[PATCH v3] drm/doc: add rfc section for small BAR uapi - dri-devel - freedesktop.org experimental mailing list

16 May 2022

Add an entry for the new uapi needed for small BAR on DG2+.
v2:
  - Some spelling fixes and other small tweaks. (Akeem & Thomas)
  - Rework error capture interactions, including no longer needing
    NEEDS_CPU_ACCESS for objects marked for capture. (Thomas)
  - Add probed_cpu_visible_size. (Lionel)
v3:
  - Drop the vma query for now.
  - Add unallocated_cpu_visible_size as part of the region query.
  - Improve the docs some more, including documenting the expected
    behaviour on older kernels, since this came up in some offline
    discussion.
Signed-off-by: Matthew Auld matthew.auld@intel.com
Cc: Thomas Hellström thomas.hellstrom@linux.intel.com
Cc: Lionel Landwerlin lionel.g.landwerlin@intel.com
Cc: Tvrtko Ursulin tvrtko.ursulin@linux.intel.com
Cc: Jon Bloomfield jon.bloomfield@intel.com
Cc: Daniel Vetter daniel.vetter@ffwll.ch
Cc: Jon Bloomfield jon.bloomfield@intel.com
Cc: Jordan Justen jordan.l.justen@intel.com
Cc: Kenneth Graunke kenneth@whitecape.org
Cc: Akeem G Abodunrin akeem.g.abodunrin@intel.com
Cc: mesa-dev@lists.freedesktop.org
---
 Documentation/gpu/rfc/i915_small_bar.h   | 164 +++++++++++++++++++++++
 Documentation/gpu/rfc/i915_small_bar.rst |  47 +++++++
 Documentation/gpu/rfc/index.rst          |   4 +
 3 files changed, 215 insertions(+)
 create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
 create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst

