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Mario Kleiner d65bfb1782 drm/amd/display: Change dither policy for 10 bpc output back to dithering
Commit d5df648ec8 ("drm/amd/display: Change dither policy for 10bpc to
round") degraded display of 12 bpc color precision output to 10 bpc sinks
by switching 10 bpc output from dithering to "truncate to 10 bpc".

I don't find the argumentation in that commit convincing, but the
consequences highly unfortunate, especially for applications that
require effective > 10 bpc precision output of > 10 bpc framebuffers.

The argument wasn't something strong like "there are hardware design
defects or limitations which require us to work around broken dithering
to 10 bpc", or "there are some special use cases which do require
truncation to 10 bpc", but essentially "at some point in the past we
used truncation in Polaris/Vega times and it looks like it got
inadvertently changed for Navi, so let's do that again". I couldn't find
evidence for that in the git commit logs for this. The commit message also
acknowledges that using dithering "...makes some sense for FP16...
...but not for ARGB2101010 surfaces..."

The problem with this is that it makes fp16 surfaces, and especially
rgba16 fixed point surfaces, less useful. These are now well
supported by Mesa 25.3 and later via OpenGL + EGL, Vulkan/WSI, and by
OSS AMDVLK Vulkan/WSI/display, and also by GNOME 50 mutter under Wayland,
and they used to provide more than 10 bpc effective precision at the
output.

Even for 8 or 10 bpc surfaces, the color pipeline behind the framebuffer,
e.g., gamma tables, CTM, can be used for color correction and will
benefit from an effective > 10 bpc output precision via dithering,
retaining some precision that would get lost on the way through the
pipeline, e.g., due to non-linear gamma functions.

Scientific apps rely on this for > 10 bpc display precision. Truncating
to 10 bpc, instead of dithering the pipeline internal 12 bpc precision
down to 10 bpc, causes a serious loss of precision. This also creates the
undesirable and slightly absurd situation that using a cheap monitor
with only 8 bpc input and display panel will yield roughly 12 bpc
precision via dithering from 12 -> 8 bpc, whereas investment into a
more expensive monitor with 10 bpc input and native 10 bpc display will
only yield 10 bpc, even if a fp16 or rgb16 framebuffer and/or a properly
set up color pipeline (gamma tables, CTM's etc. with more than 10 bpc out
precision) would allow effective 12 bpc precision output.

Therefore this patch proposes reverting that commit and going back to
dithering down to 10 bpc, consistent with the behaviour for 6 bpc or 8 bpc
output.

Successfully tested on AMD Polaris DCE 11.2 and Raven Ridge DCN 1.0 with
a native 10 bpc capable monitor, outputting a RGBA16 unorm framebuffer and
measuring resulting color precision with a photometer. No apparent visual
artifacts or problems were observed, and effective precision was measured
to be 12 bpc again, as expected.

Fixes: d5df648ec8 ("drm/amd/display: Change dither policy for 10bpc to round")
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: stable@vger.kernel.org
Cc: Aric Cyr <aric.cyr@amd.com>
Cc: Anthony Koo <anthony.koo@amd.com>
Cc: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Cc: Krunoslav Kovac <krunoslav.kovac@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
History 2026-04-03 13:57:50 -04:00
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Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
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* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


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

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