Documentation/gpu/introduction.rst - linux - Git at Google

 ============
 Introduction
 ============

 The Linux DRM layer contains code intended to support the needs of
 complex graphics devices, usually containing programmable pipelines well
 suited to 3D graphics acceleration. Graphics drivers in the kernel may
 make use of DRM functions to make tasks like memory management,
 interrupt handling and DMA easier, and provide a uniform interface to
 applications.

 A note on versions: this guide covers features found in the DRM tree,
 including the TTM memory manager, output configuration and mode setting,
 and the new vblank internals, in addition to all the regular features
 found in current kernels.

 [Insert diagram of typical DRM stack here]

 Style Guidelines
 ================

 For consistency this documentation uses American English. Abbreviations
 are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
 on. To aid in reading, documentations make full use of the markup
 characters kerneldoc provides: @parameter for function parameters,
 @member for structure members (within the same structure), &struct structure to
 reference structures and function() for functions. These all get automatically
 hyperlinked if kerneldoc for the referenced objects exists. When referencing
 entries in function vtables (and structure members in general) please use
 &vtable_name.vfunc. Unfortunately this does not yet yield a direct link to the
 member, only the structure.

 Except in special situations (to separate locked from unlocked variants)
 locking requirements for functions aren't documented in the kerneldoc.
 Instead locking should be check at runtime using e.g.
 ``WARN_ON(!mutex_is_locked(...));``. Since it's much easier to ignore
 documentation than runtime noise this provides more value. And on top of
 that runtime checks do need to be updated when the locking rules change,
 increasing the chances that they're correct. Within the documentation
 the locking rules should be explained in the relevant structures: Either
 in the comment for the lock explaining what it protects, or data fields
 need a note about which lock protects them, or both.

 Functions which have a non-\ ``void`` return value should have a section
 called "Returns" explaining the expected return values in different
 cases and their meanings. Currently there's no consensus whether that
 section name should be all upper-case or not, and whether it should end
 in a colon or not. Go with the file-local style. Other common section
 names are "Notes" with information for dangerous or tricky corner cases,
 and "FIXME" where the interface could be cleaned up.

 Also read the :ref:`guidelines for the kernel documentation at large <doc_guide>`.

 Documentation Requirements for kAPI
 -----------------------------------

 All kernel APIs exported to other modules must be documented, including their
 datastructures and at least a short introductory section explaining the overall
 concepts. Documentation should be put into the code itself as kerneldoc comments
 as much as reasonable.

 Do not blindly document everything, but document only what's relevant for driver
 authors: Internal functions of drm.ko and definitely static functions should not
 have formal kerneldoc comments. Use normal C comments if you feel like a comment
 is warranted. You may use kerneldoc syntax in the comment, but it shall not
 start with a /** kerneldoc marker. Similar for data structures, annotate
 anything entirely private with ``/* private: */`` comments as per the
 documentation guide.

 Getting Started
 ===============

 Developers interested in helping out with the DRM subsystem are very welcome.
 Often people will resort to sending in patches for various issues reported by
 checkpatch or sparse. We welcome such contributions.

 Anyone looking to kick it up a notch can find a list of janitorial tasks on
 the :ref:`TODO list <todo>`.

 Contribution Process
 ====================

 Mostly the DRM subsystem works like any other kernel subsystem, see :ref:`the
 main process guidelines and documentation <process_index>` for how things work.
 Here we just document some of the specialities of the GPU subsystem.

 Feature Merge Deadlines
 -----------------------

 All feature work must be in the linux-next tree by the -rc6 release of the
 current release cycle, otherwise they must be postponed and can't reach the next
 merge window. All patches must have landed in the drm-next tree by latest -rc7,
 but if your branch is not in linux-next then this must have happened by -rc6
 already.

 After that point only bugfixes (like after the upstream merge window has closed
 with the -rc1 release) are allowed. No new platform enabling or new drivers are
 allowed.

 This means that there's a blackout-period of about one month where feature work
 can't be merged. The recommended way to deal with that is having a -next tree
 that's always open, but making sure to not feed it into linux-next during the
 blackout period. As an example, drm-misc works like that.

 Code of Conduct
 ---------------

 As a freedesktop.org project, dri-devel, and the DRM community, follows the
 Contributor Covenant, found at: https://www.freedesktop.org/wiki/CodeOfConduct

 Please conduct yourself in a respectful and civilised manner when
 interacting with community members on mailing lists, IRC, or bug
 trackers. The community represents the project as a whole, and abusive
 or bullying behaviour is not tolerated by the project.

