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The DRI3 Extension
Version 1.2
2018-02-28
Keith Packard
keithp@keithp.com
Intel Corporation
Daniel Stone
daniels@collabora.com
Collabora
1. Introduction
The DRI3 extension provides mechanisms to translate between direct
rendered buffers and X pixmaps. When combined with the Present extension,
a complete direct rendering solution for hardware-accelerated devices
such as GPUs is provided.
The direct rendered buffers are passed across the protocol via
standard POSIX file descriptor passing mechanisms. On Linux, these
buffers are DMA-BUF objects.
DRI3 also includes a mechanism to translate between Linux Futexes
and X Sync extension Fences. This provides a synchronization mechanism
which can be used to serialize access to shared render buffers.
1.1. Acknowledgments
Eric Anholt <eric@anholt.net>
Dave Airlie <airlied@redhat.com>
Kristian Høgsberg <krh@bitplanet.net>
James Jones <jajones@nvidia.com>
Arthur Huillet <arthur.huillet@free.fr>
Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
❄ ❄ ❄ ❄ ❄ ❄ ❄
2. Data Types
The DRI3 extension uses the RandR extension Provider data type to
select among multiple GPUs on a single screen and the Sync extension
fence object to provide graphics object synchronization.
❄ ❄ ❄ ❄ ❄ ❄ ❄
4. Errors
DRI3 defines no errors.
❄ ❄ ❄ ❄ ❄ ❄ ❄
5. Events
DRI3 defines no events.
❄ ❄ ❄ ❄ ❄ ❄ ❄
6. Protocol Types
DRI3 defines no new protocol types.
❄ ❄ ❄ ❄ ❄ ❄ ❄
7. Extension Initialization
The name of this extension is "DRI3"
┌───
DRI3QueryVersion
client-major-version: CARD32
client-minor-version: CARD32
▶
major-version: CARD32
minor-version: CARD32
└───
The client sends the highest supported version to the server
and the server sends the highest version it supports, but no
higher than the requested version. Major versions changes can
introduce incompatibilities in existing functionality, minor
version changes introduce only backward compatible changes.
It is the clients responsibility to ensure that the server
supports a version which is compatible with its expectations.
Backwards compatible changes included addition of new
requests.
❄ ❄ ❄ ❄ ❄ ❄ ❄
8. Extension Requests
┌───
DRI3Open
drawable: DRAWABLE
provider: PROVIDER
▶
nfd: CARD8
device: FD
└───
Errors: Drawable, Value, Match
This requests that the X server open the direct rendering
device associated with drawable and RandR provider. The
provider must support SourceOutput or SourceOffload.
The file descriptor for the device is returned in
'device'. 'nfd' will be set to one (this is strictly a
convenience for XCB which otherwise would need
request-specific information about how many file descriptors
were associated with this reply).
┌───
DRI3PixmapFromBuffer
pixmap: PIXMAP
drawable: DRAWABLE
size: CARD32
width, height, stride: CARD16
depth, bpp: CARD8
buffer: FD
└───
Errors: Alloc, Drawable, IDChoice, Value, Match
Creates a pixmap for the direct rendering object associated
with 'buffer' and the screen associated with 'drawable'.
Changes to pixmap will be visible in that direct rendered
object and changes to the direct rendered object will be
visible in the pixmap.
'size' specifies the total size of the buffer bytes. 'width',
'height' describe the geometry (in pixels) of the underlying
buffer. 'stride' specifies the number of bytes per scanline in
the buffer. The pixels within the buffer are not required to
be arranged in a simple linear fashion, but 'size' will be at
least 'height' * 'stride'.
Precisely how any additional information about the buffer is
shared is outside the scope of this extension.
If buffer cannot be used with the screen associated with
drawable, a Match error is returned.
If depth or bpp are not supported by the screen, a Value error
is returned.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
┌───
DRI3BufferFromPixmap
pixmap: PIXMAP
▶
nfd: CARD8
size: CARD32
width, height, stride: CARD16
depth, bpp: CARD8
buffer: FD
└───
Errors: Pixmap, Match
Pass back a direct rendering object associated with
pixmap. Changes to pixmap will be visible in that
direct rendered object and changes to the direct rendered
object will be visible in the pixmap.
'size' specifies the total size of the buffer bytes. 'width',
'height' describe the geometry (in pixels) of the underlying
buffer. 'stride' specifies the number of bytes per scanline in
the buffer. The pixels within the buffer are not required to
be arranged in a simple linear fashion, but 'size' will be at
least 'height' * 'stride'.
