LGTM Reviewed-by: Nirmoy Das <nirmoy....@intel.com>

On 5/25/2022 8:43 PM, Matthew Auld wrote:
If set, force the allocation to be placed in the mappable portion of
I915_MEMORY_CLASS_DEVICE. One big restriction here is that system memory
(i.e I915_MEMORY_CLASS_SYSTEM) must be given as a potential placement for the
object, that way we can always spill the object into system memory if we
can't make space.

Testcase: igt@gem-create@create-ext-cpu-access-sanity-check
Testcase: igt@gem-create@create-ext-cpu-access-big
Signed-off-by: Matthew Auld <matthew.a...@intel.com>
Cc: Thomas Hellström <thomas.hellst...@linux.intel.com>
Cc: Lionel Landwerlin <lionel.g.landwer...@intel.com>
Cc: Jon Bloomfield <jon.bloomfi...@intel.com>
Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
Cc: Jordan Justen <jordan.l.jus...@intel.com>
Cc: Kenneth Graunke <kenn...@whitecape.org>
Cc: Akeem G Abodunrin <akeem.g.abodun...@intel.com>
---
  drivers/gpu/drm/i915/gem/i915_gem_create.c | 26 ++++++---
  include/uapi/drm/i915_drm.h                | 61 +++++++++++++++++++---
  2 files changed, 71 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_create.c 
b/drivers/gpu/drm/i915/gem/i915_gem_create.c
index d094cae0ddf1..33673fe7ee0a 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_create.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_create.c
@@ -241,6 +241,7 @@ struct create_ext {
        struct drm_i915_private *i915;
        struct intel_memory_region *placements[INTEL_REGION_UNKNOWN];
        unsigned int n_placements;
+       unsigned int placement_mask;
        unsigned long flags;
  };
@@ -337,6 +338,7 @@ static int set_placements(struct drm_i915_gem_create_ext_memory_regions *args,
        for (i = 0; i < args->num_regions; i++)
                ext_data->placements[i] = placements[i];
+ ext_data->placement_mask = mask;
        return 0;
out_dump:
@@ -411,7 +413,7 @@ i915_gem_create_ext_ioctl(struct drm_device *dev, void 
*data,
        struct drm_i915_gem_object *obj;
        int ret;
- if (args->flags)
+       if (args->flags & ~I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS)
                return -EINVAL;
ret = i915_user_extensions(u64_to_user_ptr(args->extensions),
@@ -427,13 +429,21 @@ i915_gem_create_ext_ioctl(struct drm_device *dev, void 
*data,
                ext_data.n_placements = 1;
        }
- /*
-        * TODO: add a userspace hint to force CPU_ACCESS for the object, which
-        * can override this.
-        */
-       if (ext_data.n_placements > 1 ||
-           ext_data.placements[0]->type != INTEL_MEMORY_SYSTEM)
-               ext_data.flags |= I915_BO_ALLOC_GPU_ONLY;
+       if (args->flags & I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS) {
+               if (ext_data.n_placements == 1)
+                       return -EINVAL;
+
+               /*
+                * We always need to be able to spill to system memory, if we
+                * can't place in the mappable part of LMEM.
+                */
+               if (!(ext_data.placement_mask & BIT(INTEL_REGION_SMEM)))
+                       return -EINVAL;
+       } else {
+               if (ext_data.n_placements > 1 ||
+                   ext_data.placements[0]->type != INTEL_MEMORY_SYSTEM)
+                       ext_data.flags |= I915_BO_ALLOC_GPU_ONLY;
+       }
obj = __i915_gem_object_create_user_ext(i915, args->size,
                                                ext_data.placements,
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index e30f31a440b3..5b0a10e6a1b8 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -3366,11 +3366,11 @@ struct drm_i915_query_memory_regions {
   * struct drm_i915_gem_create_ext - Existing gem_create behaviour, with added
   * extension support using struct i915_user_extension.
   *
- * Note that in the future we want to have our buffer flags 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.
+ * 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 {
        /**
@@ -3378,7 +3378,6 @@ struct drm_i915_gem_create_ext {
         *
         * The (page-aligned) allocated size for the object will be returned.
         *
-        *
         * DG2 64K min page size implications:
         *
         * On discrete platforms, starting from DG2, we have to contend with GTT
@@ -3390,7 +3389,9 @@ struct drm_i915_gem_create_ext {
         *
         * 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.
+        * such as DG2. 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.
         *
         * Special DG2 GTT address alignment requirement:
         *
@@ -3414,14 +3415,58 @@ struct drm_i915_gem_create_ext {
         * is deemed to be a good compromise.
         */
        __u64 size;
+
        /**
         * @handle: Returned handle for the object.
         *
         * Object handles are nonzero.
         */
        __u32 handle;
-       /** @flags: MBZ */
+
+       /**
+        * @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.
         *

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