drm/tegra: Add driver documentation

   drm-uapi

   i915

   meson

   tegra

   tinydrm

   vc4

   vga-switcheroo

===============================================

 drm/tegra NVIDIA Tegra GPU and display driver

===============================================

NVIDIA Tegra SoCs support a set of display, graphics and video functions via

the host1x controller. host1x supplies command streams, gathered from a push

buffer provided directly by the CPU, to its clients via channels. Software,

or blocks amongst themselves, can use syncpoints for synchronization.

Up until, but not including, Tegra124 (aka Tegra K1) the drm/tegra driver

supports the built-in GPU, comprised of the gr2d and gr3d engines. Starting

with Tegra124 the GPU is based on the NVIDIA desktop GPU architecture and

supported by the drm/nouveau driver.

The drm/tegra driver supports NVIDIA Tegra SoC generations since Tegra20. It

has three parts:

  - A host1x driver that provides infrastructure and access to the host1x

    services.

  - A KMS driver that supports the display controllers as well as a number of

    outputs, such as RGB, HDMI, DSI, and DisplayPort.

  - A set of custom userspace IOCTLs that can be used to submit jobs to the

    GPU and video engines via host1x.

Driver Infrastructure

=====================

The various host1x clients need to be bound together into a logical device in

order to expose their functionality to users. The infrastructure that supports

this is implemented in the host1x driver. When a driver is registered with the

infrastructure it provides a list of compatible strings specifying the devices

that it needs. The infrastructure creates a logical device and scan the device

tree for matching device nodes, adding the required clients to a list. Drivers

for individual clients register with the infrastructure as well and are added

to the logical host1x device.

Once all clients are available, the infrastructure will initialize the logical

device using a driver-provided function which will set up the bits specific to

the subsystem and in turn initialize each of its clients.

Similarly, when one of the clients is unregistered, the infrastructure will

destroy the logical device by calling back into the driver, which ensures that

the subsystem specific bits are torn down and the clients destroyed in turn.

Host1x Infrastructure Reference

-------------------------------

.. kernel-doc:: include/linux/host1x.h

.. kernel-doc:: drivers/gpu/host1x/bus.c

   :export:

Host1x Syncpoint Reference

--------------------------

.. kernel-doc:: drivers/gpu/host1x/syncpt.c

   :export:

KMS driver

==========

The display hardware has remained mostly backwards compatible over the various

Tegra SoC generations, up until Tegra186 which introduces several changes that

make it difficult to support with a parameterized driver.

Display Controllers

-------------------

Tegra SoCs have two display controllers, each of which can be associated with

zero or more outputs. Outputs can also share a single display controller, but

only if they run with compatible display timings. Two display controllers can

also share a single framebuffer, allowing cloned configurations even if modes

on two outputs don't match. A display controller is modelled as a CRTC in KMS

terms.

On Tegra186, the number of display controllers has been increased to three. A

display controller can no longer drive all of the outputs. While two of these

controllers can drive both DSI outputs and both SOR outputs, the third cannot

drive any DSI.

Windows

~~~~~~~

A display controller controls a set of windows that can be used to composite

multiple buffers onto the screen. While it is possible to assign arbitrary Z

ordering to individual windows (by programming the corresponding blending

registers), this is currently not supported by the driver. Instead, it will

assume a fixed Z ordering of the windows (window A is the root window, that

is, the lowest, while windows B and C are overlaid on top of window A). The

overlay windows support multiple pixel formats and can automatically convert

from YUV to RGB at scanout time. This makes them useful for displaying video

content. In KMS, each window is modelled as a plane. Each display controller

has a hardware cursor that is exposed as a cursor plane.

Outputs

-------

The type and number of supported outputs varies between Tegra SoC generations.

All generations support at least HDMI. While earlier generations supported the

very simple RGB interfaces (one per display controller), recent generations no

longer do and instead provide standard interfaces such as DSI and eDP/DP.

Outputs are modelled as a composite encoder/connector pair.

RGB/LVDS

~~~~~~~~

This interface is no longer available since Tegra124. It has been replaced by

the more standard DSI and eDP interfaces.

HDMI

~~~~

HDMI is supported on all Tegra SoCs. Starting with Tegra210, HDMI is provided

by the versatile SOR output, which supports eDP, DP and HDMI. The SOR is able

to support HDMI 2.0, though support for this is currently not merged.

DSI

~~~

Although Tegra has supported DSI since Tegra30, the controller has changed in

several ways in Tegra114. Since none of the publicly available development

boards prior to Dalmore (Tegra114) have made use of DSI, only Tegra114 and

later are supported by the drm/tegra driver.

eDP/DP

~~~~~~

eDP was first introduced in Tegra124 where it was used to drive the display

panel for notebook form factors. Tegra210 added support for full DisplayPort

support, though this is currently not implemented in the drm/tegra driver.

Userspace Interface

===================

The userspace interface provided by drm/tegra allows applications to create

GEM buffers, access and control syncpoints as well as submit command streams

to host1x.

GEM Buffers

-----------

The ``DRM_IOCTL_TEGRA_GEM_CREATE`` IOCTL is used to create a GEM buffer object

with Tegra-specific flags. This is useful for buffers that should be tiled, or

that are to be scanned out upside down (useful for 3D content).

After a GEM buffer object has been created, its memory can be mapped by an

application using the mmap offset returned by the ``DRM_IOCTL_TEGRA_GEM_MMAP``

IOCTL.

Syncpoints

----------

The current value of a syncpoint can be obtained by executing the

``DRM_IOCTL_TEGRA_SYNCPT_READ`` IOCTL. Incrementing the syncpoint is achieved

using the ``DRM_IOCTL_TEGRA_SYNCPT_INCR`` IOCTL.

Userspace can also request blocking on a syncpoint. To do so, it needs to

execute the ``DRM_IOCTL_TEGRA_SYNCPT_WAIT`` IOCTL, specifying the value of

the syncpoint to wait for. The kernel will release the application when the

syncpoint reaches that value or after a specified timeout.

Command Stream Submission

-------------------------

Before an application can submit command streams to host1x it needs to open a

channel to an engine using the ``DRM_IOCTL_TEGRA_OPEN_CHANNEL`` IOCTL. Client

IDs are used to identify the target of the channel. When a channel is no

longer needed, it can be closed using the ``DRM_IOCTL_TEGRA_CLOSE_CHANNEL``

IOCTL. To retrieve the syncpoint associated with a channel, an application

can use the ``DRM_IOCTL_TEGRA_GET_SYNCPT``.

After opening a channel, submitting command streams is easy. The application

writes commands into the memory backing a GEM buffer object and passes these

to the ``DRM_IOCTL_TEGRA_SUBMIT`` IOCTL along with various other parameters,

such as the syncpoints or relocations used in the job submission.