> -----Original Message-----
> From: Landwerlin, Lionel G <lionel.g.landwerlin@xxxxxxxxx>
> Sent: Monday, May 2, 2022 12:55 AM
> To: Auld, Matthew <matthew.auld@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx; Thomas Hellström
> <thomas.hellstrom@xxxxxxxxxxxxxxx>; Bloomfield, Jon
> <jon.bloomfield@xxxxxxxxx>; Daniel Vetter <daniel.vetter@xxxxxxxx>; Justen,
> Jordan L <jordan.l.justen@xxxxxxxxx>; Kenneth Graunke
> <kenneth@xxxxxxxxxxxxx>; Abodunrin, Akeem G
> <akeem.g.abodunrin@xxxxxxxxx>; mesa-dev@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2] drm/doc: add rfc section for small BAR uapi
>
> On 20/04/2022 20:13, Matthew Auld wrote:
> > 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)
> >
> > Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
> > Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
> > Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: Jordan Justen <jordan.l.justen@xxxxxxxxx>
> > Cc: Kenneth Graunke <kenneth@xxxxxxxxxxxxx>
> > Cc: Akeem G Abodunrin <akeem.g.abodunrin@xxxxxxxxx>
> > Cc: mesa-dev@xxxxxxxxxxxxxxxxxxxxx
> > ---
> > Documentation/gpu/rfc/i915_small_bar.h | 190
> +++++++++++++++++++++++
> > Documentation/gpu/rfc/i915_small_bar.rst | 58 +++++++
> > Documentation/gpu/rfc/index.rst | 4 +
> > 3 files changed, 252 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..7bfd0cf44d35
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.h
> > @@ -0,0 +1,190 @@
> > +/**
> > + * 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)
> */
> > + __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.
> > + */
> > + __u64 probed_cpu_visible_size;
> > + };
>
>
> Trying to implement userspace support in Vulkan for this, I have an additional
> question about the value of probed_cpu_visible_size.
>
> When is it set to -1?
I believe it is set to -1 if it is unknown, and/or not cpu accessible...
Cheers!
~Akeem
>
> I'm guessing before there is support for this value it'll be 0 (MBZ).
>
> After after it should either be the entire lmem or something smaller.
>
>
> -Lionel
>
>
> > + };
> > +};
> > +
> > +/**
> > + * 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 platforms where only some of the device
> > + * memory is directly visible or mappable through the CPU, like on DG2+.
> > + *
> > + * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM,
> to
> > + * ensure we can always spill the allocation to system memory, if we
> > + * can't place the object in the mappable part of
> > + * I915_MEMORY_CLASS_DEVICE.
> > + *
> > + * 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.
> > + */
> > +#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;
> > +};
> > +
> > +#define DRM_I915_QUERY_VMA_INFO 5
> > +
> > +/**
> > + * struct __drm_i915_query_vma_info
> > + *
> > + * Given a vm and GTT address, lookup the corresponding vma,
> > +returning its set
> > + * of attributes.
> > + *
> > + * .. code-block:: C
> > + *
> > + * struct drm_i915_query_vma_info info = {};
> > + * struct drm_i915_query_item item = {
> > + * .data_ptr = (uintptr_t)&info,
> > + * .query_id = DRM_I915_QUERY_VMA_INFO,
> > + * };
> > + * struct drm_i915_query query = {
> > + * .num_items = 1,
> > + * .items_ptr = (uintptr_t)&item,
> > + * };
> > + * int err;
> > + *
> > + * // Unlike some other types of queries, there is no need to first query
> > + * // the size of the data_ptr blob here, since we already know ahead of
> > + * // time how big this needs to be.
> > + * item.length = sizeof(info);
> > + *
> > + * // Next we fill in the vm_id and ppGTT address of the vma we wish
> > + * // to query, before then firing off the query.
> > + * info.vm_id = vm_id;
> > + * info.offset = gtt_address;
> > + * err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> > + * if (err || item.length < 0) ...
> > + *
> > + * // If all went well we can now inspect the returned attributes.
> > + * if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
> > + */
> > +struct __drm_i915_query_vma_info {
> > + /**
> > + * @vm_id: The given vm id that contains the vma. The id is the value
> > + * returned by the DRM_I915_GEM_VM_CREATE. See struct
> > + * drm_i915_gem_vm_control.vm_id.
> > + */
> > + __u32 vm_id;
> > + /** @pad: MBZ. */
> > + __u32 pad;
> > + /**
> > + * @offset: The corresponding ppGTT address of the vma which the
> kernel
> > + * will use to perform the lookup.
> > + */
> > + __u64 offset;
> > + /**
> > + * @attributes: The returned attributes for the given vma.
> > + *
> > + * Possible values:
> > + *
> > + * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages
> backing the
> > + * vma are currently CPU accessible. If this is not set then the vma is
> > + * currently backed by I915_MEMORY_CLASS_DEVICE memory, which
> the CPU
> > + * cannot directly access(this is only possible on discrete devices with
> > + * a small BAR). Attempting to MMAP and fault such an object will
> > + * require the kernel first synchronising any GPU work tied to the
> > + * object, before then migrating the pages, either to the CPU accessible
> > + * part of I915_MEMORY_CLASS_DEVICE, or
> I915_MEMORY_CLASS_SYSTEM, if the
> > + * placements permit it. See
> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
> > + *
> > + * Note that this is inherently racy.
> > + */
> > +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
> > + __u64 attributes;
> > + /** @rsvd: MBZ */
> > + __u32 rsvd[4];
> > +};
> > diff --git a/Documentation/gpu/rfc/i915_small_bar.rst
> > b/Documentation/gpu/rfc/i915_small_bar.rst
> > new file mode 100644
> > index 000000000000..be3d9bcdd86d
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> > @@ -0,0 +1,58 @@
> > +==========================
> > +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.
> > +
> > +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.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > + :functions: __drm_i915_memory_region_info
> > +
> > +DRM_I915_QUERY_VMA_INFO query
> > +-----------------------------
> > +Query the attributes of some vma. Given a vm and GTT offset, find the
> > +respective vma, and return its set of attributes. For now we only
> > +support DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the
> > +object/vma is currently placed in memory that is accessible by the
> > +CPU. This should always be set on devices where the CPU
> > +probed_cpu_visible_size of I915_MEMORY_CLASS_DEVICE matches the
> > +probed_size. If this is not set then CPU faulting the object will likely first
> require migrating the pages.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > + :functions: __drm_i915_query_vma_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
>
