[PATCH v3 1/5] drm/i915: add needs_compact_pt flag

[PATCH v3 1/5] drm/i915: add needs_compact_pt flag

[Robert Beckett]

From: Ramalingam C <ramalingam.c@intel.com>

Add a new platform flag, needs_compact_pt, to mark the requirement of
compact pt layout support for the ppGTT when using 64K GTT pages.

With this flag has_64k_pages will only indicate requirement of 64K
GTT page sizes or larger for device local memory access.

Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
---
 drivers/gpu/drm/i915/i915_drv.h          | 10 +++++++---
 drivers/gpu/drm/i915/i915_pci.c          |  2 ++
 drivers/gpu/drm/i915/intel_device_info.h |  1 +
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 890f1f6fbc49..23f4713005bb 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1512,12 +1512,16 @@ IS_SUBPLATFORM(const struct drm_i915_private *i915,
 
 /*
  * Set this flag, when platform requires 64K GTT page sizes or larger for
- * device local memory access. Also this flag implies that we require or
- * at least support the compact PT layout for the ppGTT when using the 64K
- * GTT pages.
+ * device local memory access.
  */
 #define HAS_64K_PAGES(dev_priv) (INTEL_INFO(dev_priv)->has_64k_pages)
 
+/* Set this flag when platform doesn't allow both 64k pages and 4k pages in
+ * the same PT. this flag means we need to support compact PT layout for the
+ * ppGTT when using the 64K GTT pages.
+ */
+#define NEEDS_COMPACT_PT(dev_priv) (INTEL_INFO(dev_priv)->needs_compact_pt)
+
 #define HAS_IPC(dev_priv)		 (INTEL_INFO(dev_priv)->display.has_ipc)
 
 #define HAS_REGION(i915, i) (INTEL_INFO(i915)->memory_regions & (i))
diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
index 4081fd50ba9d..799b56569ef5 100644
--- a/drivers/gpu/drm/i915/i915_pci.c
+++ b/drivers/gpu/drm/i915/i915_pci.c
@@ -1028,6 +1028,7 @@ static const struct intel_device_info xehpsdv_info = {
 	PLATFORM(INTEL_XEHPSDV),
 	.display = { },
 	.has_64k_pages = 1,
+	.needs_compact_pt = 1,
 	.platform_engine_mask =
 		BIT(RCS0) | BIT(BCS0) |
 		BIT(VECS0) | BIT(VECS1) | BIT(VECS2) | BIT(VECS3) |
@@ -1045,6 +1046,7 @@ static const struct intel_device_info dg2_info = {
 	.media.rel = 55,
 	PLATFORM(INTEL_DG2),
 	.has_64k_pages = 1,
+	.needs_compact_pt = 1,
 	.platform_engine_mask =
 		BIT(RCS0) | BIT(BCS0) |
 		BIT(VECS0) | BIT(VECS1) |
diff --git a/drivers/gpu/drm/i915/intel_device_info.h b/drivers/gpu/drm/i915/intel_device_info.h
index 3699b1c539ea..c8aaf646430c 100644
--- a/drivers/gpu/drm/i915/intel_device_info.h
+++ b/drivers/gpu/drm/i915/intel_device_info.h
@@ -130,6 +130,7 @@ enum intel_ppgtt_type {
 	/* Keep has_* in alphabetical order */ \
 	func(has_64bit_reloc); \
 	func(has_64k_pages); \
+	func(needs_compact_pt); \
 	func(gpu_reset_clobbers_display); \
 	func(has_reset_engine); \
 	func(has_global_mocs); \
-- 
2.25.1

[PATCH v3 2/5] drm/i915: enforce min GTT alignment for discrete cards

[Robert Beckett]

From: Matthew Auld <matthew.auld@intel.com>

For local-memory objects we need to align the GTT addresses
to 64K, both for the ppgtt and ggtt.

We need to support vm->min_alignment > 4K, depending
on the vm itself and the type of object we are inserting.
With this in mind update the GTT selftests to take this
into account.

For compact-pt we further align and pad lmem object GTT addresses
to 2MB to ensure PDEs contain consistent page sizes as
required by the HW.

v3:
	* use needs_compact_pt flag to discriminate between
	  64K and 64K with compact-pt
	* add i915_vm_obj_min_alignment
	* use i915_vm_obj_min_alignment to round up vma reservation
	  if compact-pt instead of hard coding

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
---
 .../i915/gem/selftests/i915_gem_client_blt.c  | 23 +++--
 drivers/gpu/drm/i915/gt/intel_gtt.c           | 12 +++
 drivers/gpu/drm/i915/gt/intel_gtt.h           | 15 +++
 drivers/gpu/drm/i915/i915_vma.c               |  9 ++
 drivers/gpu/drm/i915/selftests/i915_gem_gtt.c | 96 ++++++++++++-------
 5 files changed, 114 insertions(+), 41 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c
index c08f766e6e15..7fee95a65414 100644
--- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c
+++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c
@@ -39,6 +39,7 @@ struct tiled_blits {
 	struct blit_buffer scratch;
 	struct i915_vma *batch;
 	u64 hole;
+	u64 align;
 	u32 width;
 	u32 height;
 };
@@ -410,14 +411,21 @@ tiled_blits_create(struct intel_engine_cs *engine, struct rnd_state *prng)
 		goto err_free;
 	}
 
