drm: Add a simple generator of random permutations

When testing, we want a random but yet reproducible order in which to
process elements. Here we create an array which is a random (using the
Tausworthe PRNG) permutation of the order in which to execute.

Note these are simple helpers intended to be merged upstream in lib/

v2: Tidier code by David Herrmann
v3: Add reminder that this code is intended to be temporary, with at
least the bulk of the prandom changes going to lib/

Signed-off-by: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Cc: David Herrmann <[e-mail]>
Reviewed-by: Joonas Lahtinen <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/Kconfig

···
 	  chipset. If M is selected the module will be called savage.
 
 endif # DRM_LEGACY
+
+config DRM_LIB_RANDOM
+	bool
+	default n

drivers/gpu/drm/Makefile

···
 		drm_plane.o drm_color_mgmt.o drm_print.o \
 		drm_dumb_buffers.o drm_mode_config.o
 
+drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_GEM_CMA_HELPER) += drm_gem_cma_helper.o
 drm-$(CONFIG_PCI) += ati_pcigart.o

drivers/gpu/drm/lib/drm_random.c

···
+#include <linux/bitops.h>
+#include <linux/kernel.h>
+#include <linux/random.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+
+#include "drm_random.h"
+
+static inline u32 drm_prandom_u32_max_state(u32 ep_ro, struct rnd_state *state)
+{
+	return upper_32_bits((u64)prandom_u32_state(state) * ep_ro);
+}
+
+void drm_random_reorder(unsigned int *order, unsigned int count,
+			struct rnd_state *state)
+{
+	unsigned int i, j;
+
+	for (i = 0; i < count; ++i) {
+		BUILD_BUG_ON(sizeof(unsigned int) > sizeof(u32));
+		j = drm_prandom_u32_max_state(count, state);
+		swap(order[i], order[j]);
+	}
+}
+EXPORT_SYMBOL(drm_random_reorder);
+
+unsigned int *drm_random_order(unsigned int count, struct rnd_state *state)
+{
+	unsigned int *order, i;
+
+	order = kmalloc_array(count, sizeof(*order), GFP_TEMPORARY);
+	if (!order)
+		return order;
+
+	for (i = 0; i < count; i++)
+		order[i] = i;
+
+	drm_random_reorder(order, count, state);
+	return order;
+}
+EXPORT_SYMBOL(drm_random_order);

drivers/gpu/drm/lib/drm_random.h

···
+#ifndef __DRM_RANDOM_H__
+#define __DRM_RANDOM_H__
+
+/* This is a temporary home for a couple of utility functions that should
+ * be transposed to lib/ at the earliest convenience.
+ */
+
+#include <linux/random.h>
+
+#define DRM_RND_STATE_INITIALIZER(seed__) ({				\
+	struct rnd_state state__;					\
+	prandom_seed_state(&state__, (seed__));				\
+	state__;							\
+})
+
+#define DRM_RND_STATE(name__, seed__) \
+	struct rnd_state name__ = DRM_RND_STATE_INITIALIZER(seed__)
+
+unsigned int *drm_random_order(unsigned int count,
+			       struct rnd_state *state);
+void drm_random_reorder(unsigned int *order,
+			unsigned int count,
+			struct rnd_state *state);
+
+#endif /* !__DRM_RANDOM_H__ */