Linux DMA-BUF

Factory for creating dmabuf-based wl_buffers

Following the interfaces from: https://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_import.txt https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import_modifiers.txt and the Linux DRM sub-system's AddFb2 ioctl.

This interface offers ways to create generic dmabuf-based wl_buffers.

Clients can use the get_surface_feedback request to get dmabuf feedback for a particular surface. If the client wants to retrieve feedback not tied to a surface, they can use the get_default_feedback request.

The following are required from clients:

- Clients must ensure that either all data in the dma-buf is coherent for all subsequent read access or that coherency is correctly handled by the underlying kernel-side dma-buf implementation.

- Don't make any more attachments after sending the buffer to the compositor. Making more attachments later increases the risk of the compositor not being able to use (re-import) an existing dmabuf-based wl_buffer.

The underlying graphics stack must ensure the following:

- The dmabuf file descriptors relayed to the server will stay valid for the whole lifetime of the wl_buffer. This means the server may at any time use those fds to import the dmabuf into any kernel sub-system that might accept it.

However, when the underlying graphics stack fails to deliver the promise, because of e.g. a device hot-unplug which raises internal errors, after the wl_buffer has been successfully created the compositor must not raise protocol errors to the client when dmabuf import later fails.

To create a wl_buffer from one or more dmabufs, a client creates a zwp_linux_dmabuf_params_v1 object with a zwp_linux_dmabuf_v1.create_params request. All planes required by the intended format are added with the 'add' request. Finally, a 'create' or 'create_immed' request is issued, which has the following outcome depending on the import success.

The 'create' request, - on success, triggers a 'created' event which provides the final wl_buffer to the client. - on failure, triggers a 'failed' event to convey that the server cannot use the dmabufs received from the client.

For the 'create_immed' request, - on success, the server immediately imports the added dmabufs to create a wl_buffer. No event is sent from the server in this case. - on failure, the server can choose to either: - terminate the client by raising a fatal error. - mark the wl_buffer as failed, and send a 'failed' event to the client. If the client uses a failed wl_buffer as an argument to any request, the behaviour is compositor implementation-defined.

For all DRM formats and unless specified in another protocol extension, pre-multiplied alpha is used for pixel values.

Unless specified otherwise in another protocol extension, implicit synchronization is used. In other words, compositors and clients must wait and signal fences implicitly passed via the DMA-BUF's reservation mechanism.

destroy()
Unbind the factory

Objects created through this interface, especially wl_buffers, will remain valid.

create_params(params_id: new_id<zwp_linux_buffer_params_v1>)
Argument
Type
Description
params_idnew_id<zwp_linux_buffer_params_v1>
the new temporary
Create a temporary object for buffer parameters

This temporary object is used to collect multiple dmabuf handles into a single batch to create a wl_buffer. It can only be used once and should be destroyed after a 'created' or 'failed' event has been received.

get_default_feedback(id: new_id<zwp_linux_dmabuf_feedback_v1>)
Argument
Type
Description
idnew_id<zwp_linux_dmabuf_feedback_v1>
Get default feedback

This request creates a new wp_linux_dmabuf_feedback object not bound to a particular surface. This object will deliver feedback about dmabuf parameters to use if the client doesn't support per-surface feedback (see get_surface_feedback).

get_surface_feedback(id: new_id<zwp_linux_dmabuf_feedback_v1>, surface: object<wl_surface>)
Argument
Type
Description
idnew_id<zwp_linux_dmabuf_feedback_v1>
surfaceobject<wl_surface>
Get feedback for a surface

This request creates a new wp_linux_dmabuf_feedback object for the specified wl_surface. This object will deliver feedback about dmabuf parameters to use for buffers attached to this surface.

If the surface is destroyed before the wp_linux_dmabuf_feedback object, the feedback object becomes inert.

format(format: uint)
Argument
Type
Description
formatuint
DRM_FORMAT code
Supported buffer format

This event advertises one buffer format that the server supports. All the supported formats are advertised once when the client binds to this interface. A roundtrip after binding guarantees that the client has received all supported formats.

For the definition of the format codes, see the zwp_linux_buffer_params_v1::create request.

Starting version 4, the format event is deprecated and must not be sent by compositors. Instead, use get_default_feedback or get_surface_feedback.

modifier(format: uint, modifier_hi: uint, modifier_lo: uint)
Argument
Type
Description
formatuint
DRM_FORMAT code
modifier_hiuint
high 32 bits of layout modifier
modifier_louint
low 32 bits of layout modifier
Supported buffer format modifier

This event advertises the formats that the server supports, along with the modifiers supported for each format. All the supported modifiers for all the supported formats are advertised once when the client binds to this interface. A roundtrip after binding guarantees that the client has received all supported format-modifier pairs.