-	hole_size = 2 * PAGE_ALIGN(WIDTH * HEIGHT * 4);
+	t->align = I915_GTT_PAGE_SIZE_2M; /* XXX worst case, derive from vm! */
+	t->align = max(t->align,
+		       i915_vm_min_alignment(t->ce->vm, INTEL_MEMORY_LOCAL));
+	t->align = max(t->align,
+		       i915_vm_min_alignment(t->ce->vm, INTEL_MEMORY_SYSTEM));
+
+	hole_size = 2 * round_up(WIDTH * HEIGHT * 4, t->align);
 	hole_size *= 2; /* room to maneuver */
-	hole_size += 2 * I915_GTT_MIN_ALIGNMENT;
+	hole_size += 2 * t->align; /* padding on either side */
 
 	mutex_lock(&t->ce->vm->mutex);
 	memset(&hole, 0, sizeof(hole));
 	err = drm_mm_insert_node_in_range(&t->ce->vm->mm, &hole,
-					  hole_size, 0, I915_COLOR_UNEVICTABLE,
+					  hole_size, t->align,
+					  I915_COLOR_UNEVICTABLE,
 					  0, U64_MAX,
 					  DRM_MM_INSERT_BEST);
 	if (!err)
@@ -428,7 +436,7 @@ tiled_blits_create(struct intel_engine_cs *engine, struct rnd_state *prng)
 		goto err_put;
 	}
 
-	t->hole = hole.start + I915_GTT_MIN_ALIGNMENT;
+	t->hole = hole.start + t->align;
 	pr_info("Using hole at %llx\n", t->hole);
 
 	err = tiled_blits_create_buffers(t, WIDTH, HEIGHT, prng);
@@ -455,7 +463,7 @@ static void tiled_blits_destroy(struct tiled_blits *t)
 static int tiled_blits_prepare(struct tiled_blits *t,
 			       struct rnd_state *prng)
 {
-	u64 offset = PAGE_ALIGN(t->width * t->height * 4);
+	u64 offset = round_up(t->width * t->height * 4, t->align);
 	u32 *map;
 	int err;
 	int i;
@@ -486,8 +494,7 @@ static int tiled_blits_prepare(struct tiled_blits *t,
 
 static int tiled_blits_bounce(struct tiled_blits *t, struct rnd_state *prng)
 {
-	u64 offset =
-		round_up(t->width * t->height * 4, 2 * I915_GTT_MIN_ALIGNMENT);
+	u64 offset = round_up(t->width * t->height * 4, 2 * t->align);
 	int err;
 
 	/* We want to check position invariant tiling across GTT eviction */
@@ -500,7 +507,7 @@ static int tiled_blits_bounce(struct tiled_blits *t, struct rnd_state *prng)
 
 	/* Reposition so that we overlap the old addresses, and slightly off */
 	err = tiled_blit(t,
-			 &t->buffers[2], t->hole + I915_GTT_MIN_ALIGNMENT,
+			 &t->buffers[2], t->hole + t->align,
 			 &t->buffers[1], t->hole + 3 * offset / 2);
 	if (err)
 		return err;
diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.c b/drivers/gpu/drm/i915/gt/intel_gtt.c
index 46be4197b93f..df23ebdfc994 100644
--- a/drivers/gpu/drm/i915/gt/intel_gtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_gtt.c
@@ -223,6 +223,18 @@ void i915_address_space_init(struct i915_address_space *vm, int subclass)
 
 	GEM_BUG_ON(!vm->total);
 	drm_mm_init(&vm->mm, 0, vm->total);
+
+	memset64(vm->min_alignment, I915_GTT_MIN_ALIGNMENT,
+		 ARRAY_SIZE(vm->min_alignment));
+
+	if (HAS_64K_PAGES(vm->i915) && NEEDS_COMPACT_PT(vm->i915)) {
+		vm->min_alignment[INTEL_MEMORY_LOCAL] = I915_GTT_PAGE_SIZE_2M;
+		vm->min_alignment[INTEL_MEMORY_STOLEN_LOCAL] = I915_GTT_PAGE_SIZE_2M;
+	} else if (HAS_64K_PAGES(vm->i915)) {
+		vm->min_alignment[INTEL_MEMORY_LOCAL] = I915_GTT_PAGE_SIZE_64K;
+		vm->min_alignment[INTEL_MEMORY_STOLEN_LOCAL] = I915_GTT_PAGE_SIZE_64K;
+	}
+
 	vm->mm.head_node.color = I915_COLOR_UNEVICTABLE;
 
 	INIT_LIST_HEAD(&vm->bound_list);
diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.h b/drivers/gpu/drm/i915/gt/intel_gtt.h
index 8073438b67c8..1519c6a689b0 100644
--- a/drivers/gpu/drm/i915/gt/intel_gtt.h
+++ b/drivers/gpu/drm/i915/gt/intel_gtt.h
@@ -29,6 +29,8 @@
 #include "i915_selftest.h"
 #include "i915_vma_resource.h"
 #include "i915_vma_types.h"
+#include "i915_params.h"
+#include "intel_memory_region.h"
 
 #define I915_GFP_ALLOW_FAIL (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN)
 
@@ -223,6 +225,7 @@ struct i915_address_space {
 	struct device *dma;
 	u64 total;		/* size addr space maps (ex. 2GB for ggtt) */
 	u64 reserved;		/* size addr space reserved */
+	u64 min_alignment[INTEL_MEMORY_STOLEN_LOCAL + 1];
 
 	unsigned int bind_async_flags;
 
@@ -384,6 +387,18 @@ i915_vm_has_scratch_64K(struct i915_address_space *vm)
 	return vm->scratch_order == get_order(I915_GTT_PAGE_SIZE_64K);
 }
 
+static inline u64 i915_vm_min_alignment(struct i915_address_space *vm,
+					enum intel_memory_type type)
+{
+	return vm->min_alignment[type];
+}
+
+static inline u64 i915_vm_obj_min_alignment(struct i915_address_space *vm,
+					    struct drm_i915_gem_object  *obj)
+{
+	return i915_vm_min_alignment(vm, READ_ONCE(obj->mm.region)->type);
+}
+
 static inline bool
 i915_vm_has_cache_coloring(struct i915_address_space *vm)
 {
diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 1f15c3298112..4a3dc721cf40 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -744,6 +744,14 @@ i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 		end = min_t(u64, end, (1ULL << 32) - I915_GTT_PAGE_SIZE);
 	GEM_BUG_ON(!IS_ALIGNED(end, I915_GTT_PAGE_SIZE));
 