Precisely how any additional information about the buffer is
shared is outside the scope of this extension.
If buffer cannot be used with the screen associated with
drawable, a Match error is returned.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
┌───
DRI3FenceFromFD
drawable: DRAWABLE
fence: FENCE
initially-triggered: BOOL
fd: FD
└───
Errors: IDchoice, Drawable
Creates a Sync extension Fence that provides the regular Sync
extension semantics along with a file descriptor that provides
a device-specific mechanism to manipulate the fence directly.
Details about the mechanism used with this file descriptor are
outside the scope of the DRI3 extension.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
┌───
DRI3FDFromFence
drawable: DRAWABLE
fence: FENCE
▶
fd: FD
└───
Errors: IDchoice, Drawable, Match
Given a Sync extension Fence that provides the regular Sync
extension semantics, returns a file descriptor that provides a
device-specific mechanism to manipulate the fence directly.
Details about the mechanism used with this file descriptor are
outside the scope of the DRI3 extension. 'drawable' must be
associated with a direct rendering device that 'fence' can
work with, otherwise a Match error results.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
┌───
DRI3GetSupportedModifiers
window: WINDOW
depth: CARD8
bpp: CARD8
▶
num_window_modifiers: CARD32
num_screen_modifiers: CARD32
window_modifiers: ListOfCARD64
screen_modifiers: ListOfCARD64
└───
Errors: Window, Match
Return supported DRM FourCC modifiers for the specified
'window'.
The first list of 'window_modifiers' contains a set of
modifiers which the server considers optimal for the window's
current configuration. Using these modifiers to allocate, even
if locally suboptimal to the client driver, may result in a
more optimal display pipeline, e.g. by avoiding composition.
The second list of 'screen_modifiers', is the total set of
modifiers which are acceptable for use on the Screen associated
with 'window'. This set of modifiers will not change over the
lifetime of the client. Using this set of modifiers to allocate
may not result in a globally optimal pipeline, if separate
'window_modifiers' are available.
It is expected that a client calling this request will obtain
the modifiers for a particular window, allocate buffers using
the preferred modifier set as described above, create a Pixmap
referring to the storage of those buffers using the
DRI3BuffersFromPixmap request, then make the content visible
in the storage of those buffers visible with a request such as
the Present extension's PresentPixmap.
The meaning of any modifier is canonically defined in
drm_fourcc.h.
┌───
DRI3PixmapFromBuffers
pixmap: PIXMAP
window: WINDOW
num_buffers: CARD8
width, height: CARD16
stride0, offset0: CARD32
stride1, offset1: CARD32
stride2, offset2: CARD32
stride3, offset3: CARD32
depth, bpp: CARD8
modifier: CARD64
buffers: ListOfFD
└───
Errors: Alloc, Window, IDChoice, Value, Match
Creates a pixmap for the direct rendering object associated
with 'buffers' and the screen associated with 'window'.
Changes to pixmap will be visible in that direct rendered
object and changes to the direct rendered object will be
visible in the pixmap. The pixmap will be available for
presentation to the window.
In contrast to PixmapFromBuffer, multiple buffers may be
combined to specify a single logical source for pixel
sampling: 'num_buffers' may be set from 1 (single buffer,
akin to PixmapFromBuffer) to 4. This is the number of file
descriptors which will be sent with this request; one per
buffer.
Modifiers allow explicit specification of non-linear sources,
such as tiled or compressed buffers. The combination of bpp,
depth, and modifier allows unambiguous declaration of the
buffer layout in a manner defined by the DRM tokens.
If 'modifier' is DRM_FORMAT_MOD_INVALID, the client does
not have information on the buffer layout. In this case, the
buffer may only have a single plane. The driver may make its
own inference through unspecified means to determine the exact
buffer layout, however this is neither required nor defined
by the specification, and is considered an implementation
detail of the particular driver.
'width' and 'height' describe the geometry (in pixels) of the
logical pixel-sample source.
'strideN' and 'offsetN' define the number of bytes per logical
scanline, and the distance in bytes from the beginning of the
buffer passed for that plane until the start of the sample
source for that plane, respectively for plane N. If the plane
is not used according to the format and modifier specification,
both values for that plane must be zero.
Precisely how any additional information about the buffer (such
as memory placement) is shared is outside the scope of this
extension.
