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editing: 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 auxiliary
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>
Jesse Barnes <jbarnes@virtuousgeek.org>

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

2. DRI2 Concepts

2.1. Attachment points

Stolen from OpenGL FBOs, I guess.

2.2. Kernel rendering manager

This specification assumes a rendering architecture, 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 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 entry
points to copy contents back and forth to as necessary to ensure
consistent rendering.

The client may also use the DRI2SwapBuffers function to request a swap
of the front and back buffers.  If the display server supports it, this
operation may be preferred, since it may be easier and/or more performant
for the server to perform a simple buffer swap rather than a blit.

2.6 Synchronizing rendering

DRI2 provides several methods for synchronizing drawing with various events.
The protocol for these methods is based on the SGI_video_sync and
OML_sync_control GLX extensions.  Using the DRI2WaitMSC request, a client
can wait for a specific frame count or divisor/remainder before continuing
its processing.  With the DRI2WaitSBC request, clients can block until a given
swap count is reached (as incremented by DRI2SwapBuffers).  Finally, using
DRI2SwapBuffers, clients can limit their frame rate by specifying a swap
interval using the swap interval call (currently only available through GLX)
or by using the OML swap buffers routine.

2.7 Events

DRI2 provides an event to indicate when a DRI2SwapBuffers request has
been completed.  This can be used to throttle drawing on the client
side and tie into application main loops.

Another event is generated when the validity of the requested buffers
changes.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

3. Data Types

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

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

4. Errors

No errors are defined by the DRI2 extension.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

5. Events

The only events provided by DRI2 are DRI2_BufferSwapComplete
and DRI2InvalidateBuffers.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

6. Protocol Types

DRI2DRIVER { DRI2DriverDRI
	     DRI2DriverVDPAU }

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
		 DRI2BufferDepthStencil
		 DRI2BufferHiz }

These values describe various attachment points for DRI2
	buffers.

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

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

DRI2ATTACH_FORMAT { attachment: CARD32
		    format:     CARD32 }

The DRI2ATTACH_FORMAT describes an attachment and the associated
	format.  'attachment' describes the attachment point for the buffer,
	'format' describes an opaque, device-dependent format for the buffer.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

7. 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.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

8. 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.  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.

┌───
    DRI2SwapBuffers
	drawable: DRAWABLE
	target_msc: two CARD32s
	divisor: two CARD32s
	remainder: two CARD32s
      ▶
	swap: two CARD32s
└───
	Errors: Window

Schedule a swap of the front and back buffers with the display
	server.

Returns the swap count value when the swap will actually occur (e.g.
	the last queued swap count + (pending swap count * swap interval)).

This request is only available with protocol version 1.2 or
	later.

┌───
    DRI2GetBuffersWithFormat
	drawable: DRAWABLE
	attachments: LISTofDRI2ATTACH_FORMAT
      ▶
	width, height: CARD32
	buffers: LISTofDRI2BUFFER
└───
	Errors: Window

Get buffers for the provided attachment points with the specified
	formats for the given drawable.

If the DDX driver does not support one or more of the
	specified attachment points or formats, 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.

This request is only available with protocol version 1.1 or
	later.

┌───
    DRI2GetMSC
	drawable: DRAWABLE
      ▶
	ust, msc, sbc: CARD64
└───
	Errors: Window

Get the current media stamp counter (MSC) and swap buffer count (SBC)
	along with the unadjusted system time (UST) when the MSC was last
	incremented.

This request is only available with protocol version 1.2 or
	later.

┌───
    DRI2WaitMSC
	drawable: DRAWABLE
	target_msc: two CARD32s
	divisor: two CARD32s
	remainder: two CARD32s
      ▶
	ust, msc, sbc: CARD64
└───
	Errors: Window

Blocks the client until either the frame count reaches target_msc or,
	if the frame count is already greater than target_msc when the request
	is received, until the frame count % divisor = remainder.  If divisor
	is 0, the client will be unblocked if the frame count is greater than
	or equal to the target_msc.

Returns the current media stamp counter (MSC) and swap buffer count
	(SBC) along with the unadjusted system time (UST) when the MSC was last
	incremented.

This request is only available with protocol version 1.2 or
	later.

