[PATCH 13/14] drm/i915/uapi: document behaviour for DG2 64K support

[Ramalingam C <ramalingam.c@intel.com>]

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.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
---
 include/uapi/drm/i915_drm.h | 61 ++++++++++++++++++++++++++++++++++---
 1 file changed, 56 insertions(+), 5 deletions(-)

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index aa2a7eccfb94..d62e8b7ed8b6 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;
 
@@ -3001,11 +3007,56 @@ 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.
+	 * 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. The GTT alignment will also need be at least 64K 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 2M 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 handle the above the kernel implements a memory coloring scheme to
+	 * prevent userspace from mixing I915_MEMORY_CLASS_DEVICE and
+	 * I915_MEMORY_CLASS_SYSTEM objects in the same PDE. If the kernel is
+	 * ever unable to evict the required pages for the given PDE(different
+	 * color) when inserting the object into the GTT then it will simply
+	 * fail the request.
+	 *
+	 * Since userspace needs to manage the GTT address space themselves,
+	 * special care is needed to ensure this doesn't happen. The simplest
+	 * scheme is to simply align and round up all I915_MEMORY_CLASS_DEVICE
+	 * objects to 2M, which avoids any issues here. At the very least this
+	 * is likely needed for objects that can be placed in both
+	 * I915_MEMORY_CLASS_DEVICE and I915_MEMORY_CLASS_SYSTEM, to avoid
+	 * potential issues when the kernel needs to migrate the object behind
+	 * the scenes, since that might also involve evicting other objects.
+	 *
+	 * To summarise the GTT rules, on platforms like DG2:
+	 *
+	 *   1.) All objects that can be placed in I915_MEMORY_CLASS_DEVICE must
+	 *   have 64K alignment. The kernel will reject this otherwise.
+	 *
+	 *   2.) All I915_MEMORY_CLASS_DEVICE objects must never be placed in
+	 *   the same PDE with other I915_MEMORY_CLASS_SYSTEM objects. The
+	 *   kernel will reject this otherwise.
+	 *
+	 *   3.) Objects that can be placed in both I915_MEMORY_CLASS_DEVICE and
+	 *   I915_MEMORY_CLASS_SYSTEM should probably be aligned and padded out
+	 *   to 2M.
 	 */
 	__u64 size;
 	/**
-- 
2.20.1