drm/i915/uapi: expose GTT alignment

On some platforms we potentially have different alignment restrictions
depending on the memory type. We also now have different alignment
restrictions for the same region across different kernel versions.
Extend the region query to return the minimum required GTT alignment.

Testcase: igt@gem_create@create-ext-placement-alignment
Testcase: igt@i915_query@query-regions-sanity-check
Suggested-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Michal Mrozek <michal.mrozek@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Yang A Shi <yang.a.shi@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004114915.221708-2-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_query.c

@@ -498,6 +498,7 @@ static int query_memregion_info(struct drm_i915_private *i915,
 		info.region.memory_class = mr->type;
 		info.region.memory_instance = mr->instance;
 		info.probed_size = mr->total;
+		info.gtt_alignment = mr->min_page_size;
 
 		if (mr->type == INTEL_MEMORY_LOCAL)
 			info.probed_cpu_visible_size = mr->io_size;

include/uapi/drm/i915_drm.h

@@ -3346,8 +3346,33 @@ struct drm_i915_memory_region_info {
 	/** @region: The class:instance pair encoding */
 	struct drm_i915_gem_memory_class_instance region;
 
-	/** @rsvd0: MBZ */
-	__u32 rsvd0;
+	union {
+		/** @rsvd0: MBZ */
+		__u32 rsvd0;
+		/**
+		 * @gtt_alignment:
+		 *
+		 * The minimum required GTT alignment for this type of memory.
+		 * When allocating a GTT address it must be aligned to this
+		 * value or larger. On some platforms the kernel might opt to
+		 * using 64K pages for I915_MEMORY_CLASS_DEVICE, where 64K GTT
+		 * pages can then be used if we also use 64K GTT alignment.
+		 *
+		 * NOTE: If this is zero then this must be an older
+		 * kernel which lacks support for this field.
+		 *
+		 * Side note: For larger objects (especially for
+		 * I915_MEMORY_CLASS_DEVICE), like 2M+ in size, userspace should
+		 * consider potentially bumping the GTT alignment to say 2M,
+		 * which could potentially increase the likelihood of the kernel
+		 * being able to utilise 2M GTT pages underneath, if the layout
+		 * of the physical pages allows it.  On some configurations we
+		 * can then also use a more efficient page-table layout, if we
+		 * can't use the more desirable 2M GTT page, so long as we know
+		 * that the entire page-table will be used by this object.
+		 */
+		__u32 gtt_alignment;
+	};
 
 	/**
 	 * @probed_size: Memory probed by the driver