[Intel-gfx] [PATCH v3] drm/doc: add rfc section for small BAR uapi
Lionel Landwerlin
lionel.g.landwerlin at intel.com
Tue May 17 08:55:20 UTC 2022
On 17/05/2022 11:29, Tvrtko Ursulin wrote:
>
> On 16/05/2022 19:11, 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.
>>
>> 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 | 164 +++++++++++++++++++++++
>> Documentation/gpu/rfc/i915_small_bar.rst | 47 +++++++
>> Documentation/gpu/rfc/index.rst | 4 +
>> 3 files changed, 215 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..4079d287750b
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>> @@ -0,0 +1,164 @@
>> +/**
>> + * 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;
>
> Is -1 possible today or when it will be? For system memory it appears
> zeroes are returned today so that has to stay I think. Does it
> effectively mean userspace has to consider both 0 and -1 as unknown is
> the question.
I raised this on v2. As far as I can tell there are no situation where
we would get -1.
Is it really probed_size=0 on smem?? It's not the case on the internal
branch.
Anv is not currently handling that case.
I would very much like to not deal with 0 for smem.
It really makes it easier for userspace rather than having to fish
information from 2 different places and on top of dealing with multiple
kernel versions.
-Lionel
>
>> +
>> + /**
>> + * @unallocated_size: Estimate of memory remaining (-1 = unknown)
>> + *
>> + * Note this is only currently tracked for I915_MEMORY_CLASS_DEVICE
>> + * regions, and also requires CAP_PERFMON or CAP_SYS_ADMIN to get
>> + * reliable accounting. Without this(or if this an older kernel)
>> the
>
> s/if this an/if this is an/
>
> Also same question as above about -1.
>
>> + * value here will always match 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).
>
> Also question about -1. In this case this could be done since the
> field is yet to be added but I am curious if it ever can be -1.
>
>> + *
>> + * 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 that if the value returned here is zero, then
>> + * this must be an old kernel which lacks the relevant
>> + * small-bar uAPI support(including
>
> I have noticed you prefer no space before parentheses throughout the
> text so I guess it's just my preference to have it. Very nitpicky even
> if I am right so up to you.
>
>> + * 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 currently tracked for
>> + * I915_MEMORY_CLASS_DEVICE regions, and also requires
>> + * CAP_PERFMON or CAP_SYS_ADMIN to get reliable
>> + * accounting. Without this the value here will always
>> + * equal the @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.
>
> Is it defined how rounding up works when a list of regions is given
> (like smem+lmem) and should that be explicitly mentioned here?
>
>> + */
>> + __u64 size;
>
> Blank line here (etc below) maybe to match the previous doc block?
>
> Regards,
>
> Tvrtko
>
>> + /**
>> + * @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, where usage of this flag results in an
>> error, since
>> + * we lack the relevant small BAR uAPI(see also struct
>> + * __drm_i915_memory_region_info.probed_cpu_visible_size) 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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