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@xxxxxxxxxxxxx> > > > > 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