+	alignment = max(alignment, i915_vm_obj_min_alignment(vma->vm, vma->obj));
+	/*
+	 * for compact-pt we round up the reservation to prevent
+	 * any smaller pages being used within the same PDE
+	 */
+	if (NEEDS_COMPACT_PT(vma->vm->i915))
+		size = round_up(size, alignment);
+
 	/* If binding the object/GGTT view requires more space than the entire
 	 * aperture has, reject it early before evicting everything in a vain
 	 * attempt to find space.
@@ -756,6 +764,7 @@ i915_vma_insert(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	}
 
 	color = 0;
+
 	if (i915_vm_has_cache_coloring(vma->vm))
 		color = vma->obj->cache_level;
 
diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
index 076d860ce01a..2f3f0c01786b 100644
--- a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
@@ -238,6 +238,8 @@ static int lowlevel_hole(struct i915_address_space *vm,
 			 u64 hole_start, u64 hole_end,
 			 unsigned long end_time)
 {
+	const unsigned int min_alignment =
+		i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
 	I915_RND_STATE(seed_prng);
 	struct i915_vma_resource *mock_vma_res;
 	unsigned int size;
@@ -251,9 +253,10 @@ static int lowlevel_hole(struct i915_address_space *vm,
 		I915_RND_SUBSTATE(prng, seed_prng);
 		struct drm_i915_gem_object *obj;
 		unsigned int *order, count, n;
-		u64 hole_size;
+		u64 hole_size, aligned_size;
 
-		hole_size = (hole_end - hole_start) >> size;
+		aligned_size = max_t(u32, ilog2(min_alignment), size);
+		hole_size = (hole_end - hole_start) >> aligned_size;
 		if (hole_size > KMALLOC_MAX_SIZE / sizeof(u32))
 			hole_size = KMALLOC_MAX_SIZE / sizeof(u32);
 		count = hole_size >> 1;
@@ -274,8 +277,8 @@ static int lowlevel_hole(struct i915_address_space *vm,
 		}
 		GEM_BUG_ON(!order);
 
-		GEM_BUG_ON(count * BIT_ULL(size) > vm->total);
-		GEM_BUG_ON(hole_start + count * BIT_ULL(size) > hole_end);
+		GEM_BUG_ON(count * BIT_ULL(aligned_size) > vm->total);
+		GEM_BUG_ON(hole_start + count * BIT_ULL(aligned_size) > hole_end);
 
 		/* Ignore allocation failures (i.e. don't report them as
 		 * a test failure) as we are purposefully allocating very
@@ -298,10 +301,10 @@ static int lowlevel_hole(struct i915_address_space *vm,
 		}
 
 		for (n = 0; n < count; n++) {
-			u64 addr = hole_start + order[n] * BIT_ULL(size);
+			u64 addr = hole_start + order[n] * BIT_ULL(aligned_size);
 			intel_wakeref_t wakeref;
 
-			GEM_BUG_ON(addr + BIT_ULL(size) > vm->total);
+			GEM_BUG_ON(addr + BIT_ULL(aligned_size) > vm->total);
 
 			if (igt_timeout(end_time,
 					"%s timed out before %d/%d\n",
@@ -344,7 +347,7 @@ static int lowlevel_hole(struct i915_address_space *vm,
 			}
 
 			mock_vma_res->bi.pages = obj->mm.pages;
-			mock_vma_res->node_size = BIT_ULL(size);
+			mock_vma_res->node_size = BIT_ULL(aligned_size);
 			mock_vma_res->start = addr;
 
 			with_intel_runtime_pm(vm->gt->uncore->rpm, wakeref)
@@ -355,7 +358,7 @@ static int lowlevel_hole(struct i915_address_space *vm,
 
 		i915_random_reorder(order, count, &prng);
 		for (n = 0; n < count; n++) {
-			u64 addr = hole_start + order[n] * BIT_ULL(size);
+			u64 addr = hole_start + order[n] * BIT_ULL(aligned_size);
 			intel_wakeref_t wakeref;
 
 			GEM_BUG_ON(addr + BIT_ULL(size) > vm->total);
@@ -399,8 +402,10 @@ static int fill_hole(struct i915_address_space *vm,
 {
 	const u64 hole_size = hole_end - hole_start;
 	struct drm_i915_gem_object *obj;
+	const unsigned int min_alignment =
+		i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
 	const unsigned long max_pages =
-		min_t(u64, ULONG_MAX - 1, hole_size/2 >> PAGE_SHIFT);
+		min_t(u64, ULONG_MAX - 1, (hole_size / 2) >> ilog2(min_alignment));
 	const unsigned long max_step = max(int_sqrt(max_pages), 2UL);
 	unsigned long npages, prime, flags;
 	struct i915_vma *vma;
@@ -441,14 +446,17 @@ static int fill_hole(struct i915_address_space *vm,
 
 				offset = p->offset;
 				list_for_each_entry(obj, &objects, st_link) {
+					u64 aligned_size = round_up(obj->base.size,
+								    min_alignment);
+
 					vma = i915_vma_instance(obj, vm, NULL);
 					if (IS_ERR(vma))
 						continue;
 
 					if (p->step < 0) {
-						if (offset < hole_start + obj->base.size)
+						if (offset < hole_start + aligned_size)
 							break;
-						offset -= obj->base.size;
+						offset -= aligned_size;
 					}
 
 					err = i915_vma_pin(vma, 0, 0, offset | flags);
@@ -470,22 +478,25 @@ static int fill_hole(struct i915_address_space *vm,
 					i915_vma_unpin(vma);
 
 					if (p->step > 0) {
-						if (offset + obj->base.size > hole_end)
+						if (offset + aligned_size > hole_end)
 							break;
-						offset += obj->base.size;
+						offset += aligned_size;
 					}
 				}
 
