[BUG] PineBook Pro - HDMI output intermitently dead with compositor slowed down

Hi, sometimes I have a problem, that HDMI connected monitor (via USB-C dock) shows "No signal" after KDE starts up compositor. If I swich over to another VT (Ctrl-Alt-F2 for example), I do see mirrored content of laptop display. Similarly login manager shows picture on both laptop screen and external video output. The problem is intermittent. Sometimes it won't help to reboot (machine hangs up on reboot if external output is enabled under such conditions), sometimes it fixes itself on another attempt to enable external output. Disconnect/reconnect of USB-C dock also does not seem to help. External output is available in console always, even if KDE does not show anything on external output. If this happens and external output is enabled in KDE, compositor extremely slows down and I see following message spamming the kernel log:

[ 110.617621] ------------[ cut here ]------------
[ 110.617642] [CRTC:40:crtc-1] vblank wait timed out
[ 110.617914] WARNING: CPU: 2 PID: 441 at drm_atomic_helper_wait_for_vblanks.part.0+0x27c/0x298 [drm_kms_helper]
[ 110.617919] Modules linked in: rfcomm cdc_ether usbnet r8152 mii bnep zram joydev snd_soc_hdmi_codec snd_usb_audio snd_usbmidi_lib snd_hwdep snd_rawmidi snd_seq_device snd_soc_simple_amplifier panfrost gpu_sched dw_wdt snd_s
oc_rockchip_i2s snd_soc_rockchip_pcm btsdio hci_uart cw2015_battery btqca btbcm btintel bluetooth rockchip_rga brcmfmac ecdh_generic ecc rockchip_thermal videobuf2_dma_sg brcmutil cfg80211 hantro_vpu(C) rfkill videobuf2_dma_contig v4l2_mem2mem videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_common rockchipdrm dw_mipi_dsi dw_hdmi analogix_dp cec rc_core drm_kms_helper snd_soc_es8316 pwm_bl syscopyarea sysfillrect sysimgblt fb_sys_fops rockchip_saradc snd_soc_simple_card snd_soc_simple_card_utils panel_simple crypto_user drm drm_panel_orientation_quirks gpio_keys hid_multitouch
[ 110.618114] CPU: 2 PID: 441 Comm: kworker/u12:2 Tainted: G WC 5.6.0-2-MANJARO-ARM #1
[ 110.618120] Hardware name: Pine64 Pinebook Pro (DT)
[ 110.618233] Workqueue: events_unbound commit_work [drm_kms_helper]
[ 110.618248] pstate: 80000005 (Nzcv daif -PAN -UAO)
[ 110.618357] pc : drm_atomic_helper_wait_for_vblanks.part.0+0x27c/0x298 [drm_kms_helper]
[ 110.618469] lr : drm_atomic_helper_wait_for_vblanks.part.0+0x27c/0x298 [drm_kms_helper]
[ 110.618475] sp : ffff8000120fbca0
[ 110.618481] x29: ffff8000120fbca0 x28: 000000000000047c
[ 110.618492] x27: 0000000000000000 x26: 0000000000000001
[ 110.618502] x25: 0000000000000038 x24: 0000000000000038
[ 110.618513] x23: ffff0000ea753000 x22: 0000000000000002
[ 110.618523] x21: ffff0000ebd22080 x20: ffff0000c7ebe080
[ 110.618534] x19: 0000000000000001 x18: 0000000000000001
[ 110.618544] x17: 0000000000000000 x16: 0000000000000000
[ 110.618554] x15: ffff800011452000 x14: ffff800011509858
[ 110.618565] x13: 0000000000000000 x12: ffff800011508000
[ 110.618575] x11: ffff800011452000 x10: 0000000000000000
[ 110.618586] x9 : 0000000000000003 x8 : 0000000000000a99
[ 110.618596] x7 : 0000000000000001 x6 : 0000000000000001
[ 110.618606] x5 : 0000000000000000 x4 : ffff8000120fba80
[ 110.618616] x3 : 0000000000000000 x2 : 0000000000000007
[ 110.618626] x1 : 6124f8cccb688500 x0 : 0000000000000000
[ 110.618636] Call trace:
[ 110.618750] drm_atomic_helper_wait_for_vblanks.part.0+0x27c/0x298 [drm_kms_helper]
[ 110.618861] drm_atomic_helper_commit_tail_rpm+0x60/0x78 [drm_kms_helper]
[ 110.618969] commit_tail+0x9c/0x188 [drm_kms_helper]
[ 110.619080] commit_work+0x10/0x18 [drm_kms_helper]
[ 110.619097] process_one_work+0x1d4/0x388
[ 110.619110] worker_thread+0x144/0x4c0
[ 110.619121] kthread+0xf0/0x120
[ 110.619132] ret_from_fork+0x10/0x1c
[ 110.619141] ---[ end trace 83e04ddb87b13203 ]---
[ 112.027604] ------------[ cut here ]------------

I am running Manjaro ARM which got preinstalled with latest batch (May 2020) of PineBook Pro. System is fully upgraded. I tried to use both standard mesa package and mesa-git package but it seems that this does not make any difference, no output situation happens with both, regardless of rebooting.

kernel: 5.6.0-2-MANJARO-ARM
X.org: 1.20.8
Mesa: 20.0.7-3

So far I am not aware of any reliable workaround.

Upgraded using latest batch of updates including new kernel 5.7.0. So far it didn't happen again. Yet while the issue was intermittent, I wouldn't call this fixed yet.

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