[Intel-gfx] [PATCH] drm/i915/uapi: expose GTT alignment

[Matthew Auld]

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.

Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19398

Testcase: igt at gem_create@create-ext-placement-alignment
Testcase: igt at i915_query@query-regions-sanity-check
Suggested-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Signed-off-by: Matthew Auld <matthew.auld at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Michal Mrozek <michal.mrozek at intel.com>
Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Cc: Stuart Summers <stuart.summers at intel.com>
Cc: Jordan Justen <jordan.l.justen at intel.com>
Cc: Yang A Shi <yang.a.shi at intel.com>
Cc: Nirmoy Das <nirmoy.das at intel.com>
Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das at intel.com>
Acked-by: Jordan Justen <jordan.l.justen at intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004114915.221708-2-matthew.auld@intel.com
---
 drivers/gpu/drm/i915/i915_query.c |  1 +
 include/uapi/drm/i915_drm.h       | 29 +++++++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_query.c b/drivers/gpu/drm/i915/i915_query.c
index 6ec9c9fb7b0d..111377f210ed 100644
--- a/drivers/gpu/drm/i915/i915_query.c
+++ b/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;
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 8df261c5ab9b..c346b1923d11 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -3356,8 +3356,33 @@ 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
-- 
2.38.1