path: root/dri2proto.txt

			  The DRI2 Extension
			      Version 2.0
			      2008-09-04
      
			  Kristian Høgsberg
			    krh@redhat.com
			     Red Hat, Inc


1. Introduction

The DRI2 extension is designed to associate and access auxillary
rendering buffers with an X drawable.

DRI2 is a essentially a helper extension to support implementation of
direct rendering drivers/libraries/technologies.

The main consumer of this extension will be a direct rendering OpenGL
driver, but the DRI2 extension is not designed to be OpenGL specific.
Direct rendering implementations of OpenVG, Xv, cairo and other
graphics APIs should find the functionality exposed by this extension
helpful and hopefully sufficient.

Relation to XF86DRI


1.1. Acknowledgements

Kevin E. Martin <kem@redhat.com>
Keith Packard <keithp@keithp.com>
Eric Anholt <eric@anholt.net>
Keith Whitwell <keith@tungstengraphics.com>
Jerome Glisse <glisse@freedesktop.org>
Ian Romanick <ian.d.romanick@intel.com>
Michel Dänzer <michel@tungstengraphics.com>


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2. DRI2 Concepts


2.1. Attachment points

Stolen from OpenGL FBOs, I guess.


2.2. Kernel rendering manager

This specification assumes a rendering architechture, where an
underlying kernel rendering manager that can provide 32 bit integer
handles to video memory buffers.  These handles can be passed between
processes, which, through a direct rendering driver, submit rendering
to the kernel rendering manager, targeting and/or sourcing from these
buffers.  This extension provides a means to communicate about such
buffers as associated with an X drawable.

The details of how the a direct rendering driver use the buffer names
and submit the rendering requests is outside the scope of this
specification.  However, Appendix B does discuss implementation of
this specification on the Graphics Execution Manager (GEM).


2.3. Request ordering

No ordering between swap buffers and X rendering.  X rendering to src
buffers will block if they have a vblank pending.


2.4 Authentication model

The purpose of the DRM authentication scheme is to grant access to the
kernel rendering manager buffers created by the X server if, and only
if, the client has access to the X server.  This is achieved in a
three-step protocol:

	1) The client gets a token from the kernel rendering manager
	that uniquely identifies it.  The token is a 32 bit integer.

	2) The client passes the token to the X server in the
	DRI2Authenticate request.  This request is a round trip to
	make sure the X server has received and processed the
	authentication before the client starts accessing the DRM.

	3) The X server authorizes the client by passing the token to
	the kernel rendering manager.

A kernel rendering manager can choose not to implement any
authentication and just allow access to all buffers.


2.5 Rendering to the X front buffer

OpenGL allows the client to render to the front buffer, either by
using a single-buffered configuration or but explicitly setting the
draw buffer to GL_FRONT_LEFT.  Not allowed!

The client must ask for a fake front buffer, render to that and then
use DRI2CopyRegion to copy contents back and forth between the fake
front buffer and the real front buffer.  When X and direct rendering
to a front buffer is interleaved, it is the responsibility of the
application to synchronize access using glXWaitGL and glXWaitX.  A
DRI2 implementation of direct rendering GLX, should use these enty
points to copy contents back and forth to as necessary to ensure
consistent rendering.


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3. Data Types

The server side region support specified in the Xfixes extension
version 2 is used in the CopyRegion request.


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4. Errors

No errrors defined by the DRI2 extension.


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5. Protocol Types

DRI2DRIVER { DRI2DriverDRI }

	These values describe the type of driver the client will want
	to load.  The server sends back the name of the driver to use
	for the screen in question.

DRI2ATTACHMENT { DRI2BufferFrontLeft
	       	 DRI2BufferBackLeft
	       	 DRI2BufferFrontRight
	       	 DRI2BufferBackRight
	       	 DRI2BufferDepth
	       	 DRI2BufferStencil
	       	 DRI2BufferAccum
		 DRI2BufferFakeFrontLeft
		 DRI2BufferFakeFrontRight }

	These values describe various attachment points for DRI2
	buffers.

DRI2BUFFER { attachment: CARD32
    	     name: CARD32
	     pitch: CARD32
    	     cpp: CARD32
	     flags: CARD32 }

	The DRI2BUFFER describes an auxillary rendering buffer
	associated with an X drawable.  'attachment' describes the
	attachment point for the buffer, 'name' is the name of the
	underlying kernel buffer,


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6. Extension Initialization

The name of this extension is "DRI2".

┌───
    DRI2QueryVersion
	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, but also new value types in the DRI2CopyRegion
	request.  When new values are introduced, the minor version
	will be increased so the client can know which values the X
	server understands from the version number.


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7. Extension Requests

┌───
    DRI2Connect
	window: WINDOW
	driverType: DRI2DRIVER
      ▶
	driver: STRING
	device: STRING
└───

	Returns the driver name and device file to use for the
	specified driver type for the screen associated with 'window'.

	'type' identifies the type of driver to query for.

	'driver' is the name of the driver to load.  The client is
	assumed to know where to look for the drivers and what to do
	with it.

	'device' is the filename of the DRM device file.

	If the client is not local, or the request driver type is
	unknown or not available, 'driver' and 'device' will be empty
	strings, 'group' will be '0'.  We are not using an regular X
	error here to indicate failure, which will allow the client
	fall back to other options more easily.

	ISSUE: We could add the list of supported attachments and the
	supported DRI2CopyRegion values here (just the bitmask of all
	supported values).
	
