[PATCH v2] drm/doc: add rfc section for small BAR uapi
Matthew Auld
matthew.auld at intel.com
Tue May 3 14:27:43 UTC 2022
On 03/05/2022 11:39, Lionel Landwerlin wrote:
> On 03/05/2022 13:22, Matthew Auld wrote:
>> On 02/05/2022 09:53, Lionel Landwerlin wrote:
>>> On 02/05/2022 10:54, Lionel Landwerlin wrote:
>>>> 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 at intel.com>
>>>>> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>>>> Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>>>>> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
>>>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>>>> 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 | 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'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
>>>
>>>
>>> Other pain point of this new uAPI, previously we could query the
>>> unallocated size for each heap.
>>
>> unallocated_size should always give the same value as probed_size. We
>> have the avail tracking, but we don't currently expose that through
>> unallocated_size, due to lack of real userspace/user etc.
>>
>>>
>>> Now lmem is effectively divided into 2 heaps, but unallocated_size is
>>> tracking allocation from both parts of lmem.
>>
>> Yeah, if we ever properly expose the unallocated_size, then we could
>> also just add unallocated_cpu_visible_size.
>>
>>>
>>> Is adding new I915_MEMORY_CLASS_DEVICE_NON_MAPPABLE out of question?
>>
>> I don't think it's out of the question...
>>
>> I guess user-space should be able to get the current flag behaviour
>> just by specifying: device, system. And it does give more flexibly to
>> allow something like: device, device-nm, smem.
>>
>> We can also drop the probed_cpu_visible_size, which would now just be
>> the probed_size with device/device-nm. And if we lack device-nm, then
>> the entire thing must be CPU mappable.
>>
>> One of the downsides though, is that we can no longer easily mix
>> object pages from both device + device-nm, which we could previously
>> do when we didn't specify the flag. At least according to the current
>> design/behaviour for @regions that would not be allowed. I guess some
>> kind of new flag like ALLOC_MIXED or so? Although currently that is
>> only possible with device + device-nm in ttm/i915.
>
>
> Thanks, I wasn't aware of the restrictions.
>
> Adding unallocated_cpu_visible_size would be great.
So do we want this in the next version? i.e we already have a current
real use case in mind for unallocated_size where probed_size is not good
enough?
>
>
> -Lionel
>
>
>>
>>>
>>>
>>> -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
>>>>
>>>>
>>>
>
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