 Simple DRM drivers to use as examples
 =====================================

 The DRM subsystem contains a lot of helper functions to ease writing drivers for
 simple graphic devices. For example, the `drivers/gpu/drm/tiny/` directory has a
 set of drivers that are simple enough to be implemented in a single source file.

 These drivers make use of the `struct drm_simple_display_pipe_funcs`, that hides
 any complexity of the DRM subsystem and just requires drivers to implement a few
 functions needed to operate the device. This could be used for devices that just
 need a display pipeline with one full-screen scanout buffer feeding one output.

 The tiny DRM drivers are good examples to understand how DRM drivers should look
 like. Since are just a few hundreds lines of code, they are quite easy to read.

 External References
 ===================

 Delving into a Linux kernel subsystem for the first time can be an overwhelming
 experience, one needs to get familiar with all the concepts and learn about the
 subsystem's internals, among other details.

 To shallow the learning curve, this section contains a list of presentations
 and documents that can be used to learn about DRM/KMS and graphics in general.

 There are different reasons why someone might want to get into DRM: porting an
 existing fbdev driver, write a DRM driver for a new hardware, fixing bugs that
 could face when working on the graphics user-space stack, etc. For this reason,
 the learning material covers many aspects of the Linux graphics stack. From an
 overview of the kernel and user-space stacks to very specific topics.

 The list is sorted in reverse chronological order, to keep the most up-to-date
 material at the top. But all of them contain useful information, and it can be
 valuable to go through older material to understand the rationale and context
 in which the changes to the DRM subsystem were made.

 Conference talks
 ----------------

 * `An Overview of the Linux and Userspace Graphics Stack <https://www.youtube.com/watch?v=wjAJmqwg47k>`_ - Paul Kocialkowski (2020)
 * `Getting pixels on screen on Linux: introduction to Kernel Mode Setting <https://www.youtube.com/watch?v=haes4_Xnc5Q>`_ - Simon Ser (2020)
 * `Everything Great about Upstream Graphics <https://www.youtube.com/watch?v=kVzHOgt6WGE>`_ - Simona Vetter (2019)
 * `An introduction to the Linux DRM subsystem <https://www.youtube.com/watch?v=LbDOCJcDRoo>`_ - Maxime Ripard (2017)
 * `Embrace the Atomic (Display) Age <https://www.youtube.com/watch?v=LjiB_JeDn2M>`_ - Simona Vetter (2016)
 * `Anatomy of an Atomic KMS Driver <https://www.youtube.com/watch?v=lihqR9sENpc>`_ - Laurent Pinchart (2015)
 * `Atomic Modesetting for Drivers <https://www.youtube.com/watch?v=kl9suFgbTc8>`_ - Simona Vetter (2015)
 * `Anatomy of an Embedded KMS Driver <https://www.youtube.com/watch?v=Ja8fM7rTae4>`_ - Laurent Pinchart (2013)

 Slides and articles
 -------------------

 * `The Linux graphics stack in a nutshell, part 1 <https://lwn.net/Articles/955376/>`_ - Thomas Zimmermann (2023)
 * `The Linux graphics stack in a nutshell, part 2 <https://lwn.net/Articles/955708/>`_ - Thomas Zimmermann (2023)
 * `Understanding the Linux Graphics Stack <https://bootlin.com/doc/training/graphics/graphics-slides.pdf>`_ - Bootlin (2022)
 * `DRM KMS overview <https://wiki.st.com/stm32mpu/wiki/DRM_KMS_overview>`_ - STMicroelectronics (2021)
 * `Linux graphic stack <https://studiopixl.com/2017-05-13/linux-graphic-stack-an-overview>`_ - Nathan Gauër (2017)
 * `Atomic mode setting design overview, part 1 <https://lwn.net/Articles/653071/>`_ - Simona Vetter (2015)
 * `Atomic mode setting design overview, part 2 <https://lwn.net/Articles/653466/>`_ - Simona Vetter (2015)
 * `The DRM/KMS subsystem from a newbie’s point of view <https://bootlin.com/pub/conferences/2014/elce/brezillon-drm-kms/brezillon-drm-kms.pdf>`_ - Boris Brezillon (2014)
 * `A brief introduction to the Linux graphics stack <https://blogs.igalia.com/itoral/2014/07/29/a-brief-introduction-to-the-linux-graphics-stack/>`_ - Iago Toral (2014)
 * `The Linux Graphics Stack <https://blog.mecheye.net/2012/06/the-linux-graphics-stack/>`_ - Jasper St. Pierre (2012)
	============
	Introduction
	============

	The Linux DRM layer contains code intended to support the needs of
	complex graphics devices, usually containing programmable pipelines well
	suited to 3D graphics acceleration. Graphics drivers in the kernel may
	make use of DRM functions to make tasks like memory management,
	interrupt handling and DMA easier, and provide a uniform interface to
	applications.

