Make Intel GPUs choose YCbCr420 encoding automatically when required for 4k 60Hz output

[Werner Sembach]

SRU Justification:

Impact:
On some setups, while the monitor and the GPU support display modes with pixel clocks of up to 600MHz, the connector might not. This prevents RGB encoding for 4k60Hz, but YCbCr420 encoding might still be possible. However, which color mode is used is decided before the pixel clock capabilities are checked, causing the check to fail and discarding 4k60Hz from the list of possible display modes.

Fix:
This patch fixes the problem by retrying to find a display mode with YCbCr420 enforced and using it, if it is valid. It's very similar to a patch submitted to amdgpu which fixed the same problem.

Testcase:
I personally tested on a Clevo NV40MB, but generally: Find a PC with a current Intel iGPU, but only a HDMI 1.4 output. Connect a 4k@60Hz display supporting YCbCr420 encoding to the HDMI port. Without the patch the maximum that can be set via xrandr is 3840 × 2160 30Hz. With the Patch 3840 × 2160 60Hz can be selected which will use YCbCr420 automatically.

Prerequisite (included in this email patchset as 1/4):
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c4442aa22878091f16c8d9592f5f5b6a94d1556

Patchset already got accepted upstream and reached the torvalds tree:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eacba74d4d561ea6487d944417526e1b025cbebd
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=84d95f77f4aea3f22a486cd04777afd4ab0f0ea5
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=388b863509f76f6a5ecedd7ffdaf184aa813241e
and needs only a minor modifications to apply to ubuntu-focal/hwe-5.8

Commit-hashes:
3c4442aa22878091f16c8d9592f5f5b6a94d1556
eacba74d4d561ea6487d944417526e1b025cbebd
84d95f77f4aea3f22a486cd04777afd4ab0f0ea5
388b863509f76f6a5ecedd7ffdaf184aa813241e

[Werner Sembach (matombo)]

I'm currently resolving the minor merge conflict with ubuntu-5.8 kernel and then test it

[Werner Sembach (matombo)]

The bug still exists on 5.11.

The Patchset just got merged after the 5.13 mainline release, so in mainline it will be in 5.14 onwards.

[Werner Sembach (matombo)]

Patchset ported, tested, and sent to kernel-team mailing list

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-hirsute' to 'verification-done-hirsute'. If the problem still exists, change the tag 'verification-needed-hirsute' to 'verification-failed-hirsute'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1934489

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Werner Sembach (matombo)]

Something seems to have broken automatic selection of ycbcr420 (Also in mainline linux releases).

I need more time to figure out what exactly is causing this.

[Werner Sembach (matombo)]

Ok, turns out it only works on some Intel based laptops. But for them this is still an improvement so I vote for this patch to be keept, as it has no downsides for other devices.

e.g. The Clevo NS50MU Barebone with an i7-1165G7 only outputs 4k@30Hz without this patch, but 4k@60Hz with YCbCr420 with this patch.
Accoring to the spec sheet of the CPU it should be possible to output 4k@60Hz without relying on YCbCr420, but for some reasons this fails. (Didn't check on Windows).

The Clevo L140CU with an i5-10210U cannot output 4k@60Hz with Chroma subsampling.
The spec sheet does not give clear indication whether the CPU supports HDMI 1.4 or 2.0, but it does support 4k@30Hz which should be roughly the same bandwith requirement as 4k@60Hz with chroma subsampling. So i don't know where exactly this fails yet.

TL;DR: Works on Clevo NS50MU, does not work on Clevo L140CU, but also has no negative effect on it -> Patch is still an improvent.

[Werner Sembach (matombo)]

Offtoppic, but just for reference: Checked the Clevo NS50MU on windows: There 4k@60Hz is also only possible with YCbCr420 so it's no linux specific problem, but propably some strange hardware limitation.

So this device defenetly needs this patch and there is no other fix where 4k@60Hz would run without YCbCr420.

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 5.11.0-34.36

---------------
linux (5.11.0-34.36) hirsute; urgency=medium

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