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211 changes: 209 additions & 2 deletions source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -34,14 +34,18 @@
.. Image:: /images/DSS7Lite_HW.png


The above image gives an overview of the DSS hardware.
.. ifconfig:: CONFIG_part_variant in ('J721E', 'J721S2', 'J784S4', 'J742S2', 'AM62X', 'AM65X', 'AM62AX', 'AM62PX', 'J722S')

The above image gives an overview of the DSS hardware.

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[RedHat.TermsSuggestions] Depending on the context, consider using 'earlier', 'previous', 'preceding', or 'before' rather than 'above'.

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{"message": "[RedHat.TermsSuggestions] Depending on the context, consider using 'earlier', 'previous', 'preceding', or 'before' rather than 'above'.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 39, "column": 9}}}, "severity": "INFO"}

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[RedHat.TermsSuggestions] Depending on the context, consider using 'earlier', 'previous', 'preceding', or 'before' rather than 'above'.

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{"message": "[RedHat.TermsSuggestions] Depending on the context, consider using 'earlier', 'previous', 'preceding', or 'before' rather than 'above'.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 39, "column": 9}}}, "severity": "INFO"}

.. ifconfig:: CONFIG_part_variant in ('AM62PX', 'J722S')

- The |__PART_FAMILY_NAME__| SoC has 2 instances of the DSS7-UL, connected to different display peripherals, inside the SoC.
- **Note:** The Video Pipelines from one instance of DSS **cannot** overlay image planes via the Overlay Managers of another DSS.

The arrows show how pipelines are connected to overlay managers, which are further connected to video-ports, which finally create an encoded pixel stream for display on the LCD or monitor.
.. ifconfig:: CONFIG_part_variant in ('J721E', 'J721S2', 'J784S4', 'J742S2', 'AM62X', 'AM65X', 'AM62AX', 'AM62PX', 'J722S')

The arrows show how pipelines are connected to overlay managers, which are further connected to video-ports, which finally create an encoded pixel stream for display on the LCD or monitor.

.. ifconfig:: CONFIG_part_variant in ('AM62AX')

Expand Down Expand Up @@ -508,6 +512,12 @@
+--------------------+----------+------------------------------------------------------------------------------------------------------+
| GAMMA_LUT_SIZE | crtc | Number of elements in gammma lookup table. |
+--------------------+----------+------------------------------------------------------------------------------------------------------+
| SELF_REFRESH | plane | Boolean property. When set, plays the last displayed frame in a loop from the DSS internal buffer; |
| | | the driver ignores new framebuffers submitted by userspace until userspace clears the property. |

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[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 516, "column": 58}}}, "severity": "WARNING"}

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[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 516, "column": 58}}}, "severity": "WARNING"}
+--------------------+----------+------------------------------------------------------------------------------------------------------+
| ALWAYS_ON_DISPLAY | crtc | Boolean property. When set, keeps the video port, along with its associated bridges and PHYs, |

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[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 518, "column": 123}}}, "severity": "INFO"}

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[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 518, "column": 24}}}, "severity": "WARNING"}

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[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 518, "column": 123}}}, "severity": "INFO"}

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[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 518, "column": 24}}}, "severity": "WARNING"}

@StaticRocket StaticRocket Aug 4, 2026

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CRTC is an acronym. Why is it lowercase in this file?

| | | powered on even after the last DRM client exits. |
+--------------------+----------+------------------------------------------------------------------------------------------------------+

.. _testing_tidss_properties:

Expand Down Expand Up @@ -770,6 +780,131 @@
* `<https://www.w3.org/TR/PNG-GammaAppendix.html>`__
* `<https://www.benq.com/en-us/knowledge-center/knowledge/gamma-monitor.html>`__

.. rubric:: Self Refresh

Self refresh plays the last displayed frame in a loop by using the DSS internal buffer.

