[PATCH] doc: gpu: Add document describing buffer exchange

[Robert Beckett]

On 05/09/2021 13:27, Daniel Stone wrote:
> Since there's a lot of confusion around this, document both the rules
> and the best practice around negotiating, allocating, importing, and
> using buffers when crossing context/process/device/subsystem boundaries.
> 
> This ties up all of dmabuf, formats and modifiers, and their usage.
> 
> Signed-off-by: Daniel Stone <daniels at collabora.com>
> ---
> 
> This is just a quick first draft, inspired by:
>    https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3197#note_1048637
> 
> It's not complete or perfect, but I'm off to eat a roast then have a
> nice walk in the sun, so figured it'd be better to dash it off rather
> than let it rot on my hard drive.
> 
> 
>   .../gpu/exchanging-pixel-buffers.rst          | 285 ++++++++++++++++++
>   Documentation/gpu/index.rst                   |   1 +
>   2 files changed, 286 insertions(+)
>   create mode 100644 Documentation/gpu/exchanging-pixel-buffers.rst
> 
> diff --git a/Documentation/gpu/exchanging-pixel-buffers.rst b/Documentation/gpu/exchanging-pixel-buffers.rst
> new file mode 100644
> index 000000000000..75c4de13d5c8
> --- /dev/null
> +++ b/Documentation/gpu/exchanging-pixel-buffers.rst
> @@ -0,0 +1,285 @@
> +.. Copyright 2021 Collabora Ltd.
> +
> +========================
> +Exchanging pixel buffers
> +========================
> +
> +As originally designed, the Linux graphics subsystem had extremely limited
> +support for sharing pixel-buffer allocations between processes, devices, and
> +subsystems. Modern systems require extensive integration between all three
> +classes; this document details how applications and kernel subsystems should
> +approach this sharing for two-dimensional image data.
> +
> +It is written with reference to the DRM subsystem for GPU and display devices,
> +V4L2 for media devices, and also to Vulkan, EGL and Wayland, for userspace
> +support, however any other subsystems should also follow this design and advice.
> +
> +
> +Formats and modifiers
> +=====================
> +
> +Each buffer must have an underlying format. This format describes the data which
> +can be stored and loaded for each pixel. Although each subsystem has its own
> +format descriptions (e.g. V4L2 and fbdev), the `DRM_FORMAT_*` tokens should be
> +reused wherever possible, as they are the standard descriptions used for
> +interchange.
> +
> +Each `DRM_FORMAT_*` token describes the per-pixel data available, in terms of
> +the translation between one or more pixels in memory, and the color data
> +contained within that memory. The number and type of color channels are
> +described: whether they are RGB or YUV, integer or floating-point, the size
> +of each channel and their locations within the pixel memory, and the
> +relationship between color planes.
> +
> +For example, `DRM_FORMAT_ARGB8888` describes a format in which each pixel has a
> +single 32-bit value in memory. Alpha, red, green, and blue, color channels are
> +available at 8-byte precision per channel, ordered respectively from most to

think you meant 8-bit there

> +least significant bits in little-endian storage. As a more complex example,
> +`DRM_FORMAT_NV12` describes a format in which luma and chroma YUV samples are
> +stored in separate memory planes, where the chroma plane is stored at half the
> +resolution in both dimensions (i.e. one U/V chroma sample is stored for each 2x2
> +pixel grouping).
> +
> +Format modifiers describe a translation mechanism between these per-pixel memory
> +samples, and the actual memory storage for the buffer. The most straightforward
> +modifier is `DRM_FORMAT_MOD_LINEAR`, describing a scheme in which each pixel has
> +contiguous storage beginning at (0,0); each pixel's location in memory will be
> +`base + (y * stride) + (x * bpp)`. This is considered the baseline interchange
> +format, and most convenient for CPU access.
> +
> +Modern hardware employs much more sophisticated access mechanisms, typically
> +making use of tiled access and possibly also compression. For example, the
> +`DRM_FORMAT_MOD_VIVANTE_TILED` modifier describes memory storage where pixels
> +are stored in 4x4 blocks arranged in row-major ordering, i.e. the first tile in
> +memory stores pixels (0,0) to (3,3) inclusive, and the second tile in memory
> +stores pixels (4,0) to (7,3) inclusive.
