drm/i915/selftests: Extract random_offset() for use with a prng

For selftests, we desire repeatability and so prefer using a prng with
known seed over true randomness. Extract random_offset() as a selftest
utility that can take the prng state.
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002122430.23205-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -1299,6 +1299,7 @@ static int igt_gtt_reserve(void *arg)
 {
 	struct i915_ggtt *ggtt = arg;
 	struct drm_i915_gem_object *obj, *on;
+	I915_RND_STATE(prng);
 	LIST_HEAD(objects);
 	u64 total;
 	int err = -ENODEV;
@@ -1425,9 +1426,10 @@ static int igt_gtt_reserve(void *arg)
 			goto out;
 		}
 
-		offset = random_offset(0, ggtt->vm.total,
-				       2*I915_GTT_PAGE_SIZE,
-				       I915_GTT_MIN_ALIGNMENT);
+		offset = igt_random_offset(&prng,
+					   0, ggtt->vm.total,
+					   2 * I915_GTT_PAGE_SIZE,
+					   I915_GTT_MIN_ALIGNMENT);
 
 		err = i915_gem_gtt_reserve(&ggtt->vm, &vma->node,
 					   obj->base.size,
@@ -1772,6 +1774,7 @@ static int igt_cs_tlb(void *arg)
 	struct intel_context *ce;
 	struct drm_file *file;
 	struct i915_vma *vma;
+	I915_RND_STATE(prng);
 	unsigned int i;
 	u32 *result;
 	u32 *batch;
@@ -1885,8 +1888,9 @@ static int igt_cs_tlb(void *arg)
 			struct i915_request *rq;
 			u64 offset;
 
-			offset = random_offset(0, vm->total - PAGE_SIZE,
-					       chunk_size, PAGE_SIZE);
+			offset = igt_random_offset(&prng,
+						   0, vm->total - PAGE_SIZE,
+						   chunk_size, PAGE_SIZE);
 
 			err = vm->allocate_va_range(vm, offset, chunk_size);
 			if (err)

drivers/gpu/drm/i915/selftests/i915_random.c

@@ -29,6 +29,7 @@
 #include <linux/types.h>
 
 #include "i915_random.h"
+#include "i915_utils.h"
 
 u64 i915_prandom_u64_state(struct rnd_state *rnd)
 {
@@ -87,3 +88,22 @@ unsigned int *i915_random_order(unsigned int count, struct rnd_state *state)
 	i915_random_reorder(order, count, state);
 	return order;
 }
+
+u64 igt_random_offset(struct rnd_state *state,
+		      u64 start, u64 end,
+		      u64 len, u64 align)
+{
+	u64 range, addr;
+
+	BUG_ON(range_overflows(start, len, end));
+	BUG_ON(round_up(start, align) > round_down(end - len, align));
+
+	range = round_down(end - len, align) - round_up(start, align);
+	if (range) {
+		addr = i915_prandom_u64_state(state);
+		div64_u64_rem(addr, range, &addr);
+		start += addr;
+	}
+
+	return round_up(start, align);
+}

drivers/gpu/drm/i915/selftests/i915_random.h

@@ -57,4 +57,8 @@ void i915_random_reorder(unsigned int *order,
 void i915_prandom_shuffle(void *arr, size_t elsz, size_t count,
 			  struct rnd_state *state);
 
+u64 igt_random_offset(struct rnd_state *state,
+		      u64 start, u64 end,
+		      u64 len, u64 align);
+
 #endif /* !__I915_SELFTESTS_RANDOM_H__ */