Re: [PATCH 16/16] arm64: Add work around for Arm Cortex-A55 Erratum 1024718

From: Dave Martin
Date: Fri Jan 26 2018 - 10:33:24 EST


On Tue, Jan 23, 2018 at 12:28:09PM +0000, Suzuki K Poulose wrote:
> Some variants of the Arm Cortex-55 cores (r0p0, r0p1, r1p0) suffer
> from an erratum 1024718, which causes incorrect updates when DBM/AP
> bits in a page table entry is modified without a break-before-make
> sequence. The work around is to skip enabling the hardware DBM feature
> on the affected cores. The hardware Access Flag management features
> is not affected.
>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
> Documentation/arm64/silicon-errata.txt | 1 +
> arch/arm64/Kconfig | 14 ++++++++++++++
> arch/arm64/kernel/cpufeature.c | 14 +++++++++++++-
> 3 files changed, 28 insertions(+), 1 deletion(-)

[...]

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 8af755b8219d..64f1e911c6af 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -914,9 +914,21 @@ static inline void __cpu_enable_hw_dbm(void)
> isb();
> }
>
> +static bool cpu_has_erratum_1024718(void)
> +{
> + static const struct midr_range __maybe_unused cpus[] = {

Do you need __maybe_unused? If #ifdef were used here then
__maybe_unused would be needed, but I thought that if code is optimised
out instead of conditionally copiled, this didn't apply.

> + MIDR_RANGE(MIDR_CORTEX_A55, 0, 0, 1, 0), // A55 r0p0 -r1p0
> + {},
> + };
> +
> + return IS_ENABLED(CONFIG_ARM64_ERRATUM_1024718) &&
> + is_midr_in_range_list(read_cpuid_id(), cpus);

Why have a list with just one entry? Do you expect more entries over
time?

[...]

Cheers
---Dave