 				offset = p->offset;
 				list_for_each_entry(obj, &objects, st_link) {
+					u64 aligned_size = round_up(obj->base.size,
+								    min_alignment);
+
 					vma = i915_vma_instance(obj, vm, NULL);
 					if (IS_ERR(vma))
 						continue;
 
 					if (p->step < 0) {
-						if (offset < hole_start + obj->base.size)
+						if (offset < hole_start + aligned_size)
 							break;
-						offset -= obj->base.size;
+						offset -= aligned_size;
 					}
 
 					if (!drm_mm_node_allocated(&vma->node) ||
@@ -506,22 +517,25 @@ static int fill_hole(struct i915_address_space *vm,
 					}
 
 					if (p->step > 0) {
-						if (offset + obj->base.size > hole_end)
+						if (offset + aligned_size > hole_end)
 							break;
-						offset += obj->base.size;
+						offset += aligned_size;
 					}
 				}
 
 				offset = p->offset;
 				list_for_each_entry_reverse(obj, &objects, st_link) {
+					u64 aligned_size = round_up(obj->base.size,
+								    min_alignment);
+
 					vma = i915_vma_instance(obj, vm, NULL);
 					if (IS_ERR(vma))
 						continue;
 
 					if (p->step < 0) {
-						if (offset < hole_start + obj->base.size)
+						if (offset < hole_start + aligned_size)
 							break;
-						offset -= obj->base.size;
+						offset -= aligned_size;
 					}
 
 					err = i915_vma_pin(vma, 0, 0, offset | flags);
@@ -543,22 +557,25 @@ static int fill_hole(struct i915_address_space *vm,
 					i915_vma_unpin(vma);
 
 					if (p->step > 0) {
-						if (offset + obj->base.size > hole_end)
+						if (offset + aligned_size > hole_end)
 							break;
-						offset += obj->base.size;
+						offset += aligned_size;
 					}
 				}
 
 				offset = p->offset;
 				list_for_each_entry_reverse(obj, &objects, st_link) {
+					u64 aligned_size = round_up(obj->base.size,
+								    min_alignment);
+
 					vma = i915_vma_instance(obj, vm, NULL);
 					if (IS_ERR(vma))
 						continue;
 
 					if (p->step < 0) {
-						if (offset < hole_start + obj->base.size)
+						if (offset < hole_start + aligned_size)
 							break;
-						offset -= obj->base.size;
+						offset -= aligned_size;
 					}
 
 					if (!drm_mm_node_allocated(&vma->node) ||
@@ -579,9 +596,9 @@ static int fill_hole(struct i915_address_space *vm,
 					}
 
 					if (p->step > 0) {
-						if (offset + obj->base.size > hole_end)
+						if (offset + aligned_size > hole_end)
 							break;
-						offset += obj->base.size;
+						offset += aligned_size;
 					}
 				}
 			}
@@ -611,6 +628,7 @@ static int walk_hole(struct i915_address_space *vm,
 	const u64 hole_size = hole_end - hole_start;
 	const unsigned long max_pages =
 		min_t(u64, ULONG_MAX - 1, hole_size >> PAGE_SHIFT);
+	unsigned long min_alignment;
 	unsigned long flags;
 	u64 size;
 
@@ -620,6 +638,8 @@ static int walk_hole(struct i915_address_space *vm,
 	if (i915_is_ggtt(vm))
 		flags |= PIN_GLOBAL;
 
+	min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
+
 	for_each_prime_number_from(size, 1, max_pages) {
 		struct drm_i915_gem_object *obj;
 		struct i915_vma *vma;
@@ -638,7 +658,7 @@ static int walk_hole(struct i915_address_space *vm,
 
 		for (addr = hole_start;
 		     addr + obj->base.size < hole_end;
-		     addr += obj->base.size) {
+		     addr += round_up(obj->base.size, min_alignment)) {
 			err = i915_vma_pin(vma, 0, 0, addr | flags);
 			if (err) {
 				pr_err("%s bind failed at %llx + %llx [hole %llx- %llx] with err=%d\n",
@@ -690,6 +710,7 @@ static int pot_hole(struct i915_address_space *vm,
 {
 	struct drm_i915_gem_object *obj;
 	struct i915_vma *vma;
+	unsigned int min_alignment;
 	unsigned long flags;
 	unsigned int pot;
 	int err = 0;
@@ -698,6 +719,8 @@ static int pot_hole(struct i915_address_space *vm,
 	if (i915_is_ggtt(vm))
 		flags |= PIN_GLOBAL;
 
+	min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
+
 	obj = i915_gem_object_create_internal(vm->i915, 2 * I915_GTT_PAGE_SIZE);
 	if (IS_ERR(obj))
 		return PTR_ERR(obj);
@@ -710,13 +733,13 @@ static int pot_hole(struct i915_address_space *vm,
 
 	/* Insert a pair of pages across every pot boundary within the hole */
 	for (pot = fls64(hole_end - 1) - 1;
-	     pot > ilog2(2 * I915_GTT_PAGE_SIZE);
+	     pot > ilog2(2 * min_alignment);
 	     pot--) {
 		u64 step = BIT_ULL(pot);
 		u64 addr;
 
-		for (addr = round_up(hole_start + I915_GTT_PAGE_SIZE, step) - I915_GTT_PAGE_SIZE;
-		     addr <= round_down(hole_end - 2*I915_GTT_PAGE_SIZE, step) - I915_GTT_PAGE_SIZE;
+		for (addr = round_up(hole_start + min_alignment, step) - min_alignment;
+		     addr <= round_down(hole_end - (2 * min_alignment), step) - min_alignment;
 		     addr += step) {
 			err = i915_vma_pin(vma, 0, 0, addr | flags);
 			if (err) {
@@ -761,6 +784,7 @@ static int drunk_hole(struct i915_address_space *vm,
 		      unsigned long end_time)
 {
 	I915_RND_STATE(prng);
+	unsigned int min_alignment;
 	unsigned int size;
 	unsigned long flags;
 
