[PATCH v3] ARM: wire up HWCAP2 feature bits to the CPU modalias

On 15 Feb 2017, at 21:37, Ard Biesheuvel [off-list ref] wrote:

Wire up the generic support for exposing CPU feature bits via the
modalias in /sys/device/system/cpu. This allows udev to automatically
load modules for things like crypto algorithms that are implemented
using optional instructions.

Signed-off-by: Ard Biesheuvel <redacted>
---
v3: add offset of 32 so that ELF_HWCAP could be added in the future as
   well

arch/arm/Kconfig                  |  1 +
arch/arm/include/asm/cpufeature.h | 32 ++++++++++++++++++++
2 files changed, 33 insertions(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 186c4c214e0a..53689e386c47 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig

@@ -21,6 +21,7 @@ config ARM
   select GENERIC_ALLOCATOR
   select GENERIC_ATOMIC64 if (CPU_V7M || CPU_V6 || !CPU_32v6K || !AEABI)
   select GENERIC_CLOCKEVENTS_BROADCAST if SMP
+    select GENERIC_CPU_AUTOPROBE
   select GENERIC_EARLY_IOREMAP
   select GENERIC_IDLE_POLL_SETUP
   select GENERIC_IRQ_PROBE

diff --git a/arch/arm/include/asm/cpufeature.h b/arch/arm/include/asm/cpufeature.h
new file mode 100644
index 000000000000..ec3ad45761aa
--- /dev/null
+++ b/arch/arm/include/asm/cpufeature.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2017 Linaro Ltd. <ard.biesheuvel@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef __ASM_CPUFEATURE_H
+#define __ASM_CPUFEATURE_H
+
+#include <asm/hwcap.h>
+
+/*
+ * Due to the fact that ELF_HWCAP is a 32-bit type on ARM, and given the number
+ * of optional CPU features it defines, ARM's CPU capability bits have been
+ * distributed over separate elf_hwcap and elf_hwcap2 variables, each of which
+ * covers a subset of the available CPU features.
+ *
+ * Currently, only a few of those are suitable for automatic module loading
+ * (which is the primary use case of this facility) and those happen to be all
+ * covered by HWCAP2. So let's only expose those via the CPU modalias for now.
+ */
+#define MAX_CPU_FEATURES    64
+#define cpu_feature(x)        (32 + ilog2(HWCAP2_ ## x))
+
+static inline bool cpu_have_feature(unsigned int num)
+{