> +
> +Some modifiers may modify the number of memory buffers required to store the
> +data; for example, the `I915_FORMAT_MOD_Y_TILED_CCS` modifier adds a second
> +memory buffer to RGB formats in which it stores data about the status of every
> +tile, notably including whether the tile is fully populated with pixel data, or
> +can be expanded from a single solid color.
> +
> +These extended layouts are highly vendor-specific, and even specific to
> +particular generations or configurations of devices per-vendor. For this reason,
> +support of modifiers must be explicitly enumerated and negotiated by all users
> +in order to ensure a compatible and optimal pipeline, as discussed below.
> +
> +
> +Dimensions and size
> +===================
> +
> +Each pixel buffer must be accompanied by logical pixel dimensions. This refers
> +to the number of unique samples which can be extracted from, or stored to, the
> +underlying memory storage. For example, even though a 1920x1080
> +`DRM_FORMAT_NV12` buffer has a luma plane containing 1920x1080 samples for the Y
> +component, and 960x540 samples for the U and V components, the overall buffer is
> +still described as having dimensions of 1920x1080.
> +
> +The in-memory storage of a buffer is not guaranteed to begin immediately at the
> +base address of the underlying memory, nor is it guaranteed that the memory
> +storage is tightly clipped to either dimension.
> +
> +Each plane must therefore be described with an `offset` in bytes, which will be
> +added to the base address of the memory storage before performing any per-pixel
> +calculations. This may be used to combine multiple planes into a single pixel
> +buffer; for example, `DRM_FORMAT_NV12` may be stored in a single memory buffer
> +where the luma plane's storage begins immediately at the start of the buffer
> +with an offset of 0, and the chroma plane's storage begins after the offset of
> +the luma plane as expressed through its offset.
> +
> +Each plane must also have a `stride` in bytes, expressing the offset in memory
> +between two contiguous scanlines. For example, a `DRM_FORMAT_MOD_LINEAR` buffer
> +with dimensions of 1000x1000 may have been allocated as if it were 1024x1000, in
> +order to allow for aligned access patterns. In this case, the buffer will still
> +be described with a width of 1000, however the stride will be `1024 * bpp`,
> +indicating that there are 24 pixels at the positive extreme of the x axis whose
> +values are not significant.
> +
> +Buffers may also be padded further in the y dimension, simply by allocating a
> +larger area than would ordinarily be required. For example, many media decoders
> +are not able to natively output buffers of height 1080, but instead require an
> +effective height of 1088 pixels. In this case, the buffer continues to be
> +described as having a height of 1080, with the memory allocation for each buffer
> +being increased to account for the extra padding.
> +
> +
> +Enumeration
> +===========
> +
> +Every user of pixel buffers must be able to enumerate a set of supported formats
> +and modifiers, described together. Within KMS, this is achieved with the
> +`IN_FORMATS` property on each DRM plane, listing the supported DRM formats, and
> +the modifiers supported for each format. In userspace, this is supported through
> +the `EGL_EXT_image_dma_buf_import_modifiers` extension entrypoints for EGL, the
> +`VK_EXT_image_drm_format_modifier` extension for Vulkan, and the
> +`zwp_linux_dmabuf_v1` extension for Wayland.
> +
> +Each of these interfaces allows users to query a set of supported
> +format+modifier combinations.
> +
> +
> +Negotiation
> +===========
> +
> +It is the responsibility of userspace to negotiate an acceptable format+modifier
> +combination for its usage. This is performed through a simple intersection of
> +lists. For example, if a user wants to use Vulkan to render an image to be
> +displayed on a KMS plane, it must:
> +  - query KMS for the `IN_FORMATS` property for the given plane
> +  - query Vulkan for the supported formats for its physical device
> +  - intersect these formats to determine the most appropriate one
> +  - for this format, intersect the lists of supported modifiers for both KMS and
> +    Vulkan, to obtain a final list of acceptable modifiers for that format
> +
> +This intersection must be performed for all usages. For example, if the user
> +also wishes to encode the image to a video stream, it must query the media API
> +it intends to use for encoding for the set of modifiers it supports, and
> +additionally intersect against this list.
