[PATCH v4] ARM: wire up HWCAP/HWCAP2 feature bits to the CPU modalias

[Ard Biesheuvel]

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.

Since it is non-trivial to transparantly support both HWCAP and HWCAP2
capabilities in the cpu_feature() macro (which allows a module's hwcap
dependency and init routine to be declared using a single invocation of
module_cpu_feature_match()), support only HWCAP2 for now, which covers
the capabilities that are most likely to be useful in this manner.
Module dependencies on HWCAP will need to be declared explicitly via a
MODULE_DEVICE_TABLE(cpu, ...) declaration.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
v4: expose both elf_hwcap and elf_hwcap2 via the kernel modalias, and allow
    both to be used in MODULE_DEVICE_TABLE()s, but keep cpu_feature() as
    hwcap2 specific.

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 | 37 ++++++++++++++++++++
 2 files changed, 38 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..9ac822e927ac
--- /dev/null
+++ b/arch/arm/include/asm/cpufeature.h
@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2017 Linaro Ltd. <ard.biesheuvel at 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 hardware 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 cover those via the cpu_feature()
+ * convenience macro for now (which is used by module_cpu_feature_match()).
+ * However, all capabilities are exposed via the modalias, and can be matched
+ * using an explicit MODULE_DEVICE_TABLE() that uses __hwcap_feature() directly.
+ */
+#define MAX_CPU_FEATURES	64
+#define __hwcap_feature(x)	ilog2(HWCAP_ ## x)
+#define __hwcap2_feature(x)	(32 + ilog2(HWCAP2_ ## x))
+#define cpu_feature(x)		__hwcap2_feature(x)
+
+static inline bool cpu_have_feature(unsigned int num)
+{
+	return num < 32 ? elf_hwcap & BIT(num) : elf_hwcap2 & BIT(num - 32);
+}
+
+#endif
-- 
2.7.4