	A note on versions: this guide covers features found in the DRM tree,
	including the TTM memory manager, output configuration and mode setting,
	and the new vblank internals, in addition to all the regular features
	found in current kernels.

	[Insert diagram of typical DRM stack here]

	Style Guidelines
	================

	For consistency this documentation uses American English. Abbreviations
	are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
	on. To aid in reading, documentations make full use of the markup
	characters kerneldoc provides: @parameter for function parameters,
	@member for structure members (within the same structure), &struct structure to
	reference structures and function() for functions. These all get automatically
	hyperlinked if kerneldoc for the referenced objects exists. When referencing
	entries in function vtables (and structure members in general) please use
	&vtable_name.vfunc. Unfortunately this does not yet yield a direct link to the
	member, only the structure.

	Except in special situations (to separate locked from unlocked variants)
	locking requirements for functions aren't documented in the kerneldoc.
	Instead locking should be check at runtime using e.g.
	``WARN_ON(!mutex_is_locked(...));``. Since it's much easier to ignore
	documentation than runtime noise this provides more value. And on top of
	that runtime checks do need to be updated when the locking rules change,
	increasing the chances that they're correct. Within the documentation
	the locking rules should be explained in the relevant structures: Either
	in the comment for the lock explaining what it protects, or data fields
	need a note about which lock protects them, or both.

	Functions which have a non-\ ``void`` return value should have a section
	called "Returns" explaining the expected return values in different
	cases and their meanings. Currently there's no consensus whether that
	section name should be all upper-case or not, and whether it should end
	in a colon or not. Go with the file-local style. Other common section
	names are "Notes" with information for dangerous or tricky corner cases,
	and "FIXME" where the interface could be cleaned up.

	Also read the :ref:`guidelines for the kernel documentation at large <doc_guide>`.

	Documentation Requirements for kAPI
	-----------------------------------

	All kernel APIs exported to other modules must be documented, including their
	datastructures and at least a short introductory section explaining the overall
	concepts. Documentation should be put into the code itself as kerneldoc comments
	as much as reasonable.

	Do not blindly document everything, but document only what's relevant for driver
	authors: Internal functions of drm.ko and definitely static functions should not
	have formal kerneldoc comments. Use normal C comments if you feel like a comment
	is warranted. You may use kerneldoc syntax in the comment, but it shall not
	start with a /** kerneldoc marker. Similar for data structures, annotate
	anything entirely private with ``/* private: */`` comments as per the
	documentation guide.

	Getting Started
	===============

	Developers interested in helping out with the DRM subsystem are very welcome.
	Often people will resort to sending in patches for various issues reported by
	checkpatch or sparse. We welcome such contributions.

	Anyone looking to kick it up a notch can find a list of janitorial tasks on
	the :ref:`TODO list <todo>`.

	Contribution Process
	====================

	Mostly the DRM subsystem works like any other kernel subsystem, see :ref:`the
	main process guidelines and documentation <process_index>` for how things work.
	Here we just document some of the specialities of the GPU subsystem.

	Feature Merge Deadlines
	-----------------------

	All feature work must be in the linux-next tree by the -rc6 release of the
	current release cycle, otherwise they must be postponed and can't reach the next
	merge window. All patches must have landed in the drm-next tree by latest -rc7,
	but if your branch is not in linux-next then this must have happened by -rc6
	already.

	After that point only bugfixes (like after the upstream merge window has closed
	with the -rc1 release) are allowed. No new platform enabling or new drivers are
	allowed.

	This means that there's a blackout-period of about one month where feature work
	can't be merged. The recommended way to deal with that is having a -next tree
	that's always open, but making sure to not feed it into linux-next during the
	blackout period. As an example, drm-misc works like that.

	Code of Conduct
	---------------

	As a freedesktop.org project, dri-devel, and the DRM community, follows the
	Contributor Covenant, found at: https://www.freedesktop.org/wiki/CodeOfConduct

	Please conduct yourself in a respectful and civilised manner when
	interacting with community members on mailing lists, IRC, or bug
	trackers. The community represents the project as a whole, and abusive
	or bullying behaviour is not tolerated by the project.