If the buffer(s) cannot be used with the screen associated with
'window', a Match error is returned.
If the bpp, depth, and modifier combination is not supported by
the screen, a Value error is returned.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
┌───
DRI3BuffersFromPixmap
pixmap: PIXMAP
▶
nfd: CARD8
width, height: CARD16
depth, bpp: CARD8
modifier: CARD64
strides: ListOfCARD32
offsets: ListOfCARD32
buffers: ListOfFD
└───
Errors: Pixmap, Match
Returns direct rendering objects associated with 'pixmap'.
Changes to 'pixmap' will be visible in the direct rendered
objects and changes to the direct rendered objects will be
visible in 'pixmap' after flushing and synchronization.
'width' and 'height' describe the geometry (in pixels) of the
logical pixel-sample source from combining the direct rendering
objects.
See PixmapFromBuffers for more details on DRM modifiers usage.
'nfd' describes the number of buffers returned for the pixmap,
which must be combined together according to 'depth', 'bpp', and
'modifier'.
For each buffer, there is an entry in the 'strides',
'offsets', and 'buffers' list. 'buffer' contains a single file
descriptor referring to the buffer. 'stride' specifies the
number of bytes per logical scanline for this plane, and
'offset' specifies the distance in bytes from the beginning
of 'buffer' until the start of the sample source for that
plane.
Precisely how any additional information about the buffer is
shared is outside the scope of this extension.
If buffers cannot be exported from the the screen associated
with 'pixmap', a Match error is returned.
For information on synchronization of buffer access between
the client and the X server, please see section 12.
❄ ❄ ❄ ❄ ❄ ❄ ❄
9. Extension Events
DRI3 defines no events.
❄ ❄ ❄ ❄ ❄ ❄ ❄
10. Extension Versioning
The DRI3 extension is adapted from the DRI2 extension.
1.0: First published version
1.1: Cosmetic changes
1.2: Add GetSupportedModifiers,
PixmapFromBuffers, and BuffersFromPixmap requests.
❄ ❄ ❄ ❄ ❄ ❄ ❄
11. Relationship with other extensions
As an extension designed to support other extensions, there is
naturally some interactions with other extensions.
11.1 GLX
GLX is both an application interface and an X extension. OpenGL
applications using the GLX API will use the GLX extension, DRI3 and
Present when doing direct rendering.
11.2 Present
The Present extension provides a way to synchronize the display of pixmap
contents to the screen. When used in conjunction with DRI3, they
provide a complete direct rendering solution for OpenGL or other APIs.
11.3 DRI2
DRI3 provides similar functionality to the DRI2Connect and
DRI2GetBuffersWithFormat requests, however DRI3 uses file descriptors
to refer to the direct rendering device and buffers.
Present and DRI3 are designed in conjunction to replace DRI2
11.2 XvMC / Xv
It might be nice to be able to reference YUV formatted direct rendered
objects from the X server.
❄ ❄ ❄ ❄ ❄ ❄ ❄
12. Synchronization
Synchronization of access to buffers shared between processes is not
currently explicitly controlled by this protocol.
Without the use of additional extensions not defined by the DRI3
protocol as of version 1.2, synchronization between multiple
processes and contexts is considered to follow the implicit model.
In this model, the driver is required to have a global view of
access requests issued by all processes with a reference to the
buffer, and control scheduling of all operations on that buffer,
whether performed by the CPU or auxiliary hardware.
The driver is responsible for enforcing a strict ordering to protect
against write-after-read or read-after-write hazards, such that any
reads requested by one process or context, are fulfilled before any
writes requested by another process or context, as long as that read
was definitively submitted before the write.
A similar dependency exists for reads submitted after writes: the
driver must ensure that the write is fully visible and coherent to
the read request.
As a purely illustrative example, if two processes share a buffer,
where one process reads from a buffer using an OpenGL texture
sampler and submits this work by calling 'glFlush', and the other
process submits work to the driver to write to that buffer, the
driver is responsible for ensuring that the results of the latter
write are not visible to the texture sampler.
The Sync fences provided by DRI3 control only this submission of
work and ensuing global visibility of the requests, rather than the
completion of the work within any hardware. To further the example
above, a fence used to prevent any writes to the buffer before the
sampler had completed access, the fence would be signaled when
'glFlush' had been called, at which point the request has become
globally visible to the driver's request-scheduling and
synchronization mechanisms. The logical ordering of requests made
by software has been preserved, and the driver then takes care
to ensure that these requests are scheduled such they do not
observe effects from requests made later in time.