The ``SELF_REFRESH`` plane property is a boolean property. When userspace sets it
to 1, the DSS hardware loops the last frame the plane received from its internal
buffer. The driver silently ignores any new framebuffer that userspace submits
while the property remains set. The display resumes normal operation, taking new
framebuffers into account, only after userspace clears the property (sets it back

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[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 791, "column": 1}}}, "severity": "WARNING"}

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[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'framebuffers'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 791, "column": 1}}}, "severity": "WARNING"}

@StaticRocket StaticRocket Aug 4, 2026

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Please update the vale accept.txt for the plural form of framebuffer.

[Ff]ramebuffers?

to 0).

The size of this internal buffer varies per SoC family, and therefore so does the
maximum frame size (width x height x bytes-per-pixel) that self-refresh can loop:

+----------------------------------+-----------------------------------+
| SoC Family | Self-Refresh Internal Buffer Size |
+==================================+===================================+
| AM65X | 40 KB |
+----------------------------------+-----------------------------------+
| AM62X | 40 KB |
+----------------------------------+-----------------------------------+
| AM62AX | 40 KB |
+----------------------------------+-----------------------------------+
| AM62PX / J722S | 40 KB |
+----------------------------------+-----------------------------------+
| AM62LX | 20 KB |
+----------------------------------+-----------------------------------+
| J721E / J721S2 / J784S4 / J742S2 | 64 KB |
+----------------------------------+-----------------------------------+

If the frame exceeds the internal buffer size for the given SoC, ``SELF_REFRESH``
will not activate for that plane.

@akashdeep-ti akashdeep-ti Aug 3, 2026

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So, Self refresh property will still be set to 1 but nothing will be visible on display or neither property will be set to 1 nor will anything be displayed?
Can you please add this clearly so that the user can know how to recover from such scenarios (activate is a bit confusing to understand)


.. code-block:: console

$ modetest -M tidss -w 35:SELF_REFRESH:1

In this example, ``SELF_REFRESH`` is enabled on plane 35. The plane keeps displaying
whatever frame was on screen when userspace set the property. The driver drops any
later frame that an application pushes to the plane while the property remains set.
The combined example under **Always On Display** below shows how ``kmstest``
exercises ``SELF_REFRESH`` together with ``ALWAYS_ON_DISPLAY``.

.. rubric:: Always On Display

Keep the display pipeline powered after the application exits. With additional
firmware-side support, the pipeline also stays powered across system suspend and
resume; see the note that follows.

The ``ALWAYS_ON_DISPLAY`` crtc property is a boolean property. When set to 1, the
driver keeps the video port's power domain powered, along with the power domains
of its bridges and PHYs (for example DSI and D-PHY). This holds for that video

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[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 834, "column": 20}}}, "severity": "INFO"}

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[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'PHYs') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 834, "column": 20}}}, "severity": "INFO"}
port's entire pipeline even after the last DRM client that uses the crtc exits,

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[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 835, "column": 69}}}, "severity": "WARNING"}

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[vale] reported by reviewdog 🐶
[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'crtc'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 835, "column": 69}}}, "severity": "WARNING"}
and across system suspend and resume. This avoids incurring the cost of hardware

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[RedHat.SentenceLength] Try to keep sentences to an average of 32 words or fewer.

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{"message": "[RedHat.SentenceLength] Try to keep sentences to an average of 32 words or fewer.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 836, "column": 39}}}, "severity": "INFO"}

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{"message": "[RedHat.SentenceLength] Try to keep sentences to an average of 32 words or fewer.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 836, "column": 39}}}, "severity": "INFO"}
reinitialization of the DSI and D-PHY link the next time an application opens the

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[RedHat.Definitions] Define acronyms and abbreviations (such as 'DSI') on first occurrence if they're likely to be unfamiliar.

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'DSI') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 837, "column": 25}}}, "severity": "INFO"}

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{"message": "[RedHat.Definitions] Define acronyms and abbreviations (such as 'DSI') on first occurrence if they're likely to be unfamiliar.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 837, "column": 25}}}, "severity": "INFO"}
device, at the cost of keeping that hardware powered while idle.

The driver applies two complementary PM holds to tidss and to every bridge/PHY

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[RedHat.Spelling] Verify the word 'tidss'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'tidss'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 840, "column": 50}}}, "severity": "WARNING"}

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[RedHat.Spelling] Verify the word 'tidss'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.