┌───
    DRI2WaitSBC
	drawable: DRAWABLE
	target_sbc: two CARD32s
      ▶
	ust, msc, sbc: CARD64
└───
	Errors: Window

Blocks the client until the swap buffer count reaches target_sbc.  If
	the swap buffer count is already greater than or equal to target_sbc
	when the request is received, this request will return immediately.

If target_sbc is 0, this request will block the client until all
	previous DRI2SwapBuffers requests have completed.

Returns the current media stamp counter (MSC) and swap buffer count
	(SBC) along with the unadjusted system time (UST) when the MSC was last
	incremented.

This request is only available with protocol version 1.2 or
	later.

┌───
    DRI2SwapInterval
	drawable: DRAWABLE
	interval: CARD32
      ▶
└───
	Errors: Window

Sets the swap interval for DRAWABLE.  This will throttle
	DRI2SwapBuffers requests to swap at most once per interval frames,
	which is useful useful for limiting the frame rate.

┌───
    DRI2GetParam
	drawable: DRAWABLE
	param: CARD32
      ▶
	is_param_recognized: BOOL
	value: CARD64
└───
	Errors: Drawable

Get the value of a parameter.  The parameter's value is looked up on
	the screen associated with 'drawable'.

Parameter names in which the value of the most significant byte is
	0 are reserved for the X server. Currently, no such parameter names
	are defined. (When any such names are defined, they will be defined in
	this extension specification and its associated headers).

Parameter names in which the byte's value is 1 are reserved for the
	DDX. Such names are private to each driver and shall be defined in the
	respective driver's headers.

Parameter names in which the byte's value is neither 0 nor 1 are
	reserved for future use.

Possible values of 'is_param_recognized' are true (1) and false (0).
	If false, then 'value' is undefined.

This request is only available with protocol version 1.4 or later.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

9. Extension Events

┌───
    DRI2BufferSwapComplete
      ▶
      	event_type: CARD16
	drawable: CARD32
	ust: CARD64
	msc: CARD64
	sbc: CARD64
└───

This event reports the status of the last DRI2SwapBuffers event to
	the client.  The event type should be one of DRI2_EXCHANGE_COMPLETE,
	indicating a successful buffer exchange, DRI2_BLIT_COMPLETE, indicating
	the swap was performed with a blit, and DRI2_FLIP_COMPLETE, indicating
	a full page flip was completed.

┌───
    DRI2InvalidateBuffers
      ▶
	drawable: CARD32
└───

This event is generated when the buffers the client had
	requested for 'drawable' (with DRI2GetBuffers or
	DRI2GetBuffersWithFormat) become inappropriate because they
	don't match the drawable dimensions anymore, or a buffer swap
	has been performed.

Note that the server is only required to warn the client once
	about this condition, until the client takes care of bringing
	them back up-to-date with another GetBuffers request.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

10. 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!

2.1: True excellence.  Added DRI2GetBuffersWithFormat to allow
             more flexible object creation.

2.2: Approaching perfection.  Added requests for swapbuffers,
             MSC and SBC related requests, and events.

2.3: Added the DRI2InvalidateBuffers event.

2.6: Enlightenment attained.  Added the DRI2BufferHiz attachment.

2.7: Added the DRI2GetParam request.

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

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

11. Relationship with other extensions

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

11.1 GLX

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

11.2 DBE

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

11.3 XvMC / Xv

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

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

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
	0x1	DRI2DriverVDPAU
└───

┌───
    DRI2ATTACHMENT
	0x0	DRI2BufferFrontLeft
	0x1	DRI2BufferBackLeft
	0x2	DRI2BufferFrontRight
	0x3	DRI2BufferBackRight
	0x4	DRI2BufferDepth
	0x5	DRI2BufferStencil
	0x6	DRI2BufferAccum
	0x7	DRI2BufferFakeFrontLeft
	0x8	DRI2BufferFakeFrontRight
	0x9	DRI2BufferDepthStencil
	0xa	DRI2BufferHiz
└───
	Used to encode the possible attachment points.  The attachment
	DRI2BufferDepthStencil is only available with protocol version 1.1 or
	later.

┌───
    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.

