[RFC PATCH i-g-t v4 2/4] lib: Add minimum GTT alignment helper

[Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>]

Some tests assume 4kB offset alignment 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 if incorrect detection method is used, or may always succeed
when examining invalid use patterns.

Provide a helper function that detects minimum GTT alignment.  Tests
may use it to calculate softpin offsets valid for actually used backing
store.

v2: Rename the helper, use 'minimum GTT alignment' term across the
    change (Chris),
  - use error numbers to distinguish between invalid offsets and
    addresses occupied by other users, then
  - simplify the code (Chris).

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
---
 lib/ioctl_wrappers.c | 46 ++++++++++++++++++++++++++++++++++++++++++++
 lib/ioctl_wrappers.h |  2 ++
 2 files changed, 48 insertions(+)

diff --git a/lib/ioctl_wrappers.c b/lib/ioctl_wrappers.c
index 628f8b83..f0ef8b2e 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
@@ -1158,6 +1159,51 @@ bool gem_has_softpin(int fd)
 	return has_softpin;
 }
 
+/**
+ * gem_gtt_min_alignment_order:
+ * @fd: open i915 drm file descriptor
+ *
+ * This function detects the minimum possible alignment of a soft-pinned 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 alignment order as more handy.
+ *
+ * Returns:
+ * Size order of the minimum GTT alignment
+ */
+int gem_gtt_min_alignment_order(int fd)
+{
+	struct drm_i915_gem_exec_object2 obj;
+	struct drm_i915_gem_execbuffer2 eb;
+	const uint32_t bbe = MI_BATCH_BUFFER_END;
+	int order;
+
+	/* no softpin => 4kB page size */
+	if (!gem_has_softpin(fd))
+		return 12;
+
+	memset(&obj, 0, sizeof(obj));
+	memset(&eb, 0, sizeof(eb));
+
+	obj.handle = gem_create(fd, 4096);
+	obj.flags = EXEC_OBJECT_PINNED | EXEC_OBJECT_SUPPORTS_48B_ADDRESS;
+	eb.buffers_ptr = to_user_pointer(&obj);
+	eb.buffer_count = 1;
+	gem_write(fd, obj.handle, 0, &bbe, sizeof(bbe));
+
+	for (order = 12; order < 64; order++) {
+		obj.offset = 1ull << order;
+		if (__gem_execbuf(fd, &eb) != -EINVAL)
+			break;
+	}
+	igt_assert(obj.offset < gem_aperture_size(fd));
+
+	gem_close(fd, obj.handle);
+	igt_debug("minimum GTT alignment is %#llx\n", (long long)obj.offset);
+	return 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..c8d57a7c 100644
--- a/lib/ioctl_wrappers.h
+++ b/lib/ioctl_wrappers.h
@@ -138,6 +138,8 @@ 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_gtt_min_alignment_order(int fd);
+#define gem_gtt_min_alignment(fd) (1ull << gem_gtt_min_alignment_order(fd))
 bool gem_has_exec_fence(int fd);
 
 /* check functions which auto-skip tests by calling igt_skip() */
-- 
2.21.0

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