For legacy support, DRM_FORMAT_MOD_INVALID (that is, modifier_hi == 0x00ffffff and modifier_lo == 0xffffffff) is allowed in this event. It indicates that the server can support the format with an implicit modifier. When a plane has DRM_FORMAT_MOD_INVALID as its modifier, it is as if no explicit modifier is specified. The effective modifier will be derived from the dmabuf.

A compositor that sends valid modifiers and DRM_FORMAT_MOD_INVALID for a given format supports both explicit modifiers and implicit modifiers.

For the definition of the format and modifier codes, see the zwp_linux_buffer_params_v1::create and zwp_linux_buffer_params_v1::add requests.

Starting version 4, the modifier event is deprecated and must not be sent by compositors. Instead, use get_default_feedback or get_surface_feedback.


Parameters for creating a dmabuf-based wl_buffer

This temporary object is a collection of dmabufs and other parameters that together form a single logical buffer. The temporary object may eventually create one wl_buffer unless cancelled by destroying it before requesting 'create'.

Single-planar formats only require one dmabuf, however multi-planar formats may require more than one dmabuf. For all formats, an 'add' request must be called once per plane (even if the underlying dmabuf fd is identical).

You must use consecutive plane indices ('plane_idx' argument for 'add') from zero to the number of planes used by the drm_fourcc format code. All planes required by the format must be given exactly once, but can be given in any order. Each plane index can be set only once.

destroy()
Delete this object, used or not

Cleans up the temporary data sent to the server for dmabuf-based wl_buffer creation.

add(fd: fd, plane_idx: uint, offset: uint, stride: uint, modifier_hi: uint, modifier_lo: uint)
Argument
Type
Description
fdfd
dmabuf fd
plane_idxuint
plane index
offsetuint
offset in bytes
strideuint
stride in bytes
modifier_hiuint
high 32 bits of layout modifier
modifier_louint
low 32 bits of layout modifier
Add a dmabuf to the temporary set

This request adds one dmabuf to the set in this zwp_linux_buffer_params_v1.

The 64-bit unsigned value combined from modifier_hi and modifier_lo is the dmabuf layout modifier. DRM AddFB2 ioctl calls this the fb modifier, which is defined in drm_mode.h of Linux UAPI. This is an opaque token. Drivers use this token to express tiling, compression, etc. driver-specific modifications to the base format defined by the DRM fourcc code.

Starting from version 4, the invalid_format protocol error is sent if the format + modifier pair was not advertised as supported.

Starting from version 5, the invalid_format protocol error is sent if all planes don't use the same modifier.

This request raises the PLANE_IDX error if plane_idx is too large. The error PLANE_SET is raised if attempting to set a plane that was already set.

create(width: int, height: int, format: uint, flags: uint<zwp_linux_buffer_params_v1.flags>)
Argument
Type
Description
widthint
base plane width in pixels
heightint
base plane height in pixels
formatuint
DRM_FORMAT code
flagsuint<zwp_linux_buffer_params_v1.flags>
see enum flags
Create a wl_buffer from the given dmabufs

This asks for creation of a wl_buffer from the added dmabuf buffers. The wl_buffer is not created immediately but returned via the 'created' event if the dmabuf sharing succeeds. The sharing may fail at runtime for reasons a client cannot predict, in which case the 'failed' event is triggered.

The 'format' argument is a DRM_FORMAT code, as defined by the libdrm's drm_fourcc.h. The Linux kernel's DRM sub-system is the authoritative source on how the format codes should work.

The 'flags' is a bitfield of the flags defined in enum "flags". 'y_invert' means the that the image needs to be y-flipped.

Flag 'interlaced' means that the frame in the buffer is not progressive as usual, but interlaced. An interlaced buffer as supported here must always contain both top and bottom fields. The top field always begins on the first pixel row. The temporal ordering between the two fields is top field first, unless 'bottom_first' is specified. It is undefined whether 'bottom_first' is ignored if 'interlaced' is not set.

This protocol does not convey any information about field rate, duration, or timing, other than the relative ordering between the two fields in one buffer. A compositor may have to estimate the intended field rate from the incoming buffer rate. It is undefined whether the time of receiving wl_surface.commit with a new buffer attached, applying the wl_surface state, wl_surface.frame callback trigger, presentation, or any other point in the compositor cycle is used to measure the frame or field times. There is no support for detecting missed or late frames/fields/buffers either, and there is no support whatsoever for cooperating with interlaced compositor output.

The composited image quality resulting from the use of interlaced buffers is explicitly undefined. A compositor may use elaborate hardware features or software to deinterlace and create progressive output frames from a sequence of interlaced input buffers, or it may produce substandard image quality. However, compositors that cannot guarantee reasonable image quality in all cases are recommended to just reject all interlaced buffers.

