[Intel-gfx] [PATCH v3 04/38] lib: Add a simple prime number generator

[Chris Wilson]

Prime numbers are interesting for testing components that use multiplies
and divides, such as testing DRM's struct drm_mm alignment computations.

v2: Move to lib/, add selftest
v3: Fix initial constants (exclude 0/1 from being primes)
v4: More RCU markup to keep 0day/sparse happy

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Lukas Wunner <lukas at wunner.de>
---
 include/linux/prime_numbers.h |  13 +++
 lib/Kconfig                   |   7 ++
 lib/Makefile                  |   2 +
 lib/prime_numbers.c           | 238 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 260 insertions(+)
 create mode 100644 include/linux/prime_numbers.h
 create mode 100644 lib/prime_numbers.c

diff --git a/include/linux/prime_numbers.h b/include/linux/prime_numbers.h
new file mode 100644
index 000000000000..877f6acbd0b6
--- /dev/null
+++ b/include/linux/prime_numbers.h
@@ -0,0 +1,13 @@
+#ifndef __LINUX_PRIME_NUMBERS_H
+#define __LINUX_PRIME_NUMBERS_H
+
+#include <linux/types.h>
+
+bool is_prime_number(unsigned long x);
+unsigned long next_prime_number(unsigned long x);
+
+/* A useful white-lie here is that 1 is prime. */
+#define for_each_prime_number(prime, max) \
+	for (prime = 1;	prime < (max); prime = next_prime_number(prime))
+
+#endif /* !__LINUX_PRIME_NUMBERS_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index 260a80e313b9..1788a1f50d28 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -550,4 +550,11 @@ config STACKDEPOT
 config SBITMAP
 	bool
 
+config PRIME_NUMBERS
+	tristate "Prime number generator"
+	default n
+	help
+	  Provides a helper module to generate prime numbers. Useful for writing
+	  test code, especially when checking multiplication and divison.
+
 endmenu
diff --git a/lib/Makefile b/lib/Makefile
index 50144a3aeebd..c664143fd917 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -197,6 +197,8 @@ obj-$(CONFIG_ASN1) += asn1_decoder.o
 
 obj-$(CONFIG_FONT_SUPPORT) += fonts/
 
+obj-$(CONFIG_PRIME_NUMBERS) += prime_numbers.o
+
 hostprogs-y	:= gen_crc32table
 clean-files	:= crc32table.h
 