	Simple DRM drivers to use as examples
	=====================================

	The DRM subsystem contains a lot of helper functions to ease writing drivers for
	simple graphic devices. For example, the `drivers/gpu/drm/tiny/` directory has a
	set of drivers that are simple enough to be implemented in a single source file.

	These drivers make use of the `struct drm_simple_display_pipe_funcs`, that hides
	any complexity of the DRM subsystem and just requires drivers to implement a few
	functions needed to operate the device. This could be used for devices that just
	need a display pipeline with one full-screen scanout buffer feeding one output.

	The tiny DRM drivers are good examples to understand how DRM drivers should look
	like. Since are just a few hundreds lines of code, they are quite easy to read.

	External References
	===================

	Delving into a Linux kernel subsystem for the first time can be an overwhelming
	experience, one needs to get familiar with all the concepts and learn about the
	subsystem's internals, among other details.

	To shallow the learning curve, this section contains a list of presentations
	and documents that can be used to learn about DRM/KMS and graphics in general.

	There are different reasons why someone might want to get into DRM: porting an
	existing fbdev driver, write a DRM driver for a new hardware, fixing bugs that
	could face when working on the graphics user-space stack, etc. For this reason,
	the learning material covers many aspects of the Linux graphics stack. From an
	overview of the kernel and user-space stacks to very specific topics.

	The list is sorted in reverse chronological order, to keep the most up-to-date
	material at the top. But all of them contain useful information, and it can be
	valuable to go through older material to understand the rationale and context
	in which the changes to the DRM subsystem were made.

	Conference talks
	----------------

	* `An Overview of the Linux and Userspace Graphics Stack <https://www.youtube.com/watch?v=wjAJmqwg47k>`_ - Paul Kocialkowski (2020)
	* `Getting pixels on screen on Linux: introduction to Kernel Mode Setting <https://www.youtube.com/watch?v=haes4_Xnc5Q>`_ - Simon Ser (2020)
	* `Everything Great about Upstream Graphics <https://www.youtube.com/watch?v=kVzHOgt6WGE>`_ - Simona Vetter (2019)
	* `An introduction to the Linux DRM subsystem <https://www.youtube.com/watch?v=LbDOCJcDRoo>`_ - Maxime Ripard (2017)
	* `Embrace the Atomic (Display) Age <https://www.youtube.com/watch?v=LjiB_JeDn2M>`_ - Simona Vetter (2016)
	* `Anatomy of an Atomic KMS Driver <https://www.youtube.com/watch?v=lihqR9sENpc>`_ - Laurent Pinchart (2015)
	* `Atomic Modesetting for Drivers <https://www.youtube.com/watch?v=kl9suFgbTc8>`_ - Simona Vetter (2015)
	* `Anatomy of an Embedded KMS Driver <https://www.youtube.com/watch?v=Ja8fM7rTae4>`_ - Laurent Pinchart (2013)

	Slides and articles
	-------------------

	* `The Linux graphics stack in a nutshell, part 1 <https://lwn.net/Articles/955376/>`_ - Thomas Zimmermann (2023)
	* `The Linux graphics stack in a nutshell, part 2 <https://lwn.net/Articles/955708/>`_ - Thomas Zimmermann (2023)
	* `Understanding the Linux Graphics Stack <https://bootlin.com/doc/training/graphics/graphics-slides.pdf>`_ - Bootlin (2022)
	* `DRM KMS overview <https://wiki.st.com/stm32mpu/wiki/DRM_KMS_overview>`_ - STMicroelectronics (2021)
	* `Linux graphic stack <https://studiopixl.com/2017-05-13/linux-graphic-stack-an-overview>`_ - Nathan Gauër (2017)
	* `Atomic mode setting design overview, part 1 <https://lwn.net/Articles/653071/>`_ - Simona Vetter (2015)
	* `Atomic mode setting design overview, part 2 <https://lwn.net/Articles/653466/>`_ - Simona Vetter (2015)
	* `The DRM/KMS subsystem from a newbie’s point of view <https://bootlin.com/pub/conferences/2014/elce/brezillon-drm-kms/brezillon-drm-kms.pdf>`_ - Boris Brezillon (2014)
	* `A brief introduction to the Linux graphics stack <https://blogs.igalia.com/itoral/2014/07/29/a-brief-introduction-to-the-linux-graphics-stack/>`_ - Iago Toral (2014)
	* `The Linux Graphics Stack <https://blog.mecheye.net/2012/06/the-linux-graphics-stack/>`_ - Jasper St. Pierre (2012)