From: Matthew Auld <matthew.auld@xxxxxxxxx>
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.
v2: Fixed suggestions on formatting [Daniel]
Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
Signed-off-by: Ramalingam C <ramalingam.c@xxxxxxxxx>
cc: Simon Ser <contact@xxxxxxxxxxx>
cc: Pekka Paalanen <ppaalanen@xxxxxxxxx>
---
include/uapi/drm/i915_drm.h | 67 ++++++++++++++++++++++++++++++++++---
1 file changed, 62 insertions(+), 5 deletions(-)
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 914ebd9290e5..89bcf5a77958 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;
@@ -3144,11 +3150,62 @@ 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 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
