// Copyright 2018-2021 The Khronos Group, Inc.
//
// SPDX-License-Identifier: CC-BY-4.0

include::{generated}/meta/{refprefix}VK_EXT_image_drm_format_modifier.txt[]

=== Other Extension Metadata

*Last Modified Date*::
    2018-08-29
*IP Status*::
    No known IP claims.
*Contributors*::
  - Antoine Labour, Google
  - Bas Nieuwenhuizen, Google
  - Chad Versace, Google
  - James Jones, NVIDIA
  - Jason Ekstrand, Intel
  - Jőrg Wagner, ARM
  - Kristian Høgsberg Kristensen, Google
  - Ray Smith, ARM

=== Description

This extension provides the ability to use _DRM format modifiers_ with
images, enabling Vulkan to better integrate with the Linux ecosystem of
graphics, video, and display APIs.

Its functionality closely overlaps with
`EGL_EXT_image_dma_buf_import_modifiers`^<<VK_EXT_image_drm_format_modifier-fn2,2>>^
and
`EGL_MESA_image_dma_buf_export`^<<VK_EXT_image_drm_format_modifier-fn3,3>>^.
Unlike the EGL extensions, this extension does not require the use of a
specific handle type (such as a dma_buf) for external memory and provides
more explicit control of image creation.

=== Introduction to DRM Format Modifiers

A _DRM format modifier_ is a 64-bit, vendor-prefixed, semi-opaque unsigned
integer.
Most _modifiers_ represent a concrete, vendor-specific tiling format for
images.
Some exceptions are etext:DRM_FORMAT_MOD_LINEAR (which is not
vendor-specific); etext:DRM_FORMAT_MOD_NONE (which is an alias of
etext:DRM_FORMAT_MOD_LINEAR due to historical accident); and
etext:DRM_FORMAT_MOD_INVALID (which does not represent a tiling format).
The _modifier's_ vendor prefix consists of the 8 most significant bits.
The canonical list of _modifiers_ and vendor prefixes is found in
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/drm/drm_fourcc.h[`drm_fourcc.h`]
in the Linux kernel source.
The other dominant source of _modifiers_ are vendor kernel trees.

One goal of _modifiers_ in the Linux ecosystem is to enumerate for each
vendor a reasonably sized set of tiling formats that are appropriate for
images shared across processes, APIs, and/or devices, where each
participating component may possibly be from different vendors.
A non-goal is to enumerate all tiling formats supported by all vendors.
Some tiling formats used internally by vendors are inappropriate for
sharing; no _modifiers_ should be assigned to such tiling formats.

Modifier values typically do not _describe_ memory layouts.
More precisely, a _modifier_'s lower 56 bits usually have no structure.
Instead, modifiers _name_ memory layouts; they name a small set of
vendor-preferred layouts for image sharing.
As a consequence, in each vendor namespace the modifier values are often
sequentially allocated starting at 1.

Each _modifier_ is usually supported by a single vendor and its name matches
the pattern `{VENDOR}_FORMAT_MOD_*` or `DRM_FORMAT_MOD_{VENDOR}_*`.
Examples are etext:I915_FORMAT_MOD_X_TILED and
etext:DRM_FORMAT_MOD_BROADCOM_VC4_T_TILED.
An exception is etext:DRM_FORMAT_MOD_LINEAR, which is supported by most
vendors.

Many APIs in Linux use _modifiers_ to negotiate and specify the memory
layout of shared images.
For example, a Wayland compositor and Wayland client may, by relaying
_modifiers_ over the Wayland protocol `zwp_linux_dmabuf_v1`, negotiate a
vendor-specific tiling format for a shared code:wl_buffer.
The client may allocate the underlying memory for the code:wl_buffer with
GBM, providing the chosen _modifier_ to code:gbm_bo_create_with_modifiers.
The client may then import the code:wl_buffer into Vulkan for producing
image content, providing the resource's dma_buf to
slink:VkImportMemoryFdInfoKHR and its _modifier_ to
slink:VkImageDrmFormatModifierExplicitCreateInfoEXT.
The compositor may then import the code:wl_buffer into OpenGL for sampling,
providing the resource's dma_buf and _modifier_ to code:eglCreateImage.
The compositor may also bypass OpenGL and submit the code:wl_buffer directly
to the kernel's display API, providing the dma_buf and _modifier_ through
code:drm_mode_fb_cmd2.

=== Format Translation

_Modifier_-capable APIs often pair _modifiers_ with DRM formats, which are
defined in
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/drm/drm_fourcc.h[`drm_fourcc.h`].
However, `VK_EXT_image_drm_format_modifier` uses elink:VkFormat instead of
DRM formats.
The application must convert between elink:VkFormat and DRM format when it
sends or receives a DRM format to or from an external API.

