4K at 60 YCbCr420 missing mode in usermode

[Zuo, Jerry]

Hello all:

We are working on an issue affecting 4K at 60 HDMI display not to light up, but only showing up 4K at 30 from: https://bugs.freedesktop.org/show_bug.cgi?id=106959 and others.

Some displays (e.g., ASUS PA328) HDMI port shows YCbCr420 CEA extension block with 4K at 60 supported. Such HDMI 4K at 60 is not real HDMI 2.0, but still following HDMI 1.4 spec. with maximum TMDS clock of 300MHz instead of 600MHz.
To get such 4K at 60 supported, it needs to limit the bandwidth by reducing the color space to YCbCr420 only. We've already raised YCbCr420 only flag (attached patch) from kernel side to pass the mode validation, and expose it to user space.


    We think that one of the issues that causes this problem is due to usermode pruning the 4K at 60 mode from the modelist (attached Xorg.0.log). It seems like when usermode receives all the modes, it doesn't take in account the 4K at 60 YCbCr4:2:0 specific mode. In order to pass validation of being added to usermode modelist, its pixel clk needs to be divided by 2 so that it won't exceed TMDS max physical pixel clk (300MHz). That might explain the difference in modes between our usermode and modeset.



    Such YCbCr4:2:0 4K at 60 special mode is marked in DRM by raising a flag (y420_vdb_modes) inside connector's display_info which can be seen in do_y420vdb_modes(). Usermode could rely on that flag to pick up such mode and halve the required pclk to prevent such mode getting pruned out.



We were hoping for someone helps to look at it from usermode perspective. Thanks a lot.



Regards,

Jerry Zuo

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