┌───
    DRI2ATTACH_FORMAT
	4	CARD32	attachment
	4	CARD32	format
└───
	Used to describe the attachment and format requested from the server.
	This data type is only available with protocol version 1.1 or
	later.

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			length
	4	WINDOW			window
	4	CARD32			driver type
      ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	(n+m+p+q)/4		reply length
	4	n			driver name length
	4	m			device name length
	16				unused
	n	CARD8			driver name
	p				unused, p=pad(n)
	m	CARD8			device name
	q				unused, q=pad(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	5			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	6			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	
└───

┌───
    DRI2GetBuffersWithFormat
	1	CARD8			major opcode
	1	7			DRI2 opcode
	2	3			length
	4	DRAWABLE		drawable
	4	n			number of attachments
	8n	LISTofDRI2ATTACH_FORMAT	attachments and formats
      ▶
	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
└───

┌───
    DRI2SwapBuffers
	1	CARD8			major opcode
	1	8			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
	4	CARD32			target_msc_hi
	4	CARD32			target_msc_lo
	4	CARD32			divisor_hi
	4	CARD32			divisor_lo
	4	CARD32			remainder_hi
	4	CARD32			remainder_lo
      ▶	
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			swap_hi
	4	CARD32			swap_lo
	20				unused
└───

┌───
    DRI2GetMSC
	1	CARD8			major opcode
	1	9			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
      ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			ust_hi
	4	CARD32			ust_lo
	4	CARD32			msc_hi
	4	CARD32			msc_lo
	4	CARD32			sbc_hi
	4	CARD32			sbc_lo
└───

┌───
    DRI2WaitMSC
	1	CARD8			major opcode
	1	10			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
	4	CARD32			target_msc_hi
	4	CARD32			target_msc_lo
	4	CARD32			divisor_hi
	4	CARD32			divisor_lo
	4	CARD32			remainder_hi
	4	CARD32			remainder_lo
      ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			ust_hi
	4	CARD32			ust_lo
	4	CARD32			msc_hi
	4	CARD32			msc_lo
	4	CARD32			sbc_hi
	4	CARD32			sbc_lo
└───

┌───
    DRI2WaitSBC
	1	CARD8			major opcode
	1	11			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
 	4	CARD32			swap_hi
	4	CARD32			swap_lo
     ▶
	1	1			Reply
        1				unused
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			ust_hi
	4	CARD32			ust_lo
	4	CARD32			msc_hi
	4	CARD32			msc_lo
	4	CARD32			sbc_hi
	4	CARD32			sbc_lo
└───

┌───
    DRI2SwapInterval
	1	CARD8			major opcode
	1	12			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
 	4	CARD32			interval
     ▶
└───

┌───
    DRI2GetParam
	1	CARD8			major opcode
	1	13			DRI2 opcode
	2	8			length
	4	DRAWABLE		drawable
	4	CARD32			param
     ▶
	1	1			Reply
	1	BOOL			is_param_recognized
	2	CARD16			sequence number
	4	0			reply length
	4	CARD32			value_hi
	4	CARD32			value_lo
	16				unused
└───

A.3 Protocol Events

The DRI2 extension specifies DRI2_BufferSwapComplete and
DRI2_InvalidateBuffers events.

┌───
    DRI2_BufferSwapComplete
	1	CARD8			type
	1	CARD8			extension
	2	CARD16			sequenceNumber
	2	CARD16			event_type
	4	DRAWABLE		drawable
	4	CARD32			ust_hi
	4	CARD32			ust_lo
	4	CARD32			msc_hi
	4	CARD32			msc_lo
	4	CARD32			sbc_hi
	4	CARD32			sbc_lo
└───

┌───
    DRI2_InvalidateBuffers
	1	CARD8			type
	1	CARD8			extension
	2	CARD16			sequenceNumber
	4	DRAWABLE		drawable
	4	CARD32			unused
	4	CARD32			unused
	4	CARD32			unused
	4	CARD32			unused
	4	CARD32			unused
	4	CARD32			unused
└───

A.4 Protocol Errors

The DRI2 extension specifies no errors.

⚙ ⚙ ⚙  ⚙ ⚙ ⚙

Appendix B. Implementation on GEM

Where to begin...

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