[Intel-gfx] UHD 620: How to debug a screen resolution that does not work reliably

[Jani Nikula]

On Mon, 23 Apr 2018, Ricardo Ribalda Delgado <ricardo.ribalda at gmail.com> wrote:
> Hi
>
> I have a secondary monitor connected via USB-C adapter to  HDMI. It
> can manage resolutions up to 2560x1440.
>
> Most of the time, when the system is booted the resolution is detected
> ok, but If I suspend the machine, or replug the screen, or alternate
> to the text console, the resolution is "downgraded" to Full HD.
>
> I have added the paramter drm.debug=0x06  to my kernel option and I
> have managed to catch this error when the resolution is not detected
> properly:
>
> [    4.941022] [drm:drm_mode_debug_printmodeline [drm]] Modeline
> 77:"2560x1440" 60 241500 2560 2608 2640 2720 1440 1443 1448 1481 0x48
> 0x5
> [    4.941046] [drm:drm_mode_prune_invalid [drm]] Not using 2560x1440
> mode: CLOCK_HIGH
>
>
> On the other hand, when it works fine:
>
> [   95.361838] [drm:drm_mode_debug_printmodeline [drm]] Modeline
> 85:"2560x1440" 60 241500 2560 2608 2640 2720 1440 1443 1448 1481 0x48
> 0x5
> [   95.928536] [drm:intel_dump_pipe_config [i915]] crtc timings:
> 241500 2560 2608 2640 2720 1440 1443 1448 1481, type: 0x48 flags: 0x5
> [   95.928546] [drm:intel_dump_pipe_config [i915]] port clock: 540000,
> pipe src size: 2560x1440, pixel rate 241500
>
>
> The adapter is brand new (Xiaomi) and the cable should be of good
> quality (ethernet capable).
>
>
> Any ideas about how to debug this? The EDID seems fine, it seems
> related to hock clock_high is calculated.

CLOCK_HIGH is just a consequence of the link being downgraded, there's
not enough bandwidth in the link for the mode.

You have a bug open at freedesktop.org [1], please let's keep the
discussion in one place.


BR,
Jani.


[1] https://bugs.freedesktop.org/show_bug.cgi?id=106223


-- 
Jani Nikula, Intel Open Source Technology Center