┌───
    DRI2Authenticate
	window: WINDOW
	token: CARD32
      ▶
        authenticated: CARD32
└───
	Errors: Window

	Request that the X server authenticates 'token', allowing the
	client to access the DRM buffers created by the X server on
	the screen associated with 'window'.

	Authentication shouldn't fail at this point, except if an
	invalid token is passed, in which case authenticated is False.

┌───
    DRI2GetBuffers
	drawable: DRAWABLE
	attachments: LISTofDRI2ATTACHMENTS
      ▶	
	width, height: CARD32
	buffers: LISTofDRI2BUFFER
└───
	Errors: Window

	Get buffers for the provided attachment points for the given
	drawable.

	If the DDX driver does not support one or more of the
	specified attachment points, a Value error is generated, with
	the first unsupported attachment point as the error value.

	'width' and 'height' describes the dimensions of the drawable.

	'buffers' is a list of DRI2BUFFER for the given DRI2
	attachment points.

┌───
    DRI2CopyRegion
	drawable: DRAWABLE
	region: REGION
	source: DRI2ATTACHMENT
	destination: DRI2ATTACHMENT
      ▶	
└───
	Errors: Window, Value

	Schedule a copy from one DRI2 buffer to another.

	The DRICopyRegion request has a reply but it is empty.  The
	reply is there to let the direct rendering client wait until
	the server has seen the request before proceeding with
	rendering the next frame.


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8. Extension Versioning

The DRI2 extension has undergone a number of revisions before

	1.0: Released, but never used.  Relied on a number of
		constructs from the XF86DRI extension, such as a
		shared memory area (SAREA) to communicate changes in
		cliprects and window sizes, and

	1.99.1: Move the swap buffer functionality into the X server,
		introduce SwapBuffer request to copy back buffer
		contents to the X drawable.

	1.99.2: Rethink the SwapBuffer request as an asynchronous
		request to copy a region between DRI2 buffers.  Drop
		CreateDrawable and DestroyDrawable, update Connect to
		support different driver types and to send the
		authentication group.

	1.99.3: Drop the bitmask argument intended to indicate
		presence of optional arguments for CopyRegion.

	2.0: Awesomeness!

Compatibility up to 2.0 is not preserved, but was also never released.


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10. Relationship with other extensions

As an extension designed to support other extensions, there is
naturally some interactions with other extensions.


10.1 GLX

The GL auxilary buffers map directly to the DRI2 buffers... eh


10.2 DBE

The DBE back buffer must correspond to the DRI2_BUFFER_FRONT_LEFT
DRI2 buffer for servers that support both DBE and DRI2.


10.3 XvMC / Xv

We might add a DRI2_BUFFER_YUV to do vsynced colorspace conversion
blits.  Maybe... not really sure.


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Appendix A. Protocol Encoding

Syntactic Conventions

This document uses the same syntactic conventions as the core X
protocol encoding document.


A.1 Common Types

┌───
    DRI2DRIVER
	0x0	DRI2DriverDRI
└───

┌───
    DRI2ATTACHMENT
	0x0	DRI2BufferFrontLeft
	0x1	DRI2BufferBackLeft
	0x2	DRI2BufferFrontRight
	0x3	DRI2BufferBackRight
	0x4	DRI2BufferDepth
	0x5	DRI2BufferStencil
	0x6	DRI2BufferAccum
	0x7	DRI2BufferFakeFrontLeft
	0x8	DRI2BufferFakeFrontRight
└───
	Used to encode the possible attachment points.

┌───
    DRI2BUFFER
	4	CARD32	attachment
	4	CARD32	name
	4	CARD32	pitch
	4	CARD32	cpp
	4	CARD32	flags
└───
	A DRI2 buffer specifies the attachment, the kernel memory
	manager name, the pitch and chars per pixel for a buffer
	attached to a given drawable.


A.2 Protocol Requests

┌───
    DRI2QueryVersion
	1	CARD8			major opcode
	1	0			DRI2 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	
└───

┌───
    DRI2Connect
	1	CARD8			major opcode
	1	1			DRI2 opcode
	2	3+(n+p)/4		length
	4	WINDOW			window
	4	CARD32			driver type
      ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	(n+m+p)/4		reply length
	4	n			driver name length
	4	m			device name length
	16				unused
	n	CARD8			driver name
	m	CARD8			device name
	p				unused, p=pad(n+m)
└───

┌───
    DRI2Authenticate
	1	CARD8			major opcode
	1	2			DRI2 opcode
	2	3			length
	4	WINDOW			window
	4	CARD32			authentication token
      ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			authenticated
	20				unused	
└───

┌───
    DRI2GetBuffers
	1	CARD8			major opcode
	1	3			DRI2 opcode
	2	3			length
	4	DRAWABLE		drawable
	4	n			number of attachments
	4n	LISTofDRI2ATTACHMENTS	attachments
      ▶	
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			width of drawable
	4	CARD32			height of drawable
	4	CARD32			buffer count
	12				unused	
	5n	LISTofDRI2BUFFER	buffers
└───

┌───
    DRI2CopyRegion
	1	CARD8			major opcode
	1	4			DRI2 opcode
	2	3			length
	4	DRAWABLE		drawable
	4	REGION			region
	4	DRI2ATTACHMENT		source
	4	DRI2ATTACHMENT		destination
      ▶	
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	24				unused	
└───


A.3 Protocol Events

The DRI2 extension specifies no events.


A.4 Protocol Errors

The DRI2 extension specifies no errors.


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Appendix B. Implementation on GEM

Where to begin...