[PATCH i-g-t 1/2] lib/igt_fb: Add IGT_FORMAT_MOD_PREFERRED helper modifier

[Jeevan B <jeevan.b@intel.com>]

Add a new IGT‑internal framebuffer modifier,
IGT_FORMAT_MOD_PREFERRED, allowing tests to request the preferred
non‑linear tiling for a platform without hardcoding
driver‑specific modifiers.

The modifier is resolved in igt_fb_resolve_modifier() using simple
platform rules (Intel, VC4, fallback to linear) and is never passed to
the kernel. It is defined outside the valid DRM modifier range and used
only as an internal sentinel.

This provides a driver‑agnostic way for tests to pick an optimal
modifier, avoiding per‑test hacks.

Signed-off-by: Jeevan B <jeevan.b@intel.com>
---
 lib/igt_fb.c | 27 ++++++++++++++++++++++++++-
 lib/igt_fb.h | 21 +++++++++++++++++++++
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/lib/igt_fb.c b/lib/igt_fb.c
index fa9220953..9c3d0338b 100644
--- a/lib/igt_fb.c
+++ b/lib/igt_fb.c
@@ -790,6 +790,29 @@ static int fb_num_planes(const struct igt_fb *fb)
 	return num_planes;
 }
 
+static uint64_t igt_fb_resolve_modifier(int fd, uint64_t modifier)
+{
+	if (modifier != IGT_FORMAT_MOD_PREFERRED)
+		return modifier;
+
+	if (is_xe_device(fd)) {
+		return I915_FORMAT_MOD_4_TILED;
+	} else if (is_i915_device(fd)) {
+		int ver = intel_display_ver(intel_get_drm_devid(fd));
+
+		if (ver >= 13)
+			return I915_FORMAT_MOD_4_TILED;
+		else if (ver >= 9)
+			return I915_FORMAT_MOD_Y_TILED;
+		else
+			return I915_FORMAT_MOD_X_TILED;
+	} else if (is_vc4_device(fd))
+		return DRM_FORMAT_MOD_BROADCOM_VC4_T_TILED;
+
+	/* For AMD, nouveau, and any other driver, fall back to linear. */
+	return DRM_FORMAT_MOD_LINEAR;
+}
+
 void igt_init_fb(struct igt_fb *fb, int fd, int width, int height,
 		 uint32_t drm_format, uint64_t modifier,
 		 enum igt_color_encoding color_encoding,
@@ -803,7 +826,7 @@ void igt_init_fb(struct igt_fb *fb, int fd, int width, int height,
 
 	fb->width = width;
 	fb->height = height;
-	fb->modifier = modifier;
+	fb->modifier = igt_fb_resolve_modifier(fd, modifier);
 	fb->drm_format = drm_format;
 	fb->fd = fd;
 	fb->num_planes = fb_num_planes(fb);
@@ -5266,6 +5289,8 @@ void igt_format_array_fill(uint32_t **formats_array, unsigned int *count,
 const char *igt_fb_modifier_name(uint64_t modifier)
 {
 	switch (modifier) {
+	case IGT_FORMAT_MOD_PREFERRED:
+		return "preferred";
 	case DRM_FORMAT_MOD_LINEAR:
 		return "linear";
 	case I915_FORMAT_MOD_X_TILED:
diff --git a/lib/igt_fb.h b/lib/igt_fb.h
index 8e5907dab..5380b3dfa 100644
--- a/lib/igt_fb.h
+++ b/lib/igt_fb.h
@@ -50,6 +50,27 @@ typedef struct _igt_crc igt_crc_t;
  */
 #define IGT_FORMAT_FLOAT fourcc_code('I', 'G', 'F', 'x')
 
+/**
+ * IGT_FORMAT_MOD_PREFERRED:
+ *
+ * Modifier used by tests to request the best tiling format for the
+ * current device. The actual modifier is selected inside igt_init_fb(),
+ * so tests don’t need to handle driver-specific details.
+ *
+ * Selection rules:
+ *  - Intel Xe:              4_TILED
+ *  - Intel i915 (gen ≥13):  4_TILED
+ *  - Intel i915 (gen 9–12): Y_TILED
+ *  - Intel i915 (gen <9):   X_TILED
+ *  - Broadcom VC4:          T_TILED
+ *  - Others:                LINEAR (fallback)
+ *
+ * Note: This is a special placeholder value and must never be sent to
+ * the kernel.
+ */
+#define IGT_FORMAT_MOD_PREFERRED  UINT64_C(0xfffffffffffffffe)
+
+
 #define IGT_FORMAT_FMT "%c%c%c%c(0x%08x)"
 #define IGT_FORMAT_ARGS(f) ((f) >> 0) & 0xff, ((f) >> 8) & 0xff, \
 		((f) >> 16) & 0xff, ((f) >> 24) & 0xff, (f)
-- 
2.43.0