This presents a fully coherent in-order FIFO-like model across
processes, where synchronzation is handled externally to the DRI3
client with no explicit intervention.
This restriction also applies for cross-device usage.
❄ ❄ ❄ ❄ ❄ ❄ ❄
Appendix A. Protocol Encoding
Syntactic Conventions
This document uses the same syntactic conventions as the core X
protocol encoding document.
A.1 Common Types
None.
A.2 Protocol Requests
┌───
DRI3QueryVersion
1 CARD8 major opcode
1 0 DRI3 opcode
2 3 length
4 CARD32 major version
4 CARD32 minor version
▶
1 1 Reply
1 unused
2 CARD16 sequence number
4 0 reply length
4 CARD32 major version
4 CARD32 minor version
16 unused
└───
┌───
DRI3Open
1 CARD8 major opcode
1 1 DRI3 opcode
2 4 length
4 DRAWABLE drawable
4 PROVIDER provider
▶
1 1 Reply
1 1 nfd
2 CARD16 sequence number
4 0 reply length
24 unused
0 FD device
└───
┌───
DRI3PixmapFromBuffer
1 CARD8 major opcode
1 2 DRI3 opcode
2 6 length
4 Pixmap pixmap
4 Drawable drawable
4 CARD32 size
2 CARD16 width
2 CARD16 height
2 CARD16 stride
1 CARD8 depth
1 CARD8 bpp
0 FD buffer
└───
┌───
DRI3BufferFromPixmap
1 CARD8 major opcode
1 3 DRI3 opcode
2 2 length
4 Pixmap pixmap
▶
1 1 Reply
1 1 nfd
2 CARD16 sequence number
4 0 reply length
4 CARD32 size
2 CARD16 width
2 CARD16 height
2 CARD16 stride
1 CARD8 depth
1 CARD8 bpp
12 unused
0 FD buffer
└───
┌───
DRI3FenceFromFD
1 CARD8 major opcode
1 4 DRI3 opcode
2 4 length
4 Drawable drawable
4 Fence fence
1 BOOL initially triggered
3 unused
0 FD fence fd
└───
┌───
DRI3FDFromFence
1 CARD8 major opcode
1 5 DRI3 opcode
2 3 length
4 Drawable drawable
4 Fence fence
▶
1 1 Reply
1 1 nfd
2 CARD16 sequence number
4 0 reply length
24 unused
0 FD fence fd
└───
┌───
DRI3GetSupportedModifiers
1 CARD8 major opcode
1 7 DRI3 opcode
2 3 length
4 Window window
1 CARD8 depth
1 CARD8 bpp
2 unused
▶
1 1 Reply
1 0 unused
2 CARD16 sequence number
4 CARD32 reply length
4 CARD32 num_window_modifiers
4 CARD32 num_screen_modifiers
16 unused
4 ListOfCARD64 window_modifiers[num_window_modifiers]
4 ListOfCARD64 screen_modifiers[num_screen_modifiers]
└───
┌───
DRI3PixmapFromBuffers
1 CARD8 major opcode
1 8 DRI3 opcode
2 8 length
4 Pixmap pixmap
4 Window window
1 CARD8 num_buffers
3 unused
2 CARD16 width
2 CARD16 height
4 CARD32 stride0
4 CARD32 offset0
4 CARD32 stride1
4 CARD32 offset1
4 CARD32 stride2
4 CARD32 offset2
4 CARD32 stride3
4 CARD32 offset3
1 CARD8 depth
1 CARD8 bpp
2 unused
8 CARD64 modifier
0 ListOfFD buffers[num_buffers]
└───
┌───
DRI3BuffersFromPixmap
1 CARD8 major opcode
1 9 DRI3 opcode
2 2 length
4 Pixmap pixmap
▶
1 1 Reply
1 CARD8 nfd
2 CARD16 sequence number
4 CARD32 reply length
2 CARD16 width
2 CARD16 height
4 CARD8 unused
8 CARD64 modifier
1 CARD8 depth
1 CARD8 bpp
6 unused
0 ListOfFD buffer[nfd]
4 ListOfCARD32 strides[nfd]
4 ListOfCARD32 offsets[nfd]
└───
A.3 Protocol Events
The DRI3 extension defines no events.
A.4 Protocol Errors
The DRI3 extension defines no errors.
❄ ❄ ❄ ❄ ❄ ❄ ❄