[PATCH v4] drm/doc: add rfc section for small BAR uapi
Abodunrin, Akeem G
akeem.g.abodunrin at intel.com
Tue May 17 18:02:49 UTC 2022
> -----Original Message-----
> From: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Sent: Tuesday, May 17, 2022 6:49 AM
> To: Auld, Matthew <matthew.auld at intel.com>; intel-gfx at lists.freedesktop.org
> Cc: dri-devel at lists.freedesktop.org; Thomas Hellström
> <thomas.hellstrom at linux.intel.com>; Landwerlin, Lionel G
> <lionel.g.landwerlin at intel.com>; Bloomfield, Jon <jon.bloomfield at intel.com>;
> Daniel Vetter <daniel.vetter at ffwll.ch>; Justen, Jordan L
> <jordan.l.justen at intel.com>; Kenneth Graunke <kenneth at whitecape.org>;
> Abodunrin, Akeem G <akeem.g.abodunrin at intel.com>; mesa-
> dev at lists.freedesktop.org
> Subject: Re: [PATCH v4] drm/doc: add rfc section for small BAR uapi
>
>
> On 17/05/2022 11:52, 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)
> > 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)
>
> You can ignore my previous reply, the clarifications from v4 read good to me.
>
> Acked-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
The patch looks good to me as well...
Acked-by: Akeem G Abodunrin <akeem.g.abodunrin at intel.com>
>
> Regards,
>
> Tvrtko
>
> >
> > 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: Jon Bloomfield <jon.bloomfield at intel.com>
> > 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
> > ---
> > 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..c676640b23ef
> > --- /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 (-1 = unknown)
> > + *
> > + * 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 (-1 = unknown)
> > + *
> > + * 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. (-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 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 (-1 = unknown).
> > + *
> > + * 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 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 which lack the relevant small-bar uAPI support (see
> > + * also struct
> __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..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
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