On Sun, Sep 05, 2021 at 01:27:42PM +0100, 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@xxxxxxxxxxxxx> > --- > > 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 ++++++++++++++++++ I think we should stuff this into the dma-buf.rst page instead of hiding it in gpu? Maybe then link to it from everywhere, so from a the prime stuff in gpu, and from whatever doc there is for the v4l import/export ioctls. > 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 > +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. I think it would be good to also cover the opens here. Specifically how to shovel around additional constraints, which are mostly an issue for LINEAR. Stuff like offset/stride alignment and size limitations. Usually once you have a modifier those are all implied (except maybe for size maximums). I think listening these as opens would be good, for completeness. Otherwise looks great to me. -Daniel > 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 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