@@ -768,15 +792,18 @@ static int drunk_hole(struct i915_address_space *vm,
 	if (i915_is_ggtt(vm))
 		flags |= PIN_GLOBAL;
 
+	min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
+
 	/* Keep creating larger objects until one cannot fit into the hole */
 	for (size = 12; (hole_end - hole_start) >> size; size++) {
 		struct drm_i915_gem_object *obj;
 		unsigned int *order, count, n;
 		struct i915_vma *vma;
-		u64 hole_size;
+		u64 hole_size, aligned_size;
 		int err = -ENODEV;
 
-		hole_size = (hole_end - hole_start) >> size;
+		aligned_size = max_t(u32, ilog2(min_alignment), size);
+		hole_size = (hole_end - hole_start) >> aligned_size;
 		if (hole_size > KMALLOC_MAX_SIZE / sizeof(u32))
 			hole_size = KMALLOC_MAX_SIZE / sizeof(u32);
 		count = hole_size >> 1;
@@ -816,7 +843,7 @@ static int drunk_hole(struct i915_address_space *vm,
 		GEM_BUG_ON(vma->size != BIT_ULL(size));
 
 		for (n = 0; n < count; n++) {
-			u64 addr = hole_start + order[n] * BIT_ULL(size);
+			u64 addr = hole_start + order[n] * BIT_ULL(aligned_size);
 
 			err = i915_vma_pin(vma, 0, 0, addr | flags);
 			if (err) {
@@ -868,11 +895,14 @@ static int __shrink_hole(struct i915_address_space *vm,
 {
 	struct drm_i915_gem_object *obj;
 	unsigned long flags = PIN_OFFSET_FIXED | PIN_USER;
+	unsigned int min_alignment;
 	unsigned int order = 12;
 	LIST_HEAD(objects);
 	int err = 0;
 	u64 addr;
 
+	min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM);
+
 	/* Keep creating larger objects until one cannot fit into the hole */
 	for (addr = hole_start; addr < hole_end; ) {
 		struct i915_vma *vma;
@@ -913,7 +943,7 @@ static int __shrink_hole(struct i915_address_space *vm,
 		}
 
 		i915_vma_unpin(vma);
-		addr += size;
+		addr += round_up(size, min_alignment);
 
 		/*
 		 * Since we are injecting allocation faults at random intervals,
-- 
2.25.1

[PATCH v3 3/5] drm/i915: support 64K GTT pages for discrete cards

[Robert Beckett]

From: Matthew Auld <matthew.auld@intel.com>

discrete cards optimise 64K GTT pages for local-memory, since everything
should be allocated at 64K granularity. We say goodbye to sparse
entries, and instead get a compact 256B page-table for 64K pages,
which should be more cache friendly. 4K pages for local-memory
are no longer supported by the HW.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
---
 .../gpu/drm/i915/gem/selftests/huge_pages.c   |  60 ++++++++++
 drivers/gpu/drm/i915/gt/gen8_ppgtt.c          | 108 +++++++++++++++++-
 drivers/gpu/drm/i915/gt/intel_gtt.h           |   3 +
 drivers/gpu/drm/i915/gt/intel_ppgtt.c         |   1 +
 4 files changed, 169 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
index 26f997c376a2..7efa6a598b03 100644
--- a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
+++ b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
@@ -1478,6 +1478,65 @@ static int igt_ppgtt_sanity_check(void *arg)
 	return err;
 }
 
+static int igt_ppgtt_compact(void *arg)
+{
+	struct drm_i915_private *i915 = arg;
+	struct drm_i915_gem_object *obj;
+	int err;
+
+	/*
+	 * Simple test to catch issues with compact 64K pages -- since the pt is
+	 * compacted to 256B that gives us 32 entries per pt, however since the
+	 * backing page for the pt is 4K, any extra entries we might incorrectly
+	 * write out should be ignored by the HW. If ever hit such a case this
+	 * test should catch it since some of our writes would land in scratch.
+	 */
+
+	if (!HAS_64K_PAGES(i915)) {
+		pr_info("device lacks compact 64K page support, skipping\n");
+		return 0;
+	}
+
+	if (!HAS_LMEM(i915)) {
+		pr_info("device lacks LMEM support, skipping\n");
+		return 0;
+	}
+
+	/* We want the range to cover multiple page-table boundaries. */
+	obj = i915_gem_object_create_lmem(i915, SZ_4M, 0);
+	if (IS_ERR(obj))
+		return err;
+
+	err = i915_gem_object_pin_pages_unlocked(obj);
+	if (err)
+		goto out_put;
+
+	if (obj->mm.page_sizes.phys < I915_GTT_PAGE_SIZE_64K) {
+		pr_info("LMEM compact unable to allocate huge-page(s)\n");
+		goto out_unpin;
+	}
+
+	/*
+	 * Disable 2M GTT pages by forcing the page-size to 64K for the GTT
+	 * insertion.
+	 */
+	obj->mm.page_sizes.sg = I915_GTT_PAGE_SIZE_64K;
+
+	err = igt_write_huge(i915, obj);
+	if (err)
+		pr_err("LMEM compact write-huge failed\n");
+
+out_unpin:
+	i915_gem_object_unpin_pages(obj);
+out_put:
+	i915_gem_object_put(obj);
+
+	if (err == -ENOMEM)
+		err = 0;
+
+	return err;
+}
+
 static int igt_tmpfs_fallback(void *arg)
 {
 	struct drm_i915_private *i915 = arg;
@@ -1735,6 +1794,7 @@ int i915_gem_huge_page_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(igt_tmpfs_fallback),
 		SUBTEST(igt_ppgtt_smoke_huge),
 		SUBTEST(igt_ppgtt_sanity_check),
+		SUBTEST(igt_ppgtt_compact),
 	};
 
 	if (!HAS_PPGTT(i915)) {
diff --git a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
index c43e724afa9f..62471730266c 100644
--- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
+++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
@@ -233,6 +233,8 @@ static u64 __gen8_ppgtt_clear(struct i915_address_space * const vm,
 						   start, end, lvl);
 		} else {
 			unsigned int count;
+			unsigned int pte = gen8_pd_index(start, 0);
+			unsigned int num_ptes;
 			u64 *vaddr;
 
 			count = gen8_pt_count(start, end);
@@ -242,10 +244,18 @@ static u64 __gen8_ppgtt_clear(struct i915_address_space * const vm,
 			    atomic_read(&pt->used));
 			GEM_BUG_ON(!count || count >= atomic_read(&pt->used));
 