> +
> +If the intersection of all lists is an empty list, it is not possible to share
> +buffers in this way, and an alternate strategy must be considered (e.g. using
> +CPU access routines to copy data between the different uses, with the
> +corresponding performance cost).
> +
> +The resulting modifier list is unsorted; the order is not significant.
> +
> +
> +Allocation
> +==========
> +
> +Once userspace has determined an appropriate format, and corresponding list of
> +acceptable modifiers, it must allocate the buffer. As there is no universal
> +buffer-allocation interface available at either kernel or userspace level, the
> +client makes an arbitrary choice of allocation interface such as Vulkan, GBM, or
> +a media API.
> +
> +Each allocation request must take, at a minimum: the pixel format, a list of
> +acceptable modifiers, and the buffer's width and height. Each API may extend
> +this set of properties in different ways, such as allowing allocation in more
> +than two dimensions, intended usage patterns, etc.
> +
> +The component which allocates the buffer will make an arbitrary choice of what
> +it considers the 'best' modifier within the acceptable list for the requested
> +allocation, any padding required, and further properties of the underlying
> +memory buffers such as whether they are stored in system or device-specific
> +memory, whether or not they are physically contiguous, and their cache mode.
> +These properties of the memory buffer are not visible to userspace, however the
> +`dma-heaps` API is an effort to address this.
> +
> +After allocation, the client must query the allocator to determine the actual
> +modifier selected for the buffer, as well as the per-plane offset and stride.
> +Allocators are not permitted to vary the format in use, to select a modifier not
> +provided within the acceptable list, nor to vary the pixel dimensions other than
> +the padding expressed through offset, stride, and size.
> +
> +
> +Import
> +======
> +
> +To use a buffer within a different context, device, or subsystem, the user
> +passes these parameters (format, modifier, width, height, and per-plane offset
> +and stride) to an importing API.
> +
> +Each memory plane is referred to by a buffer handle, which may be unique or
> +duplicated within a buffer. For example, a `DRM_FORMAT_NV12` buffer may have the
> +luma and chroma buffers combined into a single memory buffer by use of the
> +per-plane offset parameters, or they may be completely separate allocations in
> +memory. For this reason, each import and allocation API must provide a separate
> +handle for each plane.
> +
> +Each kernel subsystem has its own types and interfaces for buffer management.
> +DRM uses GEM buffer objects (BOs), V4L2 has its own references, etc. These types
> +are not portable between contexts, processes, devices, or subsystems.
> +
> +To address this, `dma-buf` handles are used as the universal interchange for
> +buffers. Subsystem-specific operations are used to export native buffer handles
> +to a `dma-buf` file descriptor, and to import those file descriptors into a
> +native buffer handle. dma-buf file descriptors can be transferred between
> +contexts, processes, devices, and subsystems.
> +
> +For example, a Wayland media player may use V4L2 to decode a video frame into
> +a `DRM_FORMAT_NV12` buffer. This will result in two memory planes (luma and
> +chroma) being dequeued by the user from V4L2. These planes are then exported to
> +one dma-buf file descriptor per plane, these descriptors are then sent along
> +with the metadata (format, modifier, width, height, per-plane offset and stride)
> +to the Wayland server. The Wayland server will then import these file
> +descriptors as an EGLImage for use through EGL/OpenGL (ES), a VkImage for use
> +through Vulkan, or a `drm_fb` for use through KMS; each of these import
> +operations will take the same metadata and convert the dma-buf file descriptors
> +into their native buffer handles.
> +
> +
> +Implicit modifiers
> +==================
> +
> +The concept of modifiers post-dates all of the subsystems mentioned above. As
> +such, it has been retrofitted into all of these APIs, and in order to ensure
> +backwards compatibility, support is needed for drivers and userspace which do
> +not (yet) support modifiers.
> +
> +As an example, GBM is used to allocate buffers to be shared between EGL for
> +rendering and KMS for display. It has two entrypoints for allocating buffers:
> +`gbm_bo_create` which only takes the format, width, height, and a usage token,
> +and `gbm_bo_create_with_modifiers` which extends this with a list of modifiers.