diff --git a/Documentation/gpu/rfc/i915_small_bar.h b/Documentation/gpu/rfc/i915_small_bar.h
new file mode 100644
index 000000000000..4079d287750b
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.h
@@ -0,0 +1,164 @@
+/**
+ * struct __drm_i915_memory_region_info - Describes one region as known to the
+ * driver.
+ *
+ * Note this is using both struct drm_i915_query_item and struct drm_i915_query.
+ * For this new query we are adding the new query id DRM_I915_QUERY_MEMORY_REGIONS
+ * at &drm_i915_query_item.query_id.
+ */
+struct __drm_i915_memory_region_info {
+	/** @region: The class:instance pair encoding */
+	struct drm_i915_gem_memory_class_instance region;
+
+	/** @rsvd0: MBZ */
+	__u32 rsvd0;
+
+	/** @probed_size: Memory probed by the driver (-1 = unknown) */
+	__u64 probed_size;
+
+	/**
+	 * @unallocated_size: Estimate of memory remaining (-1 = unknown)
+	 *
+	 * Note this is only currently tracked for I915_MEMORY_CLASS_DEVICE
+	 * regions, and also requires CAP_PERFMON or CAP_SYS_ADMIN to get
+	 * reliable accounting. Without this(or if this an older kernel) the
+	 * value here will always match the @probed_size.
+	 */
+	__u64 unallocated_size;
+
+	union {
+		/** @rsvd1: MBZ */
+		__u64 rsvd1[8];
+		struct {
+			/**
+			 * @probed_cpu_visible_size: Memory probed by the driver
+			 * that is CPU accessible. (-1 = unknown).
+			 *
+			 * This will be always be <= @probed_size, and the
+			 * remainder(if there is any) will not be CPU
+			 * accessible.
+			 *
+			 * On systems without small BAR, the @probed_size will
+			 * always equal the @probed_cpu_visible_size, since all
+			 * of it will be CPU accessible.
+			 *
+			 * Note that if the value returned here is zero, then
+			 * this must be an old kernel which lacks the relevant
+			 * small-bar uAPI support(including
+			 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS), but on
+			 * such systems we should never actually end up with a
+			 * small BAR configuration, assuming we are able to load
+			 * the kernel module. Hence it should be safe to treat
+			 * this the same as when @probed_cpu_visible_size ==
+			 * @probed_size.
+			 */
+			__u64 probed_cpu_visible_size;
+
+			/**
+			 * @unallocated_cpu_visible_size: Estimate of CPU
+			 * visible memory remaining (-1 = unknown).
+			 *
+			 * Note this is only currently tracked for
+			 * I915_MEMORY_CLASS_DEVICE regions, and also requires
+			 * CAP_PERFMON or CAP_SYS_ADMIN to get reliable
+			 * accounting. Without this the value here will always
+			 * equal the @probed_cpu_visible_size.
+			 */
+			__u64 unallocated_cpu_visible_size;
+		};
+	};
+};
+
+/**
+ * struct __drm_i915_gem_create_ext - Existing gem_create behaviour, with added
+ * extension support using struct i915_user_extension.
+ *
+ * Note that new buffer flags should be added here, at least for the stuff that
+ * is immutable. Previously we would have two ioctls, one to create the object
+ * with gem_create, and another to apply various parameters, however this
+ * creates some ambiguity for the params which are considered immutable. Also in
+ * general we're phasing out the various SET/GET ioctls.
+ */
+struct __drm_i915_gem_create_ext {
+	/**
+	 * @size: Requested size for the object.
+	 *
+	 * 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.
+	 */
+	__u64 size;
+	/**
+	 * @handle: Returned handle for the object.
+	 *
+	 * Object handles are nonzero.
+	 */
+	__u32 handle;
+	/**
+	 * @flags: Optional flags.
+	 *
+	 * Supported values:
+	 *
+	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the kernel that
+	 * the object will need to be accessed via the CPU.
+	 *
+	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and only
+	 * strictly required on configurations where some subset of the device
+	 * memory is directly visible/mappable through the CPU(which we also
+	 * call small BAR), like on some DG2+ systems. Note that this is quite
+	 * undesirable, but due to various factors like the client CPU, BIOS etc
+	 * it's something we can expect to see in the wild. See struct
+	 * __drm_i915_memory_region_info.probed_cpu_visible_size for how to
+	 * determine if this system applies.
+	 *
+	 * Note that one of the placements MUST be I915_MEMORY_CLASS_SYSTEM, to
+	 * ensure the kernel can always spill the allocation to system memory,
+	 * if the object can't be allocated in the mappable part of
+	 * I915_MEMORY_CLASS_DEVICE.
+	 *
+	 * Also note that since the kernel only supports flat-CCS on objects
+	 * that can *only* be placed in I915_MEMORY_CLASS_DEVICE, we therefore
+	 * don't support I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS together with
+	 * flat-CCS.
+	 *
+	 * Without this hint, the kernel will assume that non-mappable
+	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that the
+	 * kernel can still migrate the object to the mappable part, as a last
+	 * resort, if userspace ever CPU faults this object, but this might be
+	 * expensive, and so ideally should be avoided.
+	 *
+	 * On older kernels, where usage of this flag results in an error, since
+	 * we lack the relevant small BAR uAPI(see also struct
+	 * __drm_i915_memory_region_info.probed_cpu_visible_size) it should
+	 * NEVER be possible to end up with a small BAR configuration, assuming
+	 * we can also successfully load the i915 kernel module. In such cases
+	 * the entire I915_MEMORY_CLASS_DEVICE region will be CPU accessible,
+	 * and as such there are zero restrictions on where the object can be
+	 * placed.
+	 */
+#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
+	__u32 flags;
+	/**
+	 * @extensions: The chain of extensions to apply to this object.
+	 *
+	 * This will be useful in the future when we need to support several
+	 * different extensions, and we need to apply more than one when
+	 * creating the object. See struct i915_user_extension.
+	 *
+	 * If we don't supply any extensions then we get the same old gem_create
+	 * behaviour.
+	 *
+	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
+	 * struct drm_i915_gem_create_ext_memory_regions.
+	 *
+	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
+	 * struct drm_i915_gem_create_ext_protected_content.
+	 */
+#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
+#define I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
+	__u64 extensions;
+};
diff --git a/Documentation/gpu/rfc/i915_small_bar.rst b/Documentation/gpu/rfc/i915_small_bar.rst
new file mode 100644
index 000000000000..a322481cea8b
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.rst
@@ -0,0 +1,47 @@
+==========================
+I915 Small BAR RFC Section
+==========================
+Starting from DG2 we will have resizable BAR support for device local-memory(i.e
+I915_MEMORY_CLASS_DEVICE), but in some cases the final BAR size might still be
+smaller than the total probed_size. In such cases, only some subset of
+I915_MEMORY_CLASS_DEVICE will be CPU accessible(for example the first 256M),
+while the remainder is only accessible via the GPU.
+
+I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
+----------------------------------------------
+New gem_create_ext flag to tell the kernel that a BO will require CPU access.
+This becomes important when placing an object in I915_MEMORY_CLASS_DEVICE, where
+underneath the device has a small BAR, meaning only some portion of it is CPU
+accessible. Without this flag the kernel will assume that CPU access is not
+required, and prioritize using the non-CPU visible portion of
+I915_MEMORY_CLASS_DEVICE.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
+   :functions: __drm_i915_gem_create_ext
+
+probed_cpu_visible_size attribute
+---------------------------------
+New struct__drm_i915_memory_region attribute which returns the total size of the
+CPU accessible portion, for the particular region. This should only be
+applicable for I915_MEMORY_CLASS_DEVICE. We also report the
+unallocated_cpu_visible_size, alongside the unallocated_size.
+
+Vulkan will need this as part of creating a separate VkMemoryHeap with the
+VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT set, to represent the CPU visible portion,
+where the total size of the heap needs to be known. It also wants to be able to
+give a rough estimate of how memory can potentially be allocated.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
+   :functions: __drm_i915_memory_region_info
+
+Error Capture restrictions
+--------------------------
+With error capture we have two new restrictions:
+
+    1) Error capture is best effort on small BAR systems; if the pages are not
+    CPU accessible, at the time of capture, then the kernel is free to skip
+    trying to capture them.
+
+    2) On discrete we now reject error capture on recoverable contexts. In the
+    future the kernel may want to blit during error capture, when for example
+    something is not currently CPU accessible.
diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
index 91e93a705230..5a3bd3924ba6 100644
--- a/Documentation/gpu/rfc/index.rst
+++ b/Documentation/gpu/rfc/index.rst
@@ -23,3 +23,7 @@ host such documentation:
 .. toctree::
i915_scheduler.rst
+
+.. toctree::
+
+    i915_small_bar.rst
-- 
2.34.3


    