diff --git a/lib/prime_numbers.c b/lib/prime_numbers.c
new file mode 100644
index 000000000000..ec2959520653
--- /dev/null
+++ b/lib/prime_numbers.c
@@ -0,0 +1,238 @@
+#define pr_fmt(fmt) "prime numbers: " fmt "\n"
+
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/prime_numbers.h>
+#include <linux/slab.h>
+
+struct primes {
+	struct rcu_head rcu;
+	unsigned long last, sz;
+	unsigned long primes[];
+};
+
+#if BITS_PER_LONG == 64
+static const struct primes small_primes = {
+	.last = 61,
+	.sz = 64,
+	.primes = { 0x28208a20a08a28acUL }
+};
+#elif BITS_PER_LONG == 32
+static const struct primes small_primes = {
+	.last = 31,
+	.sz = 32,
+	.primes = { 0xa08a28acUL }
+};
+#else
+#error "unhandled BITS_PER_LONG"
+#endif
+
+static DEFINE_MUTEX(lock);
+static const struct primes __rcu *primes = RCU_INITIALIZER(&small_primes);
+
+static unsigned long selftest_max;
+
+static bool slow_is_prime_number(unsigned long x)
+{
+	unsigned long y = int_sqrt(x);
+
+	while (y > 1) {
+		if ((x % y) == 0)
+			break;
+		y--;
+	}
+
+	return y == 1;
+}
+
+static unsigned long slow_next_prime_number(unsigned long x)
+{
+	for (;;) {
+		if (slow_is_prime_number(++x))
+			return x;
+	}
+}
+
+static unsigned long mark_multiples(unsigned long x,
+				    unsigned long *p,
+				    unsigned long start,
+				    unsigned long end)
+{
+	unsigned long m;
+
+	m = 2 * x;
+	if (m < start)
+		m = roundup(start, x);
+
+	while (m < end) {
+		__clear_bit(m, p);
+		m += x;
+	}
+
+	return x;
+}
+
+static const struct primes *expand_to_next(unsigned long x)
+{
+	const struct primes *p;
+	struct primes *new;
+	unsigned long sz, y;
+
+	rcu_read_unlock();
+
+	/* Betrand's Theorem states:
+	 * 	For all n > 1, there exists a prime p: n < p <= 2*n.
+	 */
+	sz = 2 * x + 1;
+	if (sz < x)
+		return NULL;
+
+	sz = round_up(sz, BITS_PER_LONG);
+	new = kmalloc(sizeof(*new) + sz / sizeof(long), GFP_KERNEL);
+	if (!new)
+		return NULL;
+
+	mutex_lock(&lock);
+	p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+	if (x < p->last) {
+		kfree(new);
+		goto relock;
+	}
+
+	/* Where memory permits, track the primes using the
+	 * Sieve of Eratosthenes.
+	 */
+	memcpy(new->primes, p->primes, p->sz / BITS_PER_LONG * sizeof(long));
+	memset(new->primes + p->sz / BITS_PER_LONG,
+	       0xff, (sz - p->sz) / BITS_PER_LONG * sizeof(long));
+	for (y = 2UL; y < sz; y = find_next_bit(new->primes, sz, y + 1))
+		new->last = mark_multiples(y, new->primes, p->sz, sz);
+	new->sz = sz;
+
+	rcu_assign_pointer(primes, new);
+	if (p != &small_primes)
+		kfree_rcu((struct primes *)p, rcu);
+	p = new;
+
+relock:
+	rcu_read_lock();
+	mutex_unlock(&lock);
+	return p;
+}
+
+static const struct primes *get_primes(unsigned long x)
+{
+	const struct primes *p;
+
+	rcu_read_lock();
+	p = rcu_dereference(primes);
+	if (!p || x >= p->last)
+		p = expand_to_next(x);
+
+	/* returns under RCU iff p != NULL */
+	return p;
+}
+
+unsigned long next_prime_number(unsigned long x)
+{
+	const struct primes *p;
+
+	p = get_primes(x);
+	if (unlikely(!p))
+		return slow_next_prime_number(x);
+
+	x = find_next_bit(p->primes, p->last, x + 1);
+	rcu_read_unlock();
+
+	return x;
+}
+EXPORT_SYMBOL(next_prime_number);
+
+bool is_prime_number(unsigned long x)
+{
+	const struct primes *p;
+	bool result;
+
+	p = get_primes(x);
+	if (unlikely(!p))
+		return slow_is_prime_number(x);
+
+	result = test_bit(x, p->primes);
+	rcu_read_unlock();
+
+	return result;
+}
+EXPORT_SYMBOL(is_prime_number);
+
+static void dump_primes(void)
+{
+	const struct primes *p;
+
+	rcu_read_lock();
+	p = rcu_dereference(primes);
+	pr_info("primes.{last=%lu, .sz=%lu, .primes[]=...x%lx}",
+		p->last, p->sz, p->primes[p->sz / BITS_PER_LONG - 1]);
+	rcu_read_unlock();
+}
+
+static int selftest(unsigned long max)
+{
+	unsigned long x, last;
+
+	if (!max)
+		return 0;
+
+	for (last = 0, x = 2; x < max; x++) {
+		bool slow = slow_is_prime_number(x);
+		bool fast = is_prime_number(x);
+
+		if (slow != fast) {
+			pr_err("inconsistent result for is-prime(%lu): slow=%s, fast=%s!",
+			       x, slow ? "yes" : "no", fast ? "yes" : "no");
+			goto err;
+		}
+
+		if (!slow)
+			continue;
+
+		if (next_prime_number(last) != x) {
+			pr_err("incorrect result for next-prime(%lu): expected %lu, got %lu",
+			       last, x, next_prime_number(last));
+			goto err;
+		}
+		last = x;
+	}
+
+	pr_info("selftest(%lu) passed, last prime was %lu", x, last);
+	return 0;
+
+err:
+	dump_primes();
+	return -EINVAL;
+}
+
+static int __init primes_init(void)
+{
+	return selftest(selftest_max);
+}
+
+static void __exit primes_exit(void)
+{
+	const struct primes *p;
+
+	mutex_lock(&lock);
+	p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+	if (p != &small_primes) {
+		kfree_rcu((struct primes *)p, rcu);
+		rcu_assign_pointer(p, &small_primes);
+	}
+	mutex_unlock(&lock);
+}
+
+module_init(primes_init);
+module_exit(primes_exit);
+
+module_param_named(selftest, selftest_max, ulong, 0400);
+
+MODULE_AUTHOR("Intel Corporation");
+MODULE_LICENSE("GPL");
-- 
2.11.0