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

Tue Nov 9 09:13:36 UTC 2021

On Mon, Nov 08, 2021 at 04:18:22PM -0800, James Jones wrote:
> On 9/6/21 5:28 AM, Simon Ser 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>
> > 
> > Thanks a lot for this write-up! This looks very good to me, a few comments
> > below.
> 
> Agreed, it would be awesome if this were merged somewhere. IMHO, a lot of
> the non-trivial/typo suggestions below could be taken care of as follow-on
> patches, as the content here is better in than out, even if it could be
> clarified a bit.

Seconded on just landing this without trying to perfect it first, because
I was just looking for it and didn't find it anywhere :-/
-Daniel

> 
> Further feedback inline:
> 
> > > ---
> > > 
> > > 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
> > 
> > RST uses double backticks for inline code blocks (applies to the whole document).
> > 
> > > +reused wherever possible, as they are the standard descriptions used for
> > > +interchange.
> > 
> > Maybe mention that the canonical source of formats and modifiers can be found
> > in include/uapi/drm/drm_fourcc.h.
> > 
> > > +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
> > 
> > Pekka uses the term "color value", which I find a bit better than repeating
> > "data".
> > 
> > > +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
> > > +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.
> > 
> > Hm, maybe in more simple terms we could explain that the pixels are stored
> > sequentially row-by-row from the top-left corner to the bottom-right one?
> 
> I wouldn't mention top-left. I'm not clear DRM_FORMAT_MOD_LINEAR excludes
> GL-style bottom-left-oriented images.
> 
> > Maybe we can drop the "base" from the formula and say that each pixel's
> > location in memory will be at offset `y * stride + x * bpp`? Or maybe this is
> > confusing with offset being mentioned below as an additional parameter?
> > 
> > > +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
> > 
> > Hm. I think that mentioning a "memory buffer" here is a bit confusing. It seems
> > like this document is about exchanging "pixel buffers", each being composed of
> > one or more "memory buffers". Maybe we can use "image" instead of "buffer" for
> > the higher-concept of "bunch of pixel values which can be displayed on screen"?
> > That would align with user-space APIs like Vulkan and EGL.
> > 
> > > +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.
> > 
> > Is it a requirement that these two memory planes must be separate memory buffers
> > for I915_FORMAT_MOD_Y_TILED_CCS?
> 
> I think a few decisions need to be made here:
> 
> - Can the general statement be made that separate memory planes (term used
> above) can always be either separate allocations or offsets within one or
> more allocations?
> 
> - Can this auxiliary, modifier-specific data always be used with the same
> semantics as an image plane?
> 
> If the answer to both is yes, I think the best way to describe
> modifier-specific planes would just be to generalize the memory plane
> language above and note "some modifiers introduce additional planes," rather
> than trying to describe auxiliary data as a separate concept. Then, the
> whole discussion about plane offsets in the dimension and size section below
> will clearly apply to auxiliary planes as well.
> 
> > > +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.
> > 
> > "and the chroma plane's storage follows, with its offset set to the size of the
> > preceding luma plane"
> > 
> > is maybe a bit clearer?
> > 
> > > +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
> > 
> > Is "scanline" a better word than "row"? I personally find "row" a bit more
> > descriptive, but maybe "scanline" is technically more accurate.
> 
> scanline is a scanout-specific term IMHO.  I agree "row" is more natural for
> a generalized discussion.
> 
> > > +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
> > 
> > … with the right VkImageUsageFlagBits and VkImageCreateFlagBits set? (Just to
> > make it clear the lists really depend on usage.)
> 
> Agreed. Very subtle and very easy to mess this up given the structure of the
> Vulkan API, so worth pointing out explicitly.
> 
> > > +  - 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.
> 
> I think it's also worth pointing out that because the list is unsorted,
> selection of a final modifier from the resulting list is best left to
> drivers, which may have more information available than the modifier list
> represents on its own. E.g., don't pass in (&modifiers[0], 1), pass in
> (modifiers, <count>) and let the allocator pick its favorite for the
> specified local usage. This is especially true of APIs like Vulkan that
> allow you to specify the local usage in great detail.
> 
> > > +
> > > +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.
> 
> Extending the thought above, once some sort of constraints API is worked
> out, the advice should probably be to allocate using the API with the most
> expressive usage whenever possible, but it's premature to recommend that
> right now.
> 
> > > +
> > > +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.
> > 
> > Vulkan doesn't quite do this, by default it only allows one memory buffer per
> > pixel buffer, and requires the driver to implement an additional extension when
> > the image is "disjoint". Later on, should we mention the inode as a way to
> > figure out whether all DMA-BUFs refer to the same memory buffer? Or maybe it's
> > better to mention that in the Vulkan docs…
> 
> Examining inodes doesn't seem like a Vulkan-specific concept. However, it
> doesn't seem specific to format modifiers either. Should that be mentioned
> in the dmabuf docs?
> 
> > > +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.
> > 
> > It would be nice to mention that even if the intersected modifier list wasn't
> > empty, the import can fail if the buffer doesn't have the right constraints for
> > the intended usage (e.g. bad alignment).
> 
> Agreed. In general, is there any guarantee that device A can import an
> arbitrary dma-buf FD? It seems to me it can fail for various reasons, and in
> addition, mapping it to some specific usage on that device during the import
> itself or some subsequent operation can also fail for the reasons mentioned
> above.
> 
> > > +
> > > +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.
> > 
> > Just to drive the point home, maybe mention explicitly that INVALID != LINEAR?
> 
> Agreed. Obvious to grey beards, easy error when first approaching these
> concepts.
> 
> > > +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
> > 
> > These are good rules, but only Wayland uses them. For instance GBM will ignore
> > INVALID in modifier lists, and iirc KMS will error out if INVALID is supplied
> > at import time?
> 
> While I've observed a few other exceptions myself, this is unfortunate, as
> inconsistency here is what prompted this work in the first place. Would it
> be possible to work towards making these rules universal, or is the current
> behavior too ingrained in the ABIs/protocols/etc.?
> 
> > > +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.
> > 
> > Hm, I wonder if there's a good use-case for this upgrade? I feel like things
> > would be simpler without the exception.
> 
> IIRC, the Tegra Mesa driver relied on this "upgrade" to allocate buffers
> from the non-modifier-aware nouveau gallium driver it layers on top of and
> map them into the modifier-aware tegra-drm driver later, as there was no way
> to share implicit layout information between the nouveau and tegra-drm
> drivers in the kernel. However, I think trying to perform the same "upgrade"
> in the modesetting X driver caused issues with my nouveau format modifier
> patches for reasons I don't recall at the moment.
> 
> > > +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.
> > 
> > Maybe spell out that "users" may mean different APIs or different devices.
> > Sharing a pixel buffer between two separate devices via GBM will only work
> > if USE_LINEAR is provided.
> 
> Yes, I always try to differentiate between actual users (people) and
> applications/components/libraries/etc. (code).
> 
> > > +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
> > > --
> > > 2.31.1
> 
> Thanks again for writing this all up.
> 
> -James

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch