[igt-dev] [RFC PATCH i-g-t v3 2/4] lib: Add GEM minimum page size helper

[Janusz Krzysztofik]

Some tests assume 4kB page size while using softpin.  That assumption
may be wrong on future GEM backends with possibly larger minimum page
sizes.  As a result, those tests may either fail on softpin at offsets
which are incorrectly aligned, may silently skip such incorrectly
aligned addresses assuming them occupied by other users, or may always
succeed when examining invalid use patterns.

Provide a helper function that detects minimum page size and returns
the size order.  Tests may use it to calculate softpin offsets suitable
for actually used backing store.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Cc: Stuart Summers <stuart.summers at intel.com>
---
 lib/ioctl_wrappers.c | 82 ++++++++++++++++++++++++++++++++++++++++++++
 lib/ioctl_wrappers.h |  1 +
 2 files changed, 83 insertions(+)

diff --git a/lib/ioctl_wrappers.c b/lib/ioctl_wrappers.c
index 280fdd62..a4313832 100644
--- a/lib/ioctl_wrappers.c
+++ b/lib/ioctl_wrappers.c
@@ -54,6 +54,7 @@
 #include "intel_io.h"
 #include "igt_debugfs.h"
 #include "igt_sysfs.h"
+#include "igt_x86.h"
 #include "config.h"
 
 #ifdef HAVE_VALGRIND
@@ -1157,6 +1158,87 @@ bool gem_has_softpin(int fd)
 	return has_softpin;
 }
 
+static int __min_page_size_order(int fd, struct drm_i915_gem_exec_object2 *obj,
+				 struct drm_i915_gem_execbuffer2 *eb,
+				 uint64_t offset, int min_order, int max_order)
+{
+	static const uint32_t bbe = MI_BATCH_BUFFER_END;
+	uint64_t page_size = 1ull << max_order;
+	int order;
+
+	if (max_order > min_order) {
+		/* explore upper half of the max_order at offset area */
+		order = __min_page_size_order(fd, obj, eb, offset, min_order,
+					      max_order - 1);
+		if (order < max_order)
+			return order;
+	}
+
+	obj->offset = gen8_canonical_addr(offset - page_size);
+	gem_write(fd, obj->handle, 0, &bbe, sizeof(bbe));
+	if (!__gem_execbuf(fd, eb)) {
+		/* upper half not occupied, must be the minimum */
+		igt_debug("found min page size=%#llx, size order=%d\n",
+			  (long long)page_size, max_order);
+		return max_order;
+	}
+
+	if (max_order > min_order) {
+		/* explore lower half of in case the upper half was occupied */
+		page_size >>= 1;
+		order = __min_page_size_order(fd, obj, eb, offset - page_size,
+					      min_order, max_order - 1);
+		if (order < max_order)
+			return order;
+	}
+
+	return max_order + 1;
+}
+
+/**
+ * gem_min_page_size_order:
+ * @fd: open i915 drm file descriptor
+ *
+ * This function detects the minimum size of a gem object allocated from
+ * a default backing store.  It is useful for calculating correctly aligned
+ * softpin offsets.
+ * Since size order to size conversion (size = 1 << order) is less trivial
+ * than the opposite, the function returns the size order as more handy.
+ *
+ * Returns:
+ * Size order of the minimum page size
+ */
+int gem_min_page_size_order(int fd)
+{
+	struct drm_i915_gem_exec_object2 obj;
+	struct drm_i915_gem_execbuffer2 eb;
+	uint64_t gtt_size = gem_aperture_size(fd);
+	int min_order = 12;	/* current I915_GTT_PAGE_SIZE equivalent */
+	uint64_t page_size = 1ull << min_order;
+	int max_order = 21;	/* current I915_GTT_MAX_PAGE_SIZE equivalent */
+
+	/* no softpin => 4kB page size */
+	if (!gem_has_softpin(fd))
+		return min_order;
+
+	memset(&obj, 0, sizeof(obj));
+	memset(&eb, 0, sizeof(eb));
+
+	obj.handle = gem_create(fd, page_size);
+	obj.flags = EXEC_OBJECT_PINNED | EXEC_OBJECT_SUPPORTS_48B_ADDRESS;
+	eb.buffers_ptr = to_user_pointer(&obj);
+	eb.buffer_count = 1;
+
+	min_order = __min_page_size_order(fd, &obj, &eb, gtt_size, min_order,
+					  max_order);
+
+	gem_close(fd, obj.handle);
+
+	igt_require(min_order <= max_order);
+
+	return min_order;
+}
+
 /**
  * gem_has_exec_fence:
  * @fd: open i915 drm file descriptor
diff --git a/lib/ioctl_wrappers.h b/lib/ioctl_wrappers.h
index 03211c97..91690847 100644
--- a/lib/ioctl_wrappers.h
+++ b/lib/ioctl_wrappers.h
@@ -138,6 +138,7 @@ uint64_t gem_aperture_size(int fd);
 uint64_t gem_global_aperture_size(int fd);
 uint64_t gem_mappable_aperture_size(void);
 bool gem_has_softpin(int fd);
+int gem_min_page_size_order(int fd);
 bool gem_has_exec_fence(int fd);
 
 /* check functions which auto-skip tests by calling igt_skip() */
-- 
2.21.0