[patch 22/38] random: Provide CONFIG_ARCH_HAS_RANDOM_ENTROPY

[Thomas Gleixner]

Chasing down to what random_get_entropy() resolves is a 'spot the mouse'
game through a maze of #ifdeffery. Also the placement in timex.h is
non-obvious and has just been chosen because it provides conveniant access
to the underlying get_cycles() #ifdeffery.

Provide a config switch which is selectable by the architecture and a
temporary #ifdef guard in timex.h. Architectures which select the config
switch must provide asm/random.h with the architecture specific
implementation.

Update all usage sites to include linux/random.h so that the gradual
conversion does not cause build regressions.

This is part of a larger effort to remove get_cycles() usage from
non-architecture code.

Signed-off-by: Thomas Gleixner <tglx at kernel.org>
Cc: "Jason A. Donenfeld" <Jason at zx2c4.com>
---
 arch/Kconfig                     |    3 +++
 arch/loongarch/kernel/relocate.c |    1 +
 arch/mips/kernel/relocate.c      |    1 +
 crypto/jitterentropy-kcapi.c     |    1 +
 include/linux/random.h           |   15 +++++++++++++++
 include/linux/timex.h            |    2 ++
 kernel/kcsan/core.c              |    1 +
 7 files changed, 24 insertions(+)

--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -370,6 +370,9 @@ config ARCH_HAS_CPU_FINALIZE_INIT
 config ARCH_HAS_DELAY_TIMER
 	bool
 
+config ARCH_HAS_RANDOM_ENTROPY
+	bool
+
 # The architecture has a per-task state that includes the mm's PASID
 config ARCH_HAS_CPU_PASID
 	bool
--- a/arch/loongarch/kernel/relocate.c
+++ b/arch/loongarch/kernel/relocate.c
@@ -9,6 +9,7 @@
 #include <linux/kernel.h>
 #include <linux/printk.h>
 #include <linux/panic_notifier.h>
+#include <linux/random.h>
 #include <linux/start_kernel.h>
 #include <asm/bootinfo.h>
 #include <asm/early_ioremap.h>
--- a/arch/mips/kernel/relocate.c
+++ b/arch/mips/kernel/relocate.c
@@ -19,6 +19,7 @@
 #include <linux/libfdt.h>
 #include <linux/of_fdt.h>
 #include <linux/panic_notifier.h>
+#include <linux/random.h>
 #include <linux/sched/task.h>
 #include <linux/start_kernel.h>
 #include <linux/string.h>
--- a/crypto/jitterentropy-kcapi.c
+++ b/crypto/jitterentropy-kcapi.c
@@ -42,6 +42,7 @@
 #include <linux/fips.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/random.h>
 #include <linux/slab.h>
 #include <linux/time.h>
 #include <crypto/internal/rng.h>
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -139,4 +139,19 @@ int random_online_cpu(unsigned int cpu);
 extern const struct file_operations random_fops, urandom_fops;
 #endif
 
+unsigned long random_get_entropy_fallback(void);
+
+/*
+ * random_get_entropy() is used by the /dev/random driver in order to extract
+ * entropy via the relative unpredictability of when an interrupt takes places
+ * versus a high speed, fine-grained timing source or cycle counter.  Since it
+ * will be occurred on every single interrupt, it must have a very low
+ * cost/overhead.
+ *
+ * If an architecture does not provide it, then use random_get_entropy_fallback().
+ */
+#ifdef CONFIG_ARCH_HAS_RANDOM_ENTROPY
+#include <asm/random.h>
+#endif
+
 #endif /* _LINUX_RANDOM_H */
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -66,6 +66,7 @@ unsigned long random_get_entropy_fallbac
 
 #include <asm/timex.h>
 
+#ifndef CONFIG_ARCH_HAS_RANDOM_ENTROPY
 #ifndef random_get_entropy
 /*
  * The random_get_entropy() function is used by the /dev/random driver
@@ -85,6 +86,7 @@ unsigned long random_get_entropy_fallbac
 #define random_get_entropy()	random_get_entropy_fallback()
 #endif
 #endif
+#endif
 
 /*
  * SHIFT_PLL is used as a dampening factor to define how much we
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -18,6 +18,7 @@
 #include <linux/moduleparam.h>
 #include <linux/percpu.h>
 #include <linux/preempt.h>
+#include <linux/random.h>
 #include <linux/sched.h>
 #include <linux/string.h>
 #include <linux/uaccess.h>