The mapping from elink:VkFormat to DRM format is lossy.
Therefore, when receiving a DRM format from an external API, often the
application must use information from the external API to accurately map the
DRM format to a elink:VkFormat.
For example, DRM formats do not distinguish between RGB and sRGB (as of
2018-03-28); external information is required to identify the image's
colorspace.

The mapping between elink:VkFormat and DRM format is also incomplete.
For some DRM formats there exist no corresponding Vulkan format, and for
some Vulkan formats there exist no corresponding DRM format.

=== Usage Patterns

Three primary usage patterns are intended for this extension:

  * *Negotiation.* The application negotiates with _modifier_-aware,
    external components to determine sets of image creation parameters
    supported among all components.
+
--
In the Linux ecosystem, the negotiation usually assumes the image is a 2D,
single-sampled, non-mipmapped, non-array image; this extension permits that
assumption but does not require it.
The result of the negotiation usually resembles a set of tuples such as
_(drmFormat, drmFormatModifier)_, where each participating component
supports all tuples in the set.

Many details of this negotiation—such as the protocol used during
negotiation, the set of image creation parameters expressable in the
protocol, and how the protocol chooses which process and which API will
create the image—are outside the scope of this specification.

In this extension, flink:vkGetPhysicalDeviceFormatProperties2 with
slink:VkDrmFormatModifierPropertiesListEXT serves a primary role during the
negotiation, and flink:vkGetPhysicalDeviceImageFormatProperties2 with
slink:VkPhysicalDeviceImageDrmFormatModifierInfoEXT serves a secondary role.
--

  * *Import.* The application imports an image with a _modifier_.
+
--
In this pattern, the application receives from an external source the
image's memory and its creation parameters, which are often the result of
the negotiation described above.
Some image creation parameters are implicitly defined by the external
source; for example, ename:VK_IMAGE_TYPE_2D is often assumed.
Some image creation parameters are usually explicit, such as the image's
pname:format, pname:drmFormatModifier, and pname:extent; and each plane's
pname:offset and pname:rowPitch.

Before creating the image, the application first verifies that the physical
device supports the received creation parameters by querying
flink:vkGetPhysicalDeviceFormatProperties2 with
slink:VkDrmFormatModifierPropertiesListEXT and
flink:vkGetPhysicalDeviceImageFormatProperties2 with
slink:VkPhysicalDeviceImageDrmFormatModifierInfoEXT.
Then the application creates the image by chaining
slink:VkImageDrmFormatModifierExplicitCreateInfoEXT and
slink:VkExternalMemoryImageCreateInfo onto slink:VkImageCreateInfo.
--

  * *Export.* The application creates an image and allocates its memory.
    Then the application exports to _modifier_-aware consumers the image's
    memory handles; its creation parameters; its _modifier_; and the
    <<VkSubresourceLayout,pname:offset>>,
    <<VkSubresourceLayout,pname:size>>, and
    <<VkSubresourceLayout,pname:rowPitch>> of each _memory plane_.
+
--
In this pattern, the Vulkan device is the authority for the image; it is the
allocator of the image's memory and the decider of the image's creation
parameters.
When choosing the image's creation parameters, the application usually
chooses a tuple _(format, drmFormatModifier)_ from the result of the
negotiation described above.
The negotiation's result often contains multiple tuples that share the same
format but differ in their _modifier_.
In this case, the application should defer the choice of the image's
_modifier_ to the Vulkan implementation by providing all such _modifiers_ to
slink:VkImageDrmFormatModifierListCreateInfoEXT::pname:pDrmFormatModifiers;
and the implementation should choose from pname:pDrmFormatModifiers the
optimal _modifier_ in consideration with the other image parameters.

The application creates the image by chaining
slink:VkImageDrmFormatModifierListCreateInfoEXT and
slink:VkExternalMemoryImageCreateInfo onto slink:VkImageCreateInfo.
The protocol and APIs by which the application will share the image with
external consumers will likely determine the value of
slink:VkExternalMemoryImageCreateInfo::pname:handleTypes.
The implementation chooses for the image an optimal _modifier_ from
slink:VkImageDrmFormatModifierListCreateInfoEXT::pname:pDrmFormatModifiers.
The application then queries the implementation-chosen _modifier_ with
flink:vkGetImageDrmFormatModifierPropertiesEXT, and queries the memory
layout of each plane with flink:vkGetImageSubresourceLayout.

The application then allocates the image's memory with
slink:VkMemoryAllocateInfo, adding chained extending structures for external
memory; binds it to the image; and exports the memory, for example, with
flink:vkGetMemoryFdKHR.