Any argument errors, including non-positive width or height, mismatch between the number of planes and the format, bad format, bad offset or stride, may be indicated by fatal protocol errors: INCOMPLETE, INVALID_FORMAT, INVALID_DIMENSIONS, OUT_OF_BOUNDS.

Dmabuf import errors in the server that are not obvious client bugs are returned via the 'failed' event as non-fatal. This allows attempting dmabuf sharing and falling back in the client if it fails.

This request can be sent only once in the object's lifetime, after which the only legal request is destroy. This object should be destroyed after issuing a 'create' request. Attempting to use this object after issuing 'create' raises ALREADY_USED protocol error.

It is not mandatory to issue 'create'. If a client wants to cancel the buffer creation, it can just destroy this object.

create_immed(buffer_id: new_id<wl_buffer>, width: int, height: int, format: uint, flags: uint<zwp_linux_buffer_params_v1.flags>)
Argument
Type
Description
buffer_idnew_id<wl_buffer>
id for the newly created wl_buffer
widthint
base plane width in pixels
heightint
base plane height in pixels
formatuint
DRM_FORMAT code
flagsuint<zwp_linux_buffer_params_v1.flags>
see enum flags
Immediately create a wl_buffer from the given dmabufs

This asks for immediate creation of a wl_buffer by importing the added dmabufs.

In case of import success, no event is sent from the server, and the wl_buffer is ready to be used by the client.

Upon import failure, either of the following may happen, as seen fit by the implementation: - the client is terminated with one of the following fatal protocol errors: - INCOMPLETE, INVALID_FORMAT, INVALID_DIMENSIONS, OUT_OF_BOUNDS, in case of argument errors such as mismatch between the number of planes and the format, bad format, non-positive width or height, or bad offset or stride. - INVALID_WL_BUFFER, in case the cause for failure is unknown or plaform specific. - the server creates an invalid wl_buffer, marks it as failed and sends a 'failed' event to the client. The result of using this invalid wl_buffer as an argument in any request by the client is defined by the compositor implementation.

This takes the same arguments as a 'create' request, and obeys the same restrictions.

created(buffer: new_id<wl_buffer>)
Argument
Type
Description
buffernew_id<wl_buffer>
the newly created wl_buffer
Buffer creation succeeded

This event indicates that the attempted buffer creation was successful. It provides the new wl_buffer referencing the dmabuf(s).

Upon receiving this event, the client should destroy the zwp_linux_buffer_params_v1 object.

failed()
Buffer creation failed

This event indicates that the attempted buffer creation has failed. It usually means that one of the dmabuf constraints has not been fulfilled.

Upon receiving this event, the client should destroy the zwp_linux_buffer_params_v1 object.

Argument
Value
Description
already_used0
the dmabuf_batch object has already been used to create a wl_buffer
plane_idx1
plane index out of bounds
plane_set2
the plane index was already set
incomplete3
missing or too many planes to create a buffer
invalid_format4
format not supported
invalid_dimensions5
invalid width or height
out_of_bounds6
offset + stride * height goes out of dmabuf bounds
invalid_wl_buffer7
invalid wl_buffer resulted from importing dmabufs via the create_immed request on given buffer_params
Argument
Value
Description
y_invert1
contents are y-inverted
interlaced2
content is interlaced
bottom_first4
bottom field first

Dmabuf feedback

This object advertises dmabuf parameters feedback. This includes the preferred devices and the supported formats/modifiers.

The parameters are sent once when this object is created and whenever they change. The done event is always sent once after all parameters have been sent. When a single parameter changes, all parameters are re-sent by the compositor.

Compositors can re-send the parameters when the current client buffer allocations are sub-optimal. Compositors should not re-send the parameters if re-allocating the buffers would not result in a more optimal configuration. In particular, compositors should avoid sending the exact same parameters multiple times in a row.

The tranche_target_device and tranche_formats events are grouped by tranches of preference. For each tranche, a tranche_target_device, one tranche_flags and one or more tranche_formats events are sent, followed by a tranche_done event finishing the list. The tranches are sent in descending order of preference. All formats and modifiers in the same tranche have the same preference.

To send parameters, the compositor sends one main_device event, tranches (each consisting of one tranche_target_device event, one tranche_flags event, tranche_formats events and then a tranche_done event), then one done event.

destroy()
Destroy the feedback object

Using this request a client can tell the server that it is not going to use the wp_linux_dmabuf_feedback object anymore.

done()
All feedback has been sent

This event is sent after all parameters of a wp_linux_dmabuf_feedback object have been sent.

This allows changes to the wp_linux_dmabuf_feedback parameters to be seen as atomic, even if they happen via multiple events.

format_table(fd: fd, size: uint)
Argument
Type
Description
fdfd
table file descriptor
sizeuint
table size, in bytes
Format and modifier table

This event provides a file descriptor which can be memory-mapped to access the format and modifier table.

