ACK/Cmnt: [SRU][G][PATCH v2 0/4] Make Intel GPUs choose YCbCr420 encoding automatically when required for 4k 60Hz output

Tim Gardner tim.gardner at
Tue Jul 6 13:16:08 UTC 2021

Acked-by: Tim Gardner <tim.gardner at>

It is helpful to the reviewers if you note what non-trivial changes were 
required in the backport. For example, patch 3 dropped an unused static 
function. Notes of this type are generally right after the 'backported 
from' line, e,g,

[ Dropped unused static function to prevent compiler warnings ]

Patch 4 was also a clean cherry-pick for me. Perhaps our merge fuzz 
settings are different.


On 7/5/21 9:14 AM, Werner Sembach wrote:
> BugLink:
> 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 at 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):
> Patchset already got accepted upstream and reached the torvalds tree:
> and needs only a minor modifications to apply to ubuntu-focal/hwe-5.8
> Commit-hashes:
> 3c4442aa22878091f16c8d9592f5f5b6a94d1556
> eacba74d4d561ea6487d944417526e1b025cbebd
> 84d95f77f4aea3f22a486cd04777afd4ab0f0ea5
> 388b863509f76f6a5ecedd7ffdaf184aa813241e

Tim Gardner
Canonical, Inc

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