[PATCH v3 01/13] drm/doc: add rfc section for small BAR uapi
Matthew Auld
matthew.auld at intel.com
Wed Jun 29 12:14:15 UTC 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.
v4:
- Various improvements all over. (Tvrtko)
v5:
- Include newer integrated platforms when applying the non-recoverable
context and error capture restriction. (Thomas)
Signed-off-by: Matthew Auld <matthew.auld at intel.com>
Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Jordan Justen <jordan.l.justen at intel.com>
Cc: Kenneth Graunke <kenneth at whitecape.org>
Cc: Akeem G Abodunrin <akeem.g.abodunrin at intel.com>
Cc: mesa-dev at lists.freedesktop.org
Acked-by: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Acked-by: Akeem G Abodunrin <akeem.g.abodunrin at intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Acked-by: Jordan Justen <jordan.l.justen at intel.com>
---
Documentation/gpu/rfc/i915_small_bar.h | 189 +++++++++++++++++++++++
Documentation/gpu/rfc/i915_small_bar.rst | 47 ++++++
Documentation/gpu/rfc/index.rst | 4 +
3 files changed, 240 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..6003c81d5aa4
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.h
@@ -0,0 +1,189 @@
+/**
+ * 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
+ *
+ * Note that it should not be possible to ever encounter a zero value
+ * here, also note that no current region type will ever return -1 here.
+ * Although for future region types, this might be a possibility. The
+ * same applies to the other size fields.
+ */
+ __u64 probed_size;
+
+ /**
+ * @unallocated_size: Estimate of memory remaining
+ *
+ * Requires CAP_PERFMON or CAP_SYS_ADMIN to get reliable accounting.
+ * Without this (or if this is an older kernel) the value here will
+ * always equal the @probed_size. Note this is only currently tracked
+ * for I915_MEMORY_CLASS_DEVICE regions (for other types the value here
+ * will always equal 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.
+ *
+ * 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 this is only tracked for
+ * I915_MEMORY_CLASS_DEVICE regions (for other types the
+ * value here will always equal the @probed_size).
+ *
+ * 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
+ *
+ * Note this is only tracked for
+ * I915_MEMORY_CLASS_DEVICE regions (for other types the
+ * value here will always equal the
+ * @probed_cpu_visible_size).
+ *
+ * Requires CAP_PERFMON or CAP_SYS_ADMIN to get reliable
+ * accounting. Without this the value here will always
+ * equal the @probed_cpu_visible_size. Note this is only
+ * currently tracked for I915_MEMORY_CLASS_DEVICE
+ * regions (for other types the value here will also
+ * always equal the @probed_cpu_visible_size).
+ *
+ * If this is an older kernel the value here will be
+ * zero, see also @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. The kernel will
+ * always select the largest minimum page-size for the set of possible
+ * placements as the value to use when rounding up the @size.
+ */
+ __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
+ * &__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 which lack the relevant small-bar uAPI support (see
+ * also &__drm_i915_memory_region_info.probed_cpu_visible_size),
+ * usage of the flag will result in an error, but 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..d6c03ce3b862
--- /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 and newer integrated platforms 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
--
2.36.1
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