> +
> +In the latter case, the allocation is as discussed above, being provided with a
> +list of acceptable modifiers that the implementation can choose from (or fail if
> +it is not possible to allocate within those constraints). In the former case
> +where modifiers are not provided, the GBM implementation must make its own
> +choice as to what is likely to be the 'best' layout. Such a choice is entirely
> +implementation-specific: some will internally use tiled layouts which are not
> +CPU-accessible if the implementation decides that is a good idea through
> +whatever heuristic. It is the implementation's responsibility to ensure that
> +this choice is appropriate.
> +
> +To support this case where the layout is not known because there is no awareness
> +of modifiers, a special `DRM_FORMAT_MOD_INVALID` token has been defined. This
> +pseudo-modifier declares that the layout is not known, and that the driver
> +should use its own logic to determine what the underlying layout may be.
> +
> +There are four cases where this token may be used:
> +  - during enumeration, an interface may return `DRM_FORMAT_MOD_INVALID`, either
> +    as the sole member of a modifier list to declare that explicit modifiers are
> +    not supported, or as part of a larger list to declare that implicit modifiers
> +    may be used
> +  - during allocation, a user may supply `DRM_FORMAT_MOD_INVALID`, either as the
> +    sole member of a modifier list (equivalent to not supplying a modifier list
> +    at all) to declare that explicit modifiers are not supported and must not be
> +    used, or as part of a larger list to declare that an allocation using implicit
> +    modifiers is acceptable
> +  - in a post-allocation query, an implementation may return
> +    `DRM_FORMAT_MOD_INVALID` as the modifier of the allocated buffer to declare
> +    that the underlying layout is implementation-defined and that an explicit
> +    modifier description is not available; per the above rules, this may only be
> +    returned when the user has included `DRM_FORMAT_MOD_INVALID` as part of the
> +    list of acceptable modifiers, or not provided a list
> +  - when importing a buffer, the user may supply `DRM_FORMAT_MOD_INVALID` as the
> +    buffer modifier (or not supply a modifier) to indicate that the modifier is
> +    unknown for whatever reason; this is only acceptable when the buffer has
> +    not been allocated with an explicit modifier
> +
> +It follows from this that a buffer chain must be either fully implicit or fully
> +explicit. For example, if a user wishes to allocate a buffer for use between
> +GPU, display, and media, but the media API does not support modifiers, then the
> +user **must not** allocate the buffer with explicit modifiers and attempt to
> +import the buffer into the media API with no modifier, but either perform the
> +allocation using implicit modifiers, or allocate the buffer for media use
> +separately and copy between the two buffers.
> +
> +As one exception to the above, allocations may be 'upgraded' from implicit
> +to explicit modifiers. For example, if the buffer is allocated with
> +`gbm_bo_create` (taking no modifiers), the user may then query the modifier with
> +`gbm_bo_get_modifier` and then use this modifier as an explicit modifier token
> +if a valid modifier is returned.
> +
> +When allocating buffers for exchange between different users and modifiers are
> +not available, implementations are strongly encouraged to use
> +`DRM_FORMAT_MOD_LINEAR` for their allocation, as this is the universal baseline
> +for exchange.
> +
> +Any new users - userspace programs and protocols, kernel subsystems, etc -
> +wishing to exchange buffers must offer interoperability through dma-buf file
> +descriptors for memory planes, DRM format tokens to describe the format, DRM
> +format modifiers to describe the layout in memory, at least width and height for
> +dimensions, and at least offset and stride for each memory plane.
> diff --git a/Documentation/gpu/index.rst b/Documentation/gpu/index.rst
> index b9c1214d8f23..cb12f2654ed7 100644
> --- a/Documentation/gpu/index.rst
> +++ b/Documentation/gpu/index.rst
> @@ -10,6 +10,7 @@ Linux GPU Driver Developer's Guide
>      drm-kms
>      drm-kms-helpers
>      drm-uapi
> +   exchanging-pixel-buffers
>      driver-uapi
>      drm-client
>      drivers
>