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============
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Introduction
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============
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The Linux DRM layer contains code intended to support the needs of
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complex graphics devices, usually containing programmable pipelines well
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suited to 3D graphics acceleration. Graphics drivers in the kernel may
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make use of DRM functions to make tasks like memory management,
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interrupt handling and DMA easier, and provide a uniform interface to
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applications.
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A note on versions: this guide covers features found in the DRM tree,
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including the TTM memory manager, output configuration and mode setting,
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and the new vblank internals, in addition to all the regular features
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found in current kernels.
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[Insert diagram of typical DRM stack here]
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Style Guidelines
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================
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For consistency this documentation uses American English. Abbreviations
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are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
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on. To aid in reading, documentations make full use of the markup
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characters kerneldoc provides: @parameter for function parameters,
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@member for structure members (within the same structure), &struct structure to
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reference structures and function() for functions. These all get automatically
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hyperlinked if kerneldoc for the referenced objects exists. When referencing
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entries in function vtables (and structure members in general) please use
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&vtable_name.vfunc. Unfortunately this does not yet yield a direct link to the
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member, only the structure.
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Except in special situations (to separate locked from unlocked variants)
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locking requirements for functions aren't documented in the kerneldoc.
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Instead locking should be check at runtime using e.g.
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``WARN_ON(!mutex_is_locked(...));``. Since it's much easier to ignore
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documentation than runtime noise this provides more value. And on top of
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that runtime checks do need to be updated when the locking rules change,
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increasing the chances that they're correct. Within the documentation
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the locking rules should be explained in the relevant structures: Either
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in the comment for the lock explaining what it protects, or data fields
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need a note about which lock protects them, or both.
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Functions which have a non-\ ``void`` return value should have a section
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called "Returns" explaining the expected return values in different
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cases and their meanings. Currently there's no consensus whether that
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section name should be all upper-case or not, and whether it should end
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in a colon or not. Go with the file-local style. Other common section
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names are "Notes" with information for dangerous or tricky corner cases,
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and "FIXME" where the interface could be cleaned up.
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Also read the :ref:`guidelines for the kernel documentation at large <doc_guide>`.
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Getting Started
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===============
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Developers interested in helping out with the DRM subsystem are very welcome.
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Often people will resort to sending in patches for various issues reported by
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checkpatch or sparse. We welcome such contributions.
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Anyone looking to kick it up a notch can find a list of janitorial tasks on
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the :ref:`TODO list <todo>`.
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Contribution Process
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====================
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Mostly the DRM subsystem works like any other kernel subsystem, see :ref:`the
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main process guidelines and documentation <process_index>` for how things work.
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Here we just document some of the specialities of the GPU subsystem.
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Feature Merge Deadlines
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-----------------------
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All feature work must be in the linux-next tree by the -rc6 release of the
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current release cycle, otherwise they must be postponed and can't reach the next
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merge window. All patches must have landed in the drm-next tree by latest -rc7,
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but if your branch is not in linux-next then this must have happened by -rc6
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already.
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After that point only bugfixes (like after the upstream merge window has closed
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with the -rc1 release) are allowed. No new platform enabling or new drivers are
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allowed.
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This means that there's a blackout-period of about one month where feature work
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can't be merged. The recommended way to deal with that is having a -next tree
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that's always open, but making sure to not feed it into linux-next during the
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blackout period. As an example, drm-misc works like that.
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Code of Conduct
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---------------
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As a freedesktop.org project, dri-devel, and the DRM community, follows the
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Contributor Covenant, found at: https://www.freedesktop.org/wiki/CodeOfConduct
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Please conduct yourself in a respectful and civilised manner when
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interacting with community members on mailing lists, IRC, or bug
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trackers. The community represents the project as a whole, and abusive
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or bullying behaviour is not tolerated by the project.
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