst +++ b/docs/design/firmware-design.rst @@ -2540,8 +2540,11 @@ Armv8.1-A This Architecture Extension is targeted when ``ARM_ARCH_MAJOR`` >= 8, or when ``ARM_ARCH_MAJOR`` == 8 and ``ARM_ARCH_MINOR`` >= 1. -- The Compare and Swap instruction is used to implement spinlocks. Otherwise, - the load-/store-exclusive instruction pair is used. +- By default, a load-/store-exclusive instruction pair is used to implement + spinlocks. The ``USE_SPINLOCK_CAS`` build option when set to 1 selects the + spinlock implementation using the ARMv8.1-LSE Compare and Swap instruction. + Notice this instruction is only available in AArch64 execution state, so + the option is only available to AArch64 builds. Armv8.2-A ~~~~~~~~~ diff --git a/docs/getting_started/user-guide.rst b/docs/getting_started/user-guide.rst index 44bfb7a3..ae276f25 100644 --- a/docs/getting_started/user-guide.rst +++ b/docs/getting_started/user-guide.rst @@ -820,6 +820,10 @@ Common build options reduces SRAM usage. Refer to `Library at ROM`_ for further details. Default is 0. +- ``USE_SPINLOCK_CAS``: Setting this build flag to 1 selects the spinlock + implementation variant using the ARMv8.1-LSE compare-and-swap instruction. + Notice this option is experimental and only available to AArch64 builds. + - ``V``: Verbose build. If assigned anything other than 0, the build commands are printed. Default is 0. |