+			num_ptes = count;
+			if (pt->is_compact) {
+				GEM_BUG_ON(num_ptes % 16);
+				GEM_BUG_ON(pte % 16);
+				num_ptes /= 16;
+				pte /= 16;
+			}
+
 			vaddr = px_vaddr(pt);
-			memset64(vaddr + gen8_pd_index(start, 0),
+			memset64(vaddr + pte,
 				 vm->scratch[0]->encode,
-				 count);
+				 num_ptes);
 
 			atomic_sub(count, &pt->used);
 			start += count;
@@ -453,6 +463,95 @@ gen8_ppgtt_insert_pte(struct i915_ppgtt *ppgtt,
 	return idx;
 }
 
+static void
+xehpsdv_ppgtt_insert_huge(struct i915_address_space *vm,
+			  struct i915_vma_resource *vma_res,
+			  struct sgt_dma *iter,
+			  enum i915_cache_level cache_level,
+			  u32 flags)
+{
+	const gen8_pte_t pte_encode = vm->pte_encode(0, cache_level, flags);
+	unsigned int rem = sg_dma_len(iter->sg);
+	u64 start = vma_res->start;
+
+	GEM_BUG_ON(!i915_vm_is_4lvl(vm));
+
+	do {
+		struct i915_page_directory * const pdp =
+			gen8_pdp_for_page_address(vm, start);
+		struct i915_page_directory * const pd =
+			i915_pd_entry(pdp, __gen8_pte_index(start, 2));
+		struct i915_page_table *pt =
+			i915_pt_entry(pd, __gen8_pte_index(start, 1));
+		gen8_pte_t encode = pte_encode;
+		unsigned int page_size;
+		gen8_pte_t *vaddr;
+		u16 index, max;
+
+		max = I915_PDES;
+
+		if (vma_res->bi.page_sizes.sg & I915_GTT_PAGE_SIZE_2M &&
+		    IS_ALIGNED(iter->dma, I915_GTT_PAGE_SIZE_2M) &&
+		    rem >= I915_GTT_PAGE_SIZE_2M &&
+		    !__gen8_pte_index(start, 0)) {
+			index = __gen8_pte_index(start, 1);
+			encode |= GEN8_PDE_PS_2M;
+			page_size = I915_GTT_PAGE_SIZE_2M;
+
+			vaddr = px_vaddr(pd);
+		} else {
+			if (encode & GEN12_PPGTT_PTE_LM) {
+				GEM_BUG_ON(__gen8_pte_index(start, 0) % 16);
+				GEM_BUG_ON(rem < I915_GTT_PAGE_SIZE_64K);
+				GEM_BUG_ON(!IS_ALIGNED(iter->dma,
+						       I915_GTT_PAGE_SIZE_64K));
+
+				index = __gen8_pte_index(start, 0) / 16;
+				page_size = I915_GTT_PAGE_SIZE_64K;
+
+				max /= 16;
+
+				vaddr = px_vaddr(pd);
+				vaddr[__gen8_pte_index(start, 1)] |= GEN12_PDE_64K;
+
+				pt->is_compact = true;
+			} else {
+				GEM_BUG_ON(pt->is_compact);
+				index =  __gen8_pte_index(start, 0);
+				page_size = I915_GTT_PAGE_SIZE;
+			}
+
+			vaddr = px_vaddr(pt);
+		}
+
+		do {
+			GEM_BUG_ON(rem < page_size);
+			vaddr[index++] = encode | iter->dma;
+
+			start += page_size;
+			iter->dma += page_size;
+			rem -= page_size;
+			if (iter->dma >= iter->max) {
+				iter->sg = __sg_next(iter->sg);
+				if (!iter->sg)
+					break;
+
+				rem = sg_dma_len(iter->sg);
+				if (!rem)
+					break;
+
+				iter->dma = sg_dma_address(iter->sg);
+				iter->max = iter->dma + rem;
+
+				if (unlikely(!IS_ALIGNED(iter->dma, page_size)))
+					break;
+			}
+		} while (rem >= page_size && index < max);
+
+		vma_res->page_sizes_gtt |= page_size;
+	} while (iter->sg && sg_dma_len(iter->sg));
+}
+
 static void gen8_ppgtt_insert_huge(struct i915_address_space *vm,
 				   struct i915_vma_resource *vma_res,
 				   struct sgt_dma *iter,
@@ -586,7 +685,10 @@ static void gen8_ppgtt_insert(struct i915_address_space *vm,
 	struct sgt_dma iter = sgt_dma(vma_res);
 
 	if (vma_res->bi.page_sizes.sg > I915_GTT_PAGE_SIZE) {
-		gen8_ppgtt_insert_huge(vm, vma_res, &iter, cache_level, flags);
+		if (HAS_64K_PAGES(vm->i915))
+			xehpsdv_ppgtt_insert_huge(vm, vma_res, &iter, cache_level, flags);
+		else
+			gen8_ppgtt_insert_huge(vm, vma_res, &iter, cache_level, flags);
 	} else  {
 		u64 idx = vma_res->start >> GEN8_PTE_SHIFT;
 
diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.h b/drivers/gpu/drm/i915/gt/intel_gtt.h
index 1519c6a689b0..c306bfebd998 100644
--- a/drivers/gpu/drm/i915/gt/intel_gtt.h
+++ b/drivers/gpu/drm/i915/gt/intel_gtt.h
@@ -92,6 +92,8 @@ typedef u64 gen8_pte_t;
 