Finally, the application sends the image's creation parameters, its
_modifier_, its per-plane memory layout, and the exported memory handle to
the external consumers.
The details of how the application transmits this information to external
consumers is outside the scope of this specification.
--

=== Prior Art

Extension
`EGL_EXT_image_dma_buf_import`^<<VK_EXT_image_drm_format_modifier-fn1,1>>^
introduced the ability to create an code:EGLImage by importing for each
plane a dma_buf, offset, and row pitch.

Later, extension
`EGL_EXT_image_dma_buf_import_modifiers`^<<VK_EXT_image_drm_format_modifier-fn2,2>>^
introduced the ability to query which combination of formats and _modifiers_
the implementation supports and to specify _modifiers_ during creation of
the code:EGLImage.

Extension
`EGL_MESA_image_dma_buf_export`^<<VK_EXT_image_drm_format_modifier-fn3,3>>^
is the inverse of `EGL_EXT_image_dma_buf_import_modifiers`.

The Linux kernel modesetting API (KMS), when configuring the display's
framebuffer with `struct
drm_mode_fb_cmd2`^<<VK_EXT_image_drm_format_modifier-fn4,4>>^, allows one to
specify the frambuffer's _modifier_ as well as a per-plane memory handle,
offset, and row pitch.

GBM, a graphics buffer manager for Linux, allows creation of a `gbm_bo`
(that is, a graphics _buffer object_) by importing data similar to that in
`EGL_EXT_image_dma_buf_import_modifiers`^<<VK_EXT_image_drm_format_modifier-fn1,1>>^;
and symmetrically allows exporting the same data from the `gbm_bo`.
See the references to _modifier_ and _plane_ in
`gbm.h`^<<VK_EXT_image_drm_format_modifier-fn5,5>>^.

include::{generated}/interfaces/VK_EXT_image_drm_format_modifier.txt[]

=== Issues

1) Should this extension define a single DRM format modifier per
sname:VkImage? Or define one per plane?
+
--
*RESOLVED*: There exists a single DRM format modifier per sname:VkImage.

*DISCUSSION*: Prior art, such as
`EGL_EXT_image_dma_buf_import_modifiers`^<<VK_EXT_image_drm_format_modifier-fn2,2>>^,
`struct drm_mode_fb_cmd2`^<<VK_EXT_image_drm_format_modifier-fn4,4>>^, and
`struct
gbm_import_fd_modifier_data`^<<VK_EXT_image_drm_format_modifier-fn5,5>>^,
allows defining one _modifier_ per plane.
However, developers of the GBM and kernel APIs concede it was a mistake.
Beginning in Linux 4.10, the kernel requires that the application provide
the same DRM format _modifier_ for each plane.
(See Linux commit
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bae781b259269590109e8a4a8227331362b88212[bae781b259269590109e8a4a8227331362b88212]).
And GBM provides an entry point, code:gbm_bo_get_modifier, for querying the
_modifier_ of the image but does not provide one to query the modifier of
individual planes.
--

2) When creating an image with
slink:VkImageDrmFormatModifierExplicitCreateInfoEXT, which is typically used
when _importing_ an image, should the application explicitly provide the
size of each plane?
+
--
*RESOLVED*: No.
The application must: not provide the size.
To enforce this, the API requires that
slink:VkImageDrmFormatModifierExplicitCreateInfoEXT::pname:pPlaneLayouts->size
must: be 0.

*DISCUSSION*: Prior art, such as
`EGL_EXT_image_dma_buf_import_modifiers`^<<VK_EXT_image_drm_format_modifier-fn2,2>>^,
`struct drm_mode_fb_cmd2`^<<VK_EXT_image_drm_format_modifier-fn4,4>>^, and
`struct
gbm_import_fd_modifier_data`^<<VK_EXT_image_drm_format_modifier-fn5,5>>^,
omits from the API the size of each plane.
Instead, the APIs infer each plane's size from the import parameters, which
include the image's pixel format and a dma_buf, offset, and row pitch for
each plane.

However, Vulkan differs from EGL and GBM with regards to image creation in
the following ways:

.Differences in Image Creation

  - *Undedicated allocation by default.* When importing or exporting a set
    of dma_bufs as an code:EGLImage or code:gbm_bo, common practice mandates
    that each dma_buf's memory be dedicated (in the sense of
    `VK_KHR_dedicated_allocation`) to the image (though not necessarily
    dedicated to a single plane).
    In particular, neither the GBM documentation nor the EGL extension
    specifications explicitly state this requirement, but in light of common
    practice this is likely due to under-specification rather than
    intentional omission.
    In contrast, `VK_EXT_image_drm_format_modifier` permits, but does not
    require, the implementation to require dedicated allocations for images
    created with ename:VK_IMAGE_TILING_DRM_FORMAT_MODIFIER_EXT.

