[v2,1/2] drm/msm/dpu1: don't choke on disabling the writeback connector

[Leonard Lausen]

Dear György,

>> Do you observe this issue on every suspend-resume cycle?
> 
> I just did 10 suspend/resume cycles in a row to double check, and without this patch the screen never comes back (always have to switch VT back-and-forth to bring it back). The
> 
> [dpu error]connector not connected 3
> [drm:drm_mode_config_helper_resume] *ERROR* Failed to resume (-22)
> 
> pair of error messages also consistently appears after all resumes.
> 
> Though I think e.g. Rob Clark reported that suspend/resume already works properly for him without this patch, so this experience is not universal on the Yoga Slim 7x.

Ack. Do you mean that Rob Clark also uses Yoga Slim 7x but does not face the "screen never comes back (always have to switch VT back-and-forth to bring it back)" issue?

>> On sc7180 lazor, I do observe that this patch deterministically breaks restoring the CRTC state and functionality after resume. Can you please validate if you observe the same on Lenovo Yoga Slim 7x? Specifically, try set Night Light in your desktop environment to "Always On" and observe whether the screen remains in "Night Light" mode after resume. For lazor, "Night Light" is breaks after applying this patch and even manually toggling it off and on after resume does not restore "Night Light" / CRTC functionality.
> 
> Unfortunately I cannot test this, as color temperature adjustments seems to be completely non-functional for me in the first place. For color temperature adjustment, I use gammastep on my machines, which uses wlr_gamma_control_unstable_v1 under the hood. It outputs the following warnings:
> 
> Warning: Zero outputs support gamma adjustment.
> Warning: 1/1 output(s) do not support gamma adjustment.
> 
> I haven't dug deeper into the cause yet, based on these it seems that wlroots isn't detecting the display as being gamma-adjustable in the first place.

The cause is simple: Qualcomm SoCs don't implement GAMMA_LUT support. Your desktop environment needs to use Color Transform Matrix (CTM) on ARM/QCom devices. You can refer to https://bugs.kde.org/show_bug.cgi?id=455720 for further details. It would be great if you can validate whether this patch breaks CRTC state (which includes the CTM state) on Yoga Slim 7x, or whether that is specific to the trogdor lazor (Chromebook Acer Spin 513), though it may require you to install KDE. Gnome does not support CTM yet (https://gitlab.gnome.org/GNOME/mutter/-/issues/2318).

Best regards
Leonard