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diff --git a/Documentation/gpu/introduction.rst b/Documentation/gpu/introduction.rst new file mode 100644 index 000000000..f05eccd2c --- /dev/null +++ b/Documentation/gpu/introduction.rst @@ -0,0 +1,174 @@ +============ +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>`_ - Daniel 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>`_ - Daniel 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>`_ - Daniel Vetter (2015) +* `Anatomy of an Embedded KMS Driver <https://www.youtube.com/watch?v=Ja8fM7rTae4>`_ - Laurent Pinchart (2013) + +Slides and articles +------------------- + +* `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/>`_ - Daniel Vetter (2015) +* `Atomic mode setting design overview, part 2 <https://lwn.net/Articles/653466/>`_ - Daniel 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) |