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{"message": "[RedHat.Spelling] Verify the word 'tidss'. It is not in the American English or Red Hat terminology spelling dictionaries used by Vale.", "location": {"path": "source/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.rst", "range": {"start": {"line": 840, "column": 50}}}, "severity": "WARNING"}

@akashdeep-ti akashdeep-ti Aug 3, 2026

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PM holds is a confusing term, can we make it simpler

device in the pipeline:

- A ``pm_runtime_get_noresume()`` hold on each device, which prevents its runtime
PM ``suspend`` callback from running and therefore gates runtime autosuspend for
tidss itself, not only the external bridges/PHYs.
- ``dev_pm_genpd_set_always_on()``, which marks the device's power domain as
always-on, blocking both runtime power-off and the power-off that would
otherwise happen when the system is suspended.

Both holds are released once ``ALWAYS_ON_DISPLAY`` is cleared on all crtcs that had
it set.

.. ifconfig:: CONFIG_part_variant in ('AM62LX')

.. note::

Keeping the display pipeline powered across the Linux driver's own suspend/resume
calls is handled entirely by the ``ALWAYS_ON_DISPLAY`` property as described above.
However, surviving an actual system-wide low power state (for example
``echo mem > /sys/power/state``) additionally requires cooperation from the
device firmware, which must also be told to keep the display power rails on
during that low power state. This firmware-side support is available on
AM62LX as the :ref:`dss-plus-deepsleep` low power mode.

.. ifconfig:: CONFIG_part_variant not in ('AM62LX')

.. note::

Keeping the display pipeline powered across the Linux driver's own suspend/resume
calls is handled entirely by the ``ALWAYS_ON_DISPLAY`` property as described above.
However, surviving an actual system-wide low power state (for example
``echo mem > /sys/power/state``) additionally requires cooperation from the
device firmware, which must also be told to keep the display power rails on
during that low power state. This firmware-side support is currently only

@akashdeep-ti akashdeep-ti Aug 3, 2026

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is currently gives an impression that it is possible to add support for this to other devices in future, which is not correct.
Can we reword it to state that this is only possible on AM62L SoC.

available on AM62LX. For other SoCs,``ALWAYS_ON_DISPLAY`` keeps the pipeline
powered across application handoff, thus avoiding runtime suspend even if no
application is holding a reference, but the display should be assumed to
lose power during a full system suspend/resume cycle.

.. code-block:: console

$ modetest -M tidss -w 42:ALWAYS_ON_DISPLAY:1

In this example, crtc 42 has ``ALWAYS_ON_DISPLAY`` set to 1. Once the application
that uses this crtc exits, the video port and its associated bridge/PHY power
domains remain powered on, so a later application can reuse the pipeline without
incurring the cost of hardware reinitialization of the DSI and D-PHY link.

When ``ALWAYS_ON_DISPLAY`` is combined with ``SELF_REFRESH`` on a plane of the same
crtc, the video port itself is also kept running (instead of being disabled) once
the application exits, so the frame looped by ``SELF_REFRESH`` continues to be
displayed even after application handoff or system suspend/resume.

.. code-block:: console

$ modetest -M tidss -w 42:ALWAYS_ON_DISPLAY:1
$ modetest -M tidss -w 35:SELF_REFRESH:1
$ kmstest --flip

In this combined example, crtc 42 is first set to ``ALWAYS_ON_DISPLAY`` and plane 35
(belonging to that crtc) is set to ``SELF_REFRESH``. ``kmstest --flip`` is then run to
exercise page-flipping on the display. While ``SELF_REFRESH`` remains set, the new
frames that ``kmstest`` flips in are dropped by the driver and the plane keeps
looping the frame it was set to. When ``kmstest`` exits, the video port is not
torn down (because both properties are set together), so the same frame keeps
looping on screen even after the application exits.

