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drm/doc: add rfc section for small BAR uapi

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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)

Mesa: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16739
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@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: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Cc: mesa-dev@lists.freedesktop.org
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Acked-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220629174350.384910-1-matthew.auld@intel.com
This commit is contained in:
Matthew Auld 2022-06-29 18:43:38 +01:00
parent 971e4a9781
commit fff1d972f4
3 changed files with 240 additions and 0 deletions
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189
Documentation/gpu/rfc/i915_small_bar.h Normal file
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@ -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;
};

47
Documentation/gpu/rfc/i915_small_bar.rst Normal file
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==========================
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.

4
Documentation/gpu/rfc/index.rst
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@ -23,3 +23,7 @@ host such documentation:
.. toctree::
i915_scheduler.rst
.. toctree::
i915_small_bar.rst