drm/i915/uapi: document behaviour for DG2 64K support

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.

v4: Kdoc modification.
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>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.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>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218184752.7524-13-ramalingam.c@intel.com

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;
 
@@ -3144,11 +3150,40 @@ 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. The kernel will internally
+	 * pad them out to the next 2MB boundary. 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;
 	/**