  - *Separation of image creation and memory allocation.* When importing a
    set of dma_bufs as an code:EGLImage or code:gbm_bo, EGL and GBM create
    the image resource and bind it to memory (the dma_bufs) simultaneously.
    This allows EGL and GBM to query each dma_buf's size during image
    creation.
    In Vulkan, image creation and memory allocation are independent unless a
    dedicated allocation is used (as in `VK_KHR_dedicated_allocation`).
    Therefore, without requiring dedicated allocation, Vulkan cannot query
    the size of each dma_buf (or other external handle) when calculating the
    image's memory layout.
    Even if dedication allocation were required, Vulkan cannot calculate the
    image's memory layout until after the image is bound to its dma_ufs.

The above differences complicate the potential inference of plane size in
Vulkan.
Consider the following problematic cases:

.Problematic Plane Size Calculations

  - *Padding.* Some plane of the image may require implementation-dependent
    padding.

  - *Metadata.* For some _modifiers_, the image may have a metadata plane
    which requires a non-trivial calculation to determine its size.

  - *Mipmapped, array, and 3D images.* The implementation may support
    ename:VK_IMAGE_TILING_DRM_FORMAT_MODIFIER_EXT for images whose
    pname:mipLevels, pname:arrayLayers, or pname:depth is greater than 1.
    For such images with certain _modifiers_, the calculation of each
    plane's size may be non-trivial.

However, an application-provided plane size solves none of the above
problems.

For simplicity, consider an external image with a single memory plane.
The implementation is obviously capable calculating the image's size when
its tiling is ename:VK_IMAGE_TILING_OPTIMAL.
Likewise, any reasonable implementation is capable of calculating the
image's size when its tiling uses a supported _modifier_.

Suppose that the external image's size is smaller than the
implementation-calculated size.
If the application provided the external image's size to
flink:vkCreateImage, the implementation would observe the mismatched size
and recognize its inability to comprehend the external image's layout
(unless the implementation used the application-provided size to select a
refinement of the tiling layout indicated by the _modifier_, which is
strongly discouraged).
The implementation would observe the conflict, and reject image creation
with ename:VK_ERROR_INVALID_DRM_FORMAT_MODIFIER_PLANE_LAYOUT_EXT.
On the other hand, if the application did not provide the external image's
size to flink:vkCreateImage, then the application would observe after
calling flink:vkGetImageMemoryRequirements that the external image's size is
less than the size required by the implementation.
The application would observe the conflict and refuse to bind the
sname:VkImage to the external memory.
In both cases, the result is explicit failure.

Suppose that the external image's size is larger than the
implementation-calculated size.
If the application provided the external image's size to
flink:vkCreateImage, for reasons similar to above the implementation would
observe the mismatched size and recognize its inability to comprehend the
image data residing in the extra size.
The implementation, however, must assume that image data resides in the
entire size provided by the application.
The implementation would observe the conflict and reject image creation with
ename:VK_ERROR_INVALID_DRM_FORMAT_MODIFIER_PLANE_LAYOUT_EXT.
On the other hand, if the application did not provide the external image's
size to flink:vkCreateImage, then the application would observe after
calling flink:vkGetImageMemoryRequirements that the external image's size is
larger than the implementation-usable size.
The application would observe the conflict and refuse to bind the
sname:VkImage to the external memory.
In both cases, the result is explicit failure.

Therefore, an application-provided size provides no benefit, and this
extension should not require it.
This decision renders slink:VkSubresourceLayout::pname:size an unused field
during image creation, and thus introduces a risk that implementations may
require applications to submit sideband creation parameters in the unused
field.
To prevent implementations from relying on sideband data, this extension
_requires_ the application to set pname:size to 0.
--

==== References

  . [[VK_EXT_image_drm_format_modifier-fn1]]
    https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import.txt[`EGL_EXT_image_dma_buf_import`]
  . [[VK_EXT_image_drm_format_modifier-fn2]]
    https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import_modifiers.txt[`EGL_EXT_image_dma_buf_import_modifiers`]
  . [[VK_EXT_image_drm_format_modifier-fn3]]
    https://www.khronos.org/registry/EGL/extensions/MESA/EGL_MESA_image_dma_buf_export.txt[`EGL_MESA_image_dma_buf_export`]
  . [[VK_EXT_image_drm_format_modifier-fn4]]
    https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/uapi/drm/drm_mode.h?id=refs/tags/v4.10#n392[`struct
    drm_mode_fb_cmd2`]
  . [[VK_EXT_image_drm_format_modifier-fn5]]
    https://cgit.freedesktop.org/mesa/mesa/tree/src/gbm/main/gbm.h?id=refs/tags/mesa-18.0.0-rc1[`gbm.h`]

==== Version History

  * Revision 1, 2018-08-29 (Chad Versace)
    - First stable revision
