linux-kernel - [PATCH v3 11/21] arm64: capabilities: Introduce weak features based on local CPU

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Date:   Fri,  9 Feb 2018 17:54:52 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     dave.martin@....com, linux-kernel@...r.kernel.org,
        will.deacon@....com, catalin.marinas@....com, marc.zyngier@....com,
        mark.rutland@....com, jnair@...iumnetworks.com,
        ckadabi@...eaurora.org, robin.murphy@....com,
        ard.biesheuvel@...aro.org,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: [PATCH v3 11/21] arm64: capabilities: Introduce weak features based on local CPU

Now that we have the flexibility of defining system features based
on individual CPUs, introduce CPU feature type that can be detected
on a local SCOPE and ignores the conflict on late CPUs. This is
applicable for ARM64_HAS_NO_HW_PREFETCH, where it is fine for
the system to have CPUs without hardware prefetch turning up
later. We only suffer a performance penalty, nothing fatal.

Cc: Will Deacon <will.deacon@....com>
Reviewed-by: Dave Martin <dave.martin@....com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
---
 arch/arm64/include/asm/cpufeature.h | 8 ++++++++
 arch/arm64/kernel/cpufeature.c      | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 6e354a98bc24..49c4ae331ca7 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -230,6 +230,14 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
  */
 #define ARM64_CPUCAP_SYSTEM_FEATURE	\
 	(ARM64_CPUCAP_SCOPE_SYSTEM | ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU)
+/*
+ * CPU feature detected at boot time based on feature of one or more CPUs.
+ * All possible conflicts for a late CPU are ignored.
+ */
+#define ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE		\
+	(ARM64_CPUCAP_SCOPE_LOCAL_CPU		|	\
+	 ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU	|	\
+	 ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU)
 
 struct arm64_cpu_capabilities {
 	const char *desc;
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9b111d560b30..373173bc3ab3 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -992,7 +992,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 	{
 		.desc = "Software prefetching using PRFM",
 		.capability = ARM64_HAS_NO_HW_PREFETCH,
-		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
 		.matches = has_no_hw_prefetch,
 	},
 #ifdef CONFIG_ARM64_UAO
-- 
2.14.3