 #define GEN12_GGTT_PTE_LM	BIT_ULL(1)
 
+#define GEN12_PDE_64K BIT(6)
+
 /*
  * Cacheability Control is a 4-bit value. The low three bits are stored in bits
  * 3:1 of the PTE, while the fourth bit is stored in bit 11 of the PTE.
@@ -160,6 +162,7 @@ struct i915_page_table {
 		atomic_t used;
 		struct i915_page_table *stash;
 	};
+	bool is_compact;
 };
 
 struct i915_page_directory {
diff --git a/drivers/gpu/drm/i915/gt/intel_ppgtt.c b/drivers/gpu/drm/i915/gt/intel_ppgtt.c
index 48e6e2f87700..043652dc6892 100644
--- a/drivers/gpu/drm/i915/gt/intel_ppgtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_ppgtt.c
@@ -26,6 +26,7 @@ struct i915_page_table *alloc_pt(struct i915_address_space *vm)
 		return ERR_PTR(-ENOMEM);
 	}
 
+	pt->is_compact = false;
 	atomic_set(&pt->used, 0);
 	return pt;
 }
-- 
2.25.1

[PATCH v3 4/5] drm/i915: add gtt misalignment test

[Robert Beckett]

add test to check handling of misaligned offsets and sizes

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
---
 drivers/gpu/drm/i915/selftests/i915_gem_gtt.c | 130 ++++++++++++++++++
 1 file changed, 130 insertions(+)

diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
index 2f3f0c01786b..76696a5e547e 100644
--- a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c
@@ -22,10 +22,12 @@
  *
  */
 
+#include "gt/intel_gtt.h"
 #include <linux/list_sort.h>
 #include <linux/prime_numbers.h>
 
 #include "gem/i915_gem_context.h"
+#include "gem/i915_gem_region.h"
 #include "gem/selftests/mock_context.h"
 #include "gt/intel_context.h"
 #include "gt/intel_gpu_commands.h"
@@ -1067,6 +1069,120 @@ static int shrink_boom(struct i915_address_space *vm,
 	return err;
 }
 
+static int misaligned_case(struct i915_address_space *vm, struct intel_memory_region *mr,
+			   u64 addr, u64 size, unsigned long flags)
+{
+	struct drm_i915_gem_object *obj;
+	struct i915_vma *vma;
+	int err = 0;
+	u64 expected_vma_size, expected_node_size;
+
+	obj = i915_gem_object_create_region(mr, size, 0, 0);
+	if (IS_ERR(obj))
+		return PTR_ERR(obj);
+
+	vma = i915_vma_instance(obj, vm, NULL);
+	if (IS_ERR(vma)) {
+		err = PTR_ERR(vma);
+		goto err_put;
+	}
+
+	err = i915_vma_pin(vma, 0, 0, addr | flags);
+	if (err)
+		goto err_put;
+	i915_vma_unpin(vma);
+
+	if (!drm_mm_node_allocated(&vma->node)) {
+		err = -EINVAL;
+		goto err_put;
+	}
+
+	if (i915_vma_misplaced(vma, 0, 0, addr | flags)) {
+		err = -EINVAL;
+		goto err_put;
+	}
+
+	expected_vma_size = round_up(size, 1 << (ffs(vma->resource->page_sizes_gtt) - 1));
+	expected_node_size = expected_vma_size;
+
+	if (IS_DG2(vm->i915) && i915_gem_object_is_lmem(obj)) {
+		/* dg2 should expand lmem node to 2MB */
+		expected_vma_size = round_up(size, I915_GTT_PAGE_SIZE_64K);
+		expected_node_size = round_up(size, I915_GTT_PAGE_SIZE_2M);
+	}
+
+	if (vma->size != expected_vma_size || vma->node.size != expected_node_size) {
+		err = i915_vma_unbind(vma);
+		err = -EBADSLT;
+		goto err_put;
+	}
+
+	err = i915_vma_unbind(vma);
+	if (err)
+		goto err_put;
+
+	GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
+
+err_put:
+	i915_gem_object_put(obj);
+	cleanup_freed_objects(vm->i915);
+	return err;
+}
+
+static int misaligned_pin(struct i915_address_space *vm,
+			  u64 hole_start, u64 hole_end,
+			  unsigned long end_time)
+{
+	struct intel_memory_region *mr;
+	enum intel_region_id id;
+	unsigned long flags = PIN_OFFSET_FIXED | PIN_USER;
+	int err = 0;
+	u64 hole_size = hole_end - hole_start;
+
+	if (i915_is_ggtt(vm))
+		flags |= PIN_GLOBAL;
+
+	for_each_memory_region(mr, vm->i915, id) {
+		u64 min_alignment = i915_vm_min_alignment(vm, id);
+		u64 size = min_alignment;
+		u64 addr = round_up(hole_start + (hole_size / 2), min_alignment);
+
+		/* we can't test < 4k alignment due to flags being encoded in lower bits */
+		if (min_alignment != I915_GTT_PAGE_SIZE_4K) {
+			err = misaligned_case(vm, mr, addr + (min_alignment / 2), size, flags);
+			/* misaligned should error with -EINVAL*/
+			if (!err)
+				err = -EBADSLT;
+			if (err != -EINVAL)
+				return err;
+		}
+
+		/* test for vma->size expansion to min page size */
+		err = misaligned_case(vm, mr, addr, PAGE_SIZE, flags);
+		if (min_alignment > hole_size) {
+			if (!err)
+				err = -EBADSLT;
+			else if (err == -ENOSPC)
+				err = 0;
+		}
+		if (err)
+			return err;
+
+		/* test for intermediate size not expanding vma->size for large alignments */
+		err = misaligned_case(vm, mr, addr, size / 2, flags);
+		if (min_alignment > hole_size) {
+			if (!err)
+				err = -EBADSLT;
+			else if (err == -ENOSPC)
+				err = 0;
+		}
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 static int exercise_ppgtt(struct drm_i915_private *dev_priv,
 			  int (*func)(struct i915_address_space *vm,
 				      u64 hole_start, u64 hole_end,
@@ -1136,6 +1252,12 @@ static int igt_ppgtt_shrink_boom(void *arg)
 	return exercise_ppgtt(arg, shrink_boom);
 }
 
