I915 Small BAR RFC

Small BAR와 CPU-access BO flag

DG2부터 device local memory인 `I915_MEMORY_CLASS_DEVICE`에 resizable BAR를 지원하지만 최종 BAR size가 전체 `probed_size`보다 작을 수 있습니다. 이 경우 예를 들어 처음 256M만 CPU가 접근하고 나머지는 GPU만 접근합니다.

새 `I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS` gem_create_ext flag는 BO에 CPU access가 필요함을 kernel에 알립니다. Small BAR의 device-memory object placement에서 중요합니다.

Flag가 없으면 kernel은 CPU access가 필요 없다고 가정하고 `I915_MEMORY_CLASS_DEVICE`의 CPU-visible하지 않은 부분을 우선 사용합니다. uAPI kernel-doc은 `Documentation/gpu/rfc/i915_small_bar.h`의 `__drm_i915_gem_create_ext`를 포함합니다.

==========================
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

CPU-visible region size attribute

새 `struct__drm_i915_memory_region` attribute `probed_cpu_visible_size`는 특정 region에서 CPU가 접근 가능한 전체 크기를 반환합니다. `I915_MEMORY_CLASS_DEVICE`에만 적용해야 합니다.

전체 미할당 크기 `unallocated_size`와 함께 CPU-visible 미할당 크기 `unallocated_cpu_visible_size`도 보고합니다.

Vulkan은 CPU-visible 부분을 나타내는 `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT`가 설정된 별도 `VkMemoryHeap`을 만들 때 전체 heap size가 필요합니다. 또한 앞으로 allocation할 수 있는 memory를 대략 추정하려 합니다.

관련 kernel-doc은 `i915_small_bar.h`의 `__drm_i915_memory_region_info`입니다.

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 제한

Small BAR system의 error capture는 best effort입니다. Capture 시점에 page가 CPU-accessible하지 않으면 kernel은 해당 page capture를 생략할 수 있습니다.

Discrete와 더 최신 integrated platform에서는 recoverable context의 error capture를 거부합니다.

미래에는 현재 CPU가 접근할 수 없는 data를 capture하기 위해 kernel이 error capture 중 blit을 사용할 수 있습니다.

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.