Re: [Intel-gfx] [PATCH 2/2] drm/doc: add rfc section for small BAR uapi

Abodunrin, Akeem G
2022-02-22T17:40:09Z

> -----Original Message-----
> From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of
> Thomas Hellström
> Sent: Tuesday, February 22, 2022 2:36 AM
> To: Auld, Matthew <matthew.auld@intel.com>; intel-gfx@lists.freedesktop.org
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>; dri-devel@lists.freedesktop.org;
> Kenneth Graunke <kenneth@whitecape.org>; Bloomfield, Jon
> <jon.bloomfield@intel.com>; Justen, Jordan L <jordan.l.justen@intel.com>;
> mesa-dev@lists.freedesktop.org
> Subject: Re: [PATCH 2/2] drm/doc: add rfc section for small BAR uapi
>
>
> On 2/18/22 12:22, Matthew Auld wrote:
> > Add an entry for the new uapi needed for small BAR on DG2+.
> >
> > Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Jordan Justen <jordan.l.justen@intel.com>
> > Cc: Kenneth Graunke <kenneth@whitecape.org>
> > Cc: mesa-dev@lists.freedesktop.org
> > ---
> > Documentation/gpu/rfc/i915_small_bar.h | 153
> +++++++++++++++++++++++
> > Documentation/gpu/rfc/i915_small_bar.rst | 40 ++++++
> > Documentation/gpu/rfc/index.rst | 4 +
> > 3 files changed, 197 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..fa65835fd608
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.h
> > @@ -0,0 +1,153 @@
> > +/**
> > + * 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,
>
> Does this sentence need updating, with the flags member?
>
>
> > 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 buffers that need to be captured with
> EXEC_OBJECT_CAPTURE,
> > + * will need to enable this hint, if the object can also be placed in
> > + * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call
> will
> > + * throw an error otherwise. This also means that such objects will need
> > + * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
> > + *
> > + * 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..fea92d3d69ab
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> > @@ -0,0 +1,40 @@
> > +==========================
> > +I915 Small BAR RFC Section
> > +==========================
> > +Starting from DG2 we will have resizable BAR support for device
> > +local-memory, but in some cases the final BAR size might still be
> > +smaller than the total local-memory size. In such cases only part of
> > +local-memory will be CPU accessible, while the remainder is only accessible
> via the GPU.
... In such cases only part of local-memory will be CPU accessible/ In such cases, only {small} part of local-memory will be CPU accessible?

> > +
> > +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.
> > +The becomes important when placing an object in LMEM, where
The/This?

> > +underneath the device has a small BAR, meaning only part 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 LMEM(if
> present on the device).
> > +
> > +Related to this, we now also reject any objects marked with
> > +EXEC_OBJECT_CAPTURE, which are also not tagged with
> NEEDS_CPU_ACCESS.
> > +This only impacts DG2+.
> > +
> > +XXX: One open here is whether we should extend the memory region
> > +query to return the CPU visible size of the region. For now the IGTs
> > +just use debugfs to query the size. However, if userspace sees a real
> > +need for this then extending the region query would be a lot nicer.
>
> I guess UMD folks need to comment on this. Although I think since there might
> be a number of clients utilizing the mappable part, and a number of buffers
> pinned in there, I figure this might be of limited value outside of tests without
> some kind of cgroups support.
>
> Otherwise
>
> Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>
>
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > + :functions: __drm_i915_gem_create_ext
> > +
> > +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 attrubutes. For now we only
Attrubutes/attributes?

You might want to read the document again, and fix punctuations in addition to those typos - at least to make it more legible...

Thanks,
~Akeem

> > +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 visible size
> > +of LMEM matches the probed size. If this is not set then CPU faulting
> > +the object will first require migrating the pages.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > + :functions: __drm_i915_query_vma_info
> > diff --git a/Documentation/gpu/rfc/index.rst
> > b/Documentation/gpu/rfc/index.rst index 018a8bf317a6..5b8495bdc1fd
> > 100644
> > --- a/Documentation/gpu/rfc/index.rst
> > +++ b/Documentation/gpu/rfc/index.rst
> > @@ -19,3 +19,7 @@ host such documentation:
> > .. toctree::
> >
> > i915_scheduler.rst
> > +
> > +.. toctree::
> > +
> > + i915_small_bar.rst