The table contains a tightly packed array of consecutive format + modifier pairs. Each pair is 16 bytes wide. It contains a format as a 32-bit unsigned integer, followed by 4 bytes of unused padding, and a modifier as a 64-bit unsigned integer. The native endianness is used.

The client must map the file descriptor in read-only private mode.

Compositors are not allowed to mutate the table file contents once this event has been sent. Instead, compositors must create a new, separate table file and re-send feedback parameters. Compositors are allowed to store duplicate format + modifier pairs in the table.

main_device(device: array)
Argument
Type
Description
devicearray
device dev_t value
Preferred main device

This event advertises the main device that the server prefers to use when direct scan-out to the target device isn't possible. The advertised main device may be different for each wp_linux_dmabuf_feedback object, and may change over time.

There is exactly one main device. The compositor must send at least one preference tranche with tranche_target_device equal to main_device.

Clients need to create buffers that the main device can import and read from, otherwise creating the dmabuf wl_buffer will fail (see the wp_linux_buffer_params.create and create_immed requests for details). The main device will also likely be kept active by the compositor, so clients can use it instead of waking up another device for power savings.

In general the device is a DRM node. The DRM node type (primary vs. render) is unspecified. Clients must not rely on the compositor sending a particular node type. Clients cannot check two devices for equality by comparing the dev_t value.

If explicit modifiers are not supported and the client performs buffer allocations on a different device than the main device, then the client must force the buffer to have a linear layout.

tranche_done()
A preference tranche has been sent

This event splits tranche_target_device and tranche_formats events in preference tranches. It is sent after a set of tranche_target_device and tranche_formats events; it represents the end of a tranche. The next tranche will have a lower preference.

tranche_target_device(device: array)
Argument
Type
Description
devicearray
device dev_t value
Target device

This event advertises the target device that the server prefers to use for a buffer created given this tranche. The advertised target device may be different for each preference tranche, and may change over time.

There is exactly one target device per tranche.

The target device may be a scan-out device, for example if the compositor prefers to directly scan-out a buffer created given this tranche. The target device may be a rendering device, for example if the compositor prefers to texture from said buffer.

The client can use this hint to allocate the buffer in a way that makes it accessible from the target device, ideally directly. The buffer must still be accessible from the main device, either through direct import or through a potentially more expensive fallback path. If the buffer can't be directly imported from the main device then clients must be prepared for the compositor changing the tranche priority or making wl_buffer creation fail (see the wp_linux_buffer_params.create and create_immed requests for details).

If the device is a DRM node, the DRM node type (primary vs. render) is unspecified. Clients must not rely on the compositor sending a particular node type. Clients cannot check two devices for equality by comparing the dev_t value.

This event is tied to a preference tranche, see the tranche_done event.

tranche_formats(indices: array)
Argument
Type
Description
indicesarray
array of 16-bit indexes
Supported buffer format modifier

This event advertises the format + modifier combinations that the compositor supports.

It carries an array of indices, each referring to a format + modifier pair in the last received format table (see the format_table event). Each index is a 16-bit unsigned integer in native endianness.

For legacy support, DRM_FORMAT_MOD_INVALID is an allowed modifier. It indicates that the server can support the format with an implicit modifier. When a buffer has DRM_FORMAT_MOD_INVALID as its modifier, it is as if no explicit modifier is specified. The effective modifier will be derived from the dmabuf.

A compositor that sends valid modifiers and DRM_FORMAT_MOD_INVALID for a given format supports both explicit modifiers and implicit modifiers.

Compositors must not send duplicate format + modifier pairs within the same tranche or across two different tranches with the same target device and flags.

This event is tied to a preference tranche, see the tranche_done event.

For the definition of the format and modifier codes, see the wp_linux_buffer_params.create request.

Argument
Type
Description
flagsuint<zwp_linux_dmabuf_feedback_v1.tranche_flags>
tranche flags
Tranche flags

This event sets tranche-specific flags.

The scanout flag is a hint that direct scan-out may be attempted by the compositor on the target device if the client appropriately allocates a buffer. How to allocate a buffer that can be scanned out on the target device is implementation-defined.

This event is tied to a preference tranche, see the tranche_done event.

tranche_flags { scanout } 
Argument
Value
Description
scanout1
direct scan-out tranche

Compositor Support

Mutter
Mutter
46
KWin
KWin
6.0
Sway
Sway
1.9
Hyprland
Hyprland
0.35
Weston
Weston
13
Mir
Mir
2.17
GameScope
GameScope
3.14.3
zwp_linux_dmabuf_v1
5
4
4
4
4
3
4

Copyright © 2014, 2015 Collabora, Ltd.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice (including the next paragraph) shall be included in all copies or substantial portions of the Software.

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