+static int igt_ppgtt_misaligned_pin(void *arg)
+{
+	return exercise_ppgtt(arg, misaligned_pin);
+}
+
+
 static int sort_holes(void *priv, const struct list_head *A,
 		      const struct list_head *B)
 {
@@ -1208,6 +1330,12 @@ static int igt_ggtt_lowlevel(void *arg)
 	return exercise_ggtt(arg, lowlevel_hole);
 }
 
+static int igt_ggtt_misaligned_pin(void *arg)
+{
+	return exercise_ggtt(arg, misaligned_pin);
+}
+
+
 static int igt_ggtt_page(void *arg)
 {
 	const unsigned int count = PAGE_SIZE/sizeof(u32);
@@ -2180,12 +2308,14 @@ int i915_gem_gtt_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(igt_ppgtt_fill),
 		SUBTEST(igt_ppgtt_shrink),
 		SUBTEST(igt_ppgtt_shrink_boom),
+		SUBTEST(igt_ppgtt_misaligned_pin),
 		SUBTEST(igt_ggtt_lowlevel),
 		SUBTEST(igt_ggtt_drunk),
 		SUBTEST(igt_ggtt_walk),
 		SUBTEST(igt_ggtt_pot),
 		SUBTEST(igt_ggtt_fill),
 		SUBTEST(igt_ggtt_page),
+		SUBTEST(igt_ggtt_misaligned_pin),
 		SUBTEST(igt_cs_tlb),
 	};
 
-- 
2.25.1

[PATCH v3 5/5] drm/i915/uapi: document behaviour for DG2 64K support

[Robert Beckett]

From: Matthew Auld <matthew.auld@intel.com>

On discrete platforms like DG2, we need to support a minimum page size
of 64K when dealing with device local-memory. This is quite tricky for
various reasons, so try to document the new implicit uapi for this.

v3: fix typos and less emphasis
v2: Fixed suggestions on formatting [Daniel]

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
cc: Simon Ser <contact@emersion.fr>
cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-dev@lists.freedesktop.org
Cc: Tony Ye <tony.ye@intel.com>
Cc: Slawomir Milczarek <slawomir.milczarek@intel.com>
---
 include/uapi/drm/i915_drm.h | 44 ++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 5e678917da70..77e5e74c32c1 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1118,10 +1118,16 @@ struct drm_i915_gem_exec_object2 {
 	/**
 	 * When the EXEC_OBJECT_PINNED flag is specified this is populated by
 	 * the user with the GTT offset at which this object will be pinned.
+	 *
 	 * When the I915_EXEC_NO_RELOC flag is specified this must contain the
 	 * presumed_offset of the object.
+	 *
 	 * During execbuffer2 the kernel populates it with the value of the
 	 * current GTT offset of the object, for future presumed_offset writes.
+	 *
+	 * See struct drm_i915_gem_create_ext for the rules when dealing with
+	 * alignment restrictions with I915_MEMORY_CLASS_DEVICE, on devices with
+	 * minimum page sizes, like DG2.
 	 */
 	__u64 offset;
 
@@ -3145,11 +3151,39 @@ struct drm_i915_gem_create_ext {
 	 *
 	 * The (page-aligned) allocated size for the object will be returned.
 	 *
-	 * Note that for some devices we have might have further minimum
-	 * page-size restrictions(larger than 4K), like for device local-memory.
-	 * However in general the final size here should always reflect any
-	 * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
-	 * extension to place the object in device local-memory.
+	 *
+	 * DG2 64K min page size implications:
+	 *
+	 * On discrete platforms, starting from DG2, we have to contend with GTT
+	 * page size restrictions when dealing with I915_MEMORY_CLASS_DEVICE
+	 * objects.  Specifically the hardware only supports 64K or larger GTT
+	 * page sizes for such memory. The kernel will already ensure that all
+	 * I915_MEMORY_CLASS_DEVICE memory is allocated using 64K or larger page
+	 * sizes underneath.
+	 *
+	 * Note that the returned size here will always reflect any required
+	 * rounding up done by the kernel, i.e 4K will now become 64K on devices
+	 * such as DG2.
+	 *
+	 * Special DG2 GTT address alignment requirement:
+	 *
+	 * The GTT alignment will also need to be at least 2M for such objects.
+	 *
+	 * Note that due to how the hardware implements 64K GTT page support, we
+	 * have some further complications:
+	 *
+	 *   1) The entire PDE (which covers a 2MB virtual address range), must
+	 *   contain only 64K PTEs, i.e mixing 4K and 64K PTEs in the same
+	 *   PDE is forbidden by the hardware.
+	 *
+	 *   2) We still need to support 4K PTEs for I915_MEMORY_CLASS_SYSTEM
+	 *   objects.
+	 *
+	 * To keep things simple for userland, we mandate that any GTT mappings
+	 * must be aligned to and rounded up to 2MB. As this only wastes virtual
+	 * address space and avoids userland having to copy any needlessly
+	 * complicated PDE sharing scheme (coloring) and only affects DG2, this
+	 * is deemed to be a good compromise.
 	 */
 	__u64 size;
 	/**
-- 
2.25.1