Buffers
-------

Expand All @@ -782,3 +917,75 @@
--------------------------

DRM framework supports "emulating" the legacy fbdev API. This feature can be enabled or disabled in the kernel config (CONFIG_DRM_FBDEV_EMULATION). The fbdev emulation offers only basic feature set and the fb is shown on the first display. Fbdev emulation is mainly intended for kernel console or boot splash screens.


Abbreviations
=============

+-----------------+--------------------------------------------------------------+
| Abbreviation | Description |
+=================+==============================================================+
Comment on lines +922 to +927

@StaticRocket StaticRocket Aug 4, 2026

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Suggested change
Abbreviations
=============
+-----------------+--------------------------------------------------------------+
| Abbreviation | Description |
+=================+==============================================================+
Acronyms
========
+-----------------+--------------------------------------------------------------+
| Acronym | Description |
+=================+==============================================================+

I appreciate this effort, but it doesn't really make vale happy. Still looking at adding a global glossary for all of this. Unfortunately even with a global glossary I think we'll have domain specific overlaps. Still not entirely sure how I want that to work.

| DSS | Display SubSystem |
+-----------------+--------------------------------------------------------------+
| DISPC | Display Controller |
+-----------------+--------------------------------------------------------------+
| DRM | Direct Rendering Manager |
+-----------------+--------------------------------------------------------------+
| KMS | Kernel Mode Setting |
+-----------------+--------------------------------------------------------------+
| CRTC | CRT Controller (represents a display pipeline's timing and |
| | scanout engine in the DRM/KMS model) |
+-----------------+--------------------------------------------------------------+
| VP | Video Port |
+-----------------+--------------------------------------------------------------+
| VID / VIDL | Video pipeline types; VID pipelines support scaling, VIDL |
| | pipelines do not |
+-----------------+--------------------------------------------------------------+
| DPI | Display Parallel Interface |
+-----------------+--------------------------------------------------------------+
| DSI | Display Serial Interface (MIPI DSI) |
+-----------------+--------------------------------------------------------------+
| MIPI | Mobile Industry Processor Interface |
+-----------------+--------------------------------------------------------------+
| OLDI | Open LVDS Display Interface |
+-----------------+--------------------------------------------------------------+
| LVDS | Low-Voltage Differential Signaling |
+-----------------+--------------------------------------------------------------+
| DP | DisplayPort |
+-----------------+--------------------------------------------------------------+
| SST | Single-Stream Transport (DisplayPort mode) |
+-----------------+--------------------------------------------------------------+
| MST | Multi-Stream Transport (DisplayPort mode) |
+-----------------+--------------------------------------------------------------+
| HDMI | High-Definition Multimedia Interface |
+-----------------+--------------------------------------------------------------+
| MHDP | Multi-Host DisplayPort controller IP (Cadence) |
+-----------------+--------------------------------------------------------------+
| PHY | Physical layer transceiver |
+-----------------+--------------------------------------------------------------+
| GPU | Graphics Processing Unit |
+-----------------+--------------------------------------------------------------+
| LUT | Look-Up Table |
+-----------------+--------------------------------------------------------------+
| RGB | Red-Green-Blue color format |
+-----------------+--------------------------------------------------------------+
| YCbCr | Luma, blue-difference, red-difference chroma color format |
+-----------------+--------------------------------------------------------------+
| BT.601 / BT.709 | ITU-R color encoding standards for YCbCr conversion |
+-----------------+--------------------------------------------------------------+
| DMA | Direct Memory Access |
+-----------------+--------------------------------------------------------------+
| FB | Framebuffer |
+-----------------+--------------------------------------------------------------+
| IP | Intellectual Property (a reusable hardware block) |
+-----------------+--------------------------------------------------------------+
| EVM | Evaluation Module |
+-----------------+--------------------------------------------------------------+
| SK | Starter Kit |
+-----------------+--------------------------------------------------------------+
| DT | Device Tree |
+-----------------+--------------------------------------------------------------+
| PM | Power Management |
+-----------------+--------------------------------------------------------------+
| V4L2 | Video for Linux 2 (Linux media/video capture API) |
+-